By Chun-Ting Jake Cheng a Thesis Submitted in Conformity with The

ASSEMBLY,INTEGRATION, AND TESTOFTHE INSTRUMENTFOR SPACE ASTRONOMY USEDON-BOARDTHE BRIGHT TARGET EXPLORER CONSTELLATION OF NANOSATELLITES

Chun-Ting Jake Cheng

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science Graduate Department of Aerospace University of Toronto

Assembly, Integration, and Test of the Instrument for Space Astronomy used on-board the Bright Target Explorer Constellation of Nanosatellites

Chun-Ting Jake Cheng Master of Applied Science Graduate Department of Aerospace University of Toronto 2012

The BRIght Target Explorer (BRITE) constellation is revolutionary in the sense that the same scientiﬁc objectives can be achieved smaller (cm3 versus m3) and lighter (< 10kg versus 1, 000kg). It is a space astronomy mission, observing the variations in the apparent brightness of stars. The work presented herein focuses on the assembly, integration and test of the instrument used on-board six nanosatellites that form the constellation. The instrument is composed of an optical telescope equipped with a Charge

Coupled Device (CCD) imager and a dedicated computer. This thesis provides a particular in-depth look into the inner workings of CCD. Methods used to characterize the instrument CCD in terms of its bias level stability, gain factor determination, saturation, dark current and readout noise level evaluation are provided. These methodologies are not limited to CCDs and they provide the basis for anyone who wishes to characterize any type of imager for scientic applications.

ii Dedication

To my dear parents, especially my Mother, who had taken on such a tough role to provide care, guidance and encouragement throughout my childhood and teenage years and continues to be my most loyal supporter. Everything good I achieve in this life will be because of your nourishment.

To my friends. Thank you all for putting up with my complaints while providing me with your un- biased views. I owe it to you guys for putting some sanity back in my life during stressful times. Admittedly it can be tough to be my friend and you guys deserve my greatest gratitude for being there and unknowingly living up to my high standard for being good friends. You guys deserve all the best in your future and I have no doubt you will attain it.

To Science. It is one of my greatest desires that the BRITE mission will be a spectacular success in the scientiﬁc community. Moreover, my greed compels me to wish some of the greatest mysteries in nature would be resolved through BRITE. This way I could be content in life knowing that I had played a role in contributing to a major scientiﬁc revolution.

iii Acknowledgements

First deserving of acknowledgement is Dr. Robert Zee for offering me the opportunity to work on a real mission and putting it to space, this probably grants me the bragging rights amongst my friends for the foreseeable future. Particularly deserving of note is his support to grant me the chance to take on spacecraft assembly, a spacecraft thermal vacuum test campaign and the Small Satellite conference eye-opening experiences.

Second, to BRITE mission manager Cordell Grant, who never ceases to impress me with how he is able to manage multiple missions of such high complexity. Furthermore, I would like to credit his patience for putting up with a lot of my recurring questions which in many occasions serve as real life examples to counter proof the phrase ”there is no stupid question”.

Third, to BRITE instrument electronics designer Mihail Barbu, who deserves no less credit than any other for the amount of guidance he provided throughout. Thank you very much for explaining and helping me navigate through the vast realm of electronics. Too many times it almost seemed that your mere presence would inﬂuence the electronics and cause it to ﬁx itself (and there are times when you gently reminded me to plug it in or turn it on). But lets call it even for getting you out of the dark room.

Fourth, to fellow student Jakob Lifshits, who proves to be an invaluable asset to the lab, whose de- ductive reasoning permeates through the way he converses and shows wisdom beyond his age.

Fifth, to the overall lab guru Daniel Kekez, who had consistently shown me immense patience and complete willingness to explain things down to the very detail. Thank you very much for your assistance throughout my time in the group.

Sixth, to Dr. Stefan Mochnacki, who provided expert guidance and input on the various aspects of instrument testing. Your guided me, a complete amateur on the topic of charge coupled device, to carry out the various testing processes and provided me with sanity checks on the ideas and procedures I came up with. Thank you also for accompanying me with the outdoor ﬁeld tests and lending my your vast knowledge in astronomy.

Seventh, to Dr. Rainer Kuschnig for his collaboration, guidance, and efforts during the instrument focusing process. Your valuable inputs on the star ﬁelds to image are also much appreciated.

Eighth, to my second thesis reviewer, Dr. Slavek Rucinski, for his time, attention and valuable feed- back. Most importantly you are the father of BRITE constellation. This thesis, let alone the BRITE

iv mission, would never have existed without your successful proposal to adopt a nanosatellite platform to perform the speciﬁc science!

Last, but not least, to all the staff and students, some of you who I wish I had more chance to interact with, but I am sure I have required help from every single one of you at some point during my time at the lab. You guys have enhanced my understanding of big words such as dedication, diligence and professionalism. I wish all of you the best in your future endeavors.

v Contents

1 Introduction 1 1.1 History of Space Telescopes and the State of the Field ...... 1 1.1.1 Orbiting Astronomical Observatory 2 ...... 2 1.1.2 International Ultraviolet Explorer ...... 2 1.1.3 High Precision Parallax Collecting Satellite ...... 3 1.1.4 Hubble Space Telescope and Charge Coupled Devices Technology ...... 3 1.1.5 Microvariability and Oscillations of Stars ...... 4 1.1.6 Swift ...... 5 1.1.7 Convection Rotation et Transits Planetaires´ ...... 6 1.2 Thesis Objectives, Contributions, and Signiﬁcance ...... 6

2 The CanX-3 BRIght Target Explorer (BRITE) 8 2.1 The Generic Nanosatellite Bus (GNB) ...... 8 2.1.1 Attitude Determination and Control Subsystem ...... 9 2.1.2 Power Subsystem ...... 10 2.1.3 On-Board Computer Subsystem ...... 12 2.1.4 Communication Subsystem ...... 12 2.2 Science Objective ...... 13 2.3 Mission Operation ...... 15 2.4 Formation of the BRITE Constellation ...... 17

3 BRITE Instrument 18 3.1 Detector Selection - From CMOS to CCD ...... 19 3.2 Charge Coupled Devices (CCD) ...... 19 3.3 Fundamentals of CCD Operation ...... 20 3.3.1 Charge Generation ...... 21 3.3.2 Charge Storage ...... 21 3.3.3 Charge Transfer ...... 22

vi 3.3.4 Charge Readout ...... 23 3.4 A Two Board CCD Driver Design ...... 23 3.4.1 CCD Header Board ...... 24 3.4.2 Instrument On-Board Computer (IOBC) ...... 25 3.4.3 Analog Front End (AFE) Chain ...... 27 3.5 Instrument Telescope Assembly ...... 28 3.5.1 Stray Light Suppression ...... 28 3.5.2 Instrument Optics and Point Spread Function ...... 29 3.5.3 Header Tray and Focusing Mechanism ...... 30

4 Instrument Hardware Qualification Tests 32 4.1 Environmental Tests ...... 33 4.2 Hardware Model Definitions ...... 34 4.3 Qualification Testing ...... 35 4.3.1 Type I Inspection, Rework and Delta Inspection ...... 35 4.3.2 IOBC Unit Level Functional Test ...... 35 4.3.3 Instrument Room Temperature LFFT ...... 41 4.3.4 Payload Instrument Scientific Acceptance Test (PISAT) ...... 45 4.3.5 Type II Inspection, Rework and Delta Inspection ...... 45 4.3.6 Thermal Shock, Post Thermal Shock Inspection and LFFT ...... 45 4.3.7 Thermal Vacuum Test ...... 46 4.3.8 Vibration Testing ...... 49

5 BRITE Payload Instrument Scientiﬁc Acceptance Test 50 5.1 Test Overview ...... 50 5.2 Test Setup ...... 51 5.3 Data Types ...... 52 5.4 Bias Level and Stability Test ...... 53 5.4.1 Requirements ...... 54 5.4.2 Data Gathering ...... 55 5.4.3 Data Analysis and Results for UniBRITE Instrument ...... 55 5.5 Gain Determination ...... 62 5.6 Gain Measurement ...... 63 5.6.1 Requirement ...... 64 5.6.2 Data Gathering ...... 64 5.6.3 Data Analysis and Results ...... 65 5.7 Saturation Test ...... 72

vii 5.7.1 Requirement ...... 72 5.7.2 Data Gathering ...... 72 5.7.3 Data Analysis & Results ...... 72 5.8 Dark Current Test ...... 73 5.8.1 Requirements ...... 73 5.8.2 Data Gathering ...... 73 5.8.3 Data Analysis and Results ...... 74 5.9 Readout Noise Level Test ...... 79 5.9.1 Requirement ...... 80 5.9.2 Data Gathering ...... 80 5.9.3 Data Analysis and Results ...... 80

6 Instrument Integration 81 6.1 Creating an Artiﬁcial Star Field ...... 81 6.1.1 Collimator Setup ...... 82 6.2 Instrument Focusing ...... 83

7 Field Testing with Real Stars 88 7.1 Paramount Tracking Platform ...... 88 7.2 Mobile Instrument Assembly ...... 89 7.3 Star Observation Setup ...... 90 7.4 Observing the Stars ...... 91 7.5 Image Processing ...... 91 7.6 Differential Photometry ...... 94 7.6.1 Performing Aperture Photometry with IRAF ...... 94 7.6.2 Calculating the Differential Magnitudes ...... 95

8 Future Work, Summary and Conclusions 97 8.1 Future Work ...... 97 8.2 Summary and Conclusions ...... 97

Bibliography 99

viii List of Tables

4.1 IOBC electronics shortage check points ...... 36 4.2 LFFT test description...... 42

5.1 Bias level and stability test requirements [28]...... 54 5.2 BLS-1 verification results ...... 56 5.3 BLS − 2 verification results for the 500 lines of pixels capturing the bottom section of the full frame bias image ...... 58 5.4 BLS − 2 verification results for the 500 lines of pixels capturing the top section of the full frame bias image ...... 58 5.5 BLS − 2 verification results...... 59 5.6 BLS−3 verification results, ADU values indicated are based on the mean of the average bias levels at each temperature...... 59 5.7 BLS − 4 verification results...... 60 5.8 BLS − 5 verification results...... 61 5.9 UniBRITE payload instrument gain test results...... 69 5.10 UniBRITE payload instrument gain determination test results using gradient co-subtraction method...... 71 5.11 UniBRITE instrument full well saturation level (approximate) ...... 72 5.12 Dark current requirements [28]...... 73 5.13 Dark count threshold levels that categorize each dark current sensitive pixel groups at different exposure times, at ambient temperature (25◦C ± 2◦C)...... 74 5.14 Dark current level of the most dark current sensitive groups of pixels at different exposure times at ambient temperature (25◦C ± 2◦C)...... 76 5.15 Dark current test results in response to the requirements stated in Table 5.12...... 79 5.16 UniBRITE payload instrument readout noise results...... 80

7.1 Star identiﬁer, name, magnitude and coordinates with respect to Vega for each of the 10 stars of interest...... 93 7.2 Differential magnitudes and associated errors...... 96

ix List of Figures

2.1 GNB in BRITE configuration (courtesy of B. Johnston-Lemke - SFL) ...... 9 2.2 Simplified GNB power subsystem topology (courtesy of G. Bonin - SFL) [9] . . . . . 10 2.3 Theoretical power-voltage characteristics of triple junction solar cell pair on constant temperature lines (courtesy of G. Bonin - SFL) [9] ...... 11 2.4 Modified version of the Herzsprung-Russell diagram from Richard Powell [25] showing the brightest stars only (courtesy of W. Bode [8]) ...... 14 2.5 Top level BRITE mission operation sequence (courtesy of J. Lifshits [22]) ...... 15

3.1 BRITE instrument hardware overview ...... 18 3.2 Bucket Brigade Analogy for CCD Imager Operation ...... 20 3.3 Monochrome QE of Kodak KAI-11002 CCD imager ﬁtted with micro-lens [2]. High- lighted region represents the target spectral range that the BRITE Blue and Red optics are designed for...... 21 3.4 Charge transfer process for a true 2–phase CCD ...... 23 3.5 Left: top view of CCD Header Board with a clear view of the Kodak KAI-11002 CCD imager chip. Right: bottom view of the CCD Header Board showing the SMA and Micro-D connector mounting points ...... 24 3.6 VCCD and HCCD clocking diagram illustrating single line readout [2]...... 26 3.7 Analog Front End (AFE) chain design of CCD output signal...... 28 3.8 Component diagram of the BRITE payload instrument (courtesy of C.Grant) [14]. . . . 28 3.9 BRITE Blue and Red instrument optics design and their respective theoretical PSF at boresight (courtesy of C. Grant) [14]...... 30 3.10 Header tray focusing mechanism where ﬁne adjustment of imager plane position can be adjusted with the socket wrench (courtesy of C. Grant) [14]...... 31

4.1 IOBC hardware test setup ...... 37 4.2 IOBC bias power-up sequence diagram...... 39 4.3 Heater current limit test setup...... 40 4.4 Thermal vacuum test temperature proﬁle [32]...... 48

x 5.1 Optical Element Setup for PISAT ...... 51 5.2 Gradient image produced with the PISAT optical setup when LED is turned on. . . . . 53 5.3 Sample bias image proﬁle. The bias ramp pattern is greatly exaggerated visually in this case as the image is a capture from DS9’s Z-scale display with units in ADU...... 54 5.4 BLS-2 data process sequence illustration ...... 57 5.5 BLS-4 data process sequence illustration. Note that the contrast ratio has been exaggerated to illustrate the ramp pattern in the bias frames...... 60 5.6 Visualization of a typical difference image. Top left: difference frame. Top right: pixel distribution of difference frame...... 61 5.7 The normalized pixel distribution of the difference image showed close match to a Gaus- sian ﬁt (data shown here was based on −20◦C bias data)...... 62 5.8 Photon conversion stages of a typical CCD imager [20]...... 63 5.9 Typical process of constructing a photon transfer curve for gain measurement...... 64 5.10 An ideal gradient image raster example showing smooth, linear intensity transition from saturation to dark...... 66 5.11 Photon transfer curve constructed based on ambient temperature (25 ± 2◦C) data images. 66 5.12 Three noise regimes and full well saturation on a signal vs. noise log-log plot [20]. . . 67 5.13 Signal vs. noise data from Figure 5.11 plotted in log-log format...... 68 5.14 Photon transfer curve constructed based on ambient temperature (25 ± 2◦C) data images. 68 5.15 Photon transfer curve at ambient temperature (25±2◦C) using the gradient co-subtraction method...... 70 5.16 Figure 5.15 processed with moving average to reduce the variance spread...... 70 5.17 Signal vs. noise data represented in log-log format...... 71 5.18 Dark count evolution over exposure time for the top percentage of most dark current sensitive pixels at ambient temperature (25◦C ± 2◦C)...... 75 5.19 Pixel dark count distribution at different exposure times, at ambient (25◦C ± 2◦C). . . 76 5.20 Pixel dark count distribution at different exposure times at 60◦C...... 77 5.21 Dark image taken over 10s exposure at 60◦C. Saturated columns and hot pixels are clearly visible...... 78 5.22 Dark image taken over 30s exposure at 60◦C. Left shows original contrast and right shows the enhanced contrast version for better visualization...... 78

6.1 Focusing the ancillary telescope using a commercial CCD (not BRITE instrument) and paramount tracking platform...... 82

xi 6.2 Left: unfocused pin-hole lights as observed by the commercial CCD mounted on the ancillary telescope. Right: calibrated collimator setup that produced sharp points of collimated light...... 83 6.3 The primary telescope being used as a collimator with the BRITE instrument placed at the observing end for focusing...... 83 6.4 Left: backside interior view of the instrument header tray module. Right: backside interior view with CCD Header Board installed (Photo credit to C. Grant)...... 84 6.5 Simulated PSFs examining intra-focal vs. extra-focal options for the BRITE Red Instru- ment (courtesy of Dr. R. Kuschnig)...... 84 6.6 Initial PSF exploration of CCD placement...... 85 6.7 Fine focus adjustment made on the set point (Figure 6.6) of initial PSF exploration for the UniBRITE instrument ﬂight assembly...... 85 6.8 PSF contrast before and after CCD imager plane tilt adjustment...... 86 6.9 PSF map of UniBRITE payload instrument...... 87

7.1 The mobile instrument assembly composed of the BRITE prototype camera hardware for real star observations...... 89 7.2 Hardware setup for star observation (picture taken during the daylight)...... 90 7.3 The Lyra constellation as imaged by the engineering model instrument...... 92 7.4 Close examination of the PSF achieved with the various stars of the Lyra constellation. 92 7.5 Raster frame showing the seven stars to be used for differential photometry analysis. . 93 7.6 Visualization of the aperture and annulus chosen for each star...... 95

xii Chapter 1

Introduction

This thesis details the author’s contribution to the BRIght Target Explorer (BRITE) constellation mission designed and built by the Space Flight Laboratory (SFL) at University of Toronto Institute for Aerospace Studies (UTIAS). More specifically, the focus will be on assembly, integration and testing of the BRITE payload system - an instrument composed of a miniature optical telescope equipped with a high resolution, low noise Charge Couple Device (CCD) imager. The BRITE constellation is designed to observe either the blue or red spectral region of stars. What was originally a single spacecraft mission had attracted international funding and grew to a constellation of six jointly funded by Austria, Poland and Canada. The multi-spacecraft mission allows expanded sky coverage, mission duration, and allows the constellation to observe in both blue and red spectrum without employing any moving color filters on the individual spacecraft level. Instead, each spacecraft is equipped with different optics and filter designed for the blue or red spectrum. A detailed overview of the BRITE constellation mission is delayed to Chapter 2. This chapter will provide a review on the state of the field, the problems addressed by the author, objectives of the thesis, contribution of the author to this mission, and what makes the work herein important.

1.1 History of Space Telescopes and the State of the Field

The idea of space telescopes was ﬁrst proposed in 1923 by German scientist Hermann Oberth who suggested blasting a telescope into space aboard a rocket. Another pioneer of space telescopes was Lyman Spitzer Jr. who was a major driving force behind the Orbiting Astronomical Observatory (OAO) and cul- minated in the ﬁrst successful OAO-2 and Copernicus (OAO-3) satellites. He also contributed greatly to the approval of National Aeronautics and Space Administration’s (NASA) Large Space Telescope (LST) project in 1969 that was the pre-incarnation of the Hubble Space Telescope [3]. Space-borne telescopes are able to provide more useful imagery data over ground based observatories (whether its intended for high resolution visualization, spectroscopy, or photometry) by eliminating the great degree

1 CHAPTER 1. INTRODUCTION 2 of variability that limits the level of attainable accuracy due to the presence of the Earth’s atmosphere (also known as atmospheric seeing). In what follows is a survey of past space telescopes and their instruments limited to the optical and ultraviolet (UV) bands (as this is the proper category for BRITE) to highlight the unique niche that the BRITE constellation ﬁts within.

1.1.1 Orbiting Astronomical Observatory 2

Starting chronologically, the Orbiting Astronomical Observatory 2 (OAO-2) was launched on December 7, 1968 aboard an Altas-Centaur rocket. Its mission is to carry out astronomical observations in the far- UV spectral region. OAO-2 is the first successful space telescope (given that OAO-1 suffered a power failure before the instrument could be activated and ended the mission in three days) that operated from December 1968 to January 1973. Weighing a total of 2, 012kg, the spacecraft carried a total of 11 instruments that were divided into two categories - the Smithsonian Astrophysical Observatory (SAO) telescopes that looked out from one end of the satellite and the Wisconsin Experiment Package (WEP) that looked out from the other end [30]. The SAO package, also known as the Celescope experiment, consisted of four 12-inch optical telescopes each fitted with different passband filters (1, 200 − 1, 500A,˚ 1, 375 − 1, 800A,˚ 1, 800 − 2, 800A,˚ and 2, 850 − 3, 250A)˚ and used Uvicons to produce television pictures of star fields. The purpose of the SAO package is to measure the UV brightness of stars. The SAO instruments operated for 16 months until its operation was ceased in April 1970 due to a significant decrease in sensitivity caused by over- exposing the Uvicons to direct sunlight. Over 8, 500, 2◦ × 2◦ star field images were captured during the course of its operation [24]. The WEP package consisted of four stellar photometers (1, 000 − 4, 250A),˚ two scanning spectrom- eters (1, 000−4, 000A),˚ and one nebular photometer (2, 000−3, 300A).˚ The instruments observed over 1,200 objects in the UV range including planets, comets, stars, star clusters and galaxies. The results of the data gathered led to the discovery that comets are surrounded by hydrogen halos and that galaxies emit higher level of UV than the level suggested by the visual colors of the stars they are composed of [30].

1.1.2 International Ultraviolet Explorer

The International Ultraviolet Explorer (IUE) was a joint European Space Agency (ESA), NASA and United Kingdom (UK) effort. The mission was first proposed in 1964 by UK scientists and was launched in 1978 aboard a NASA Delta rocket. The interesting fact about IUE is that the initial planned mission lifetime was only 3 years, but the satellite stayed operational for roughly 18 years. When it was eventually shut down, it was due to financial reasons, while the telescope itself was still functioning at near original efficiency. This makes IUE the longest-lived, most productive satellite in the history of space CHAPTER 1. INTRODUCTION 3 astronomy [18]. The IUE was set out to intercept UV light given off by objects ranging from supernovae to ap- proaching comets. UV observations is one of the perks of space-borne telescopes as the faint UV signal is largely absorbed by Earth’s ozone layer. This makes ground based observation of UV objects impossible. The satellite had a total mass of 671kg with the instrument contributing to 122kg. The instrument consisted of a 0.45m diameter aperture telescope with two fine error sensors (FES), two spectrographs and four UV detectors. The FESs operatd as visible light cameras to capture the telescope’s field of view. The image was transferred to ground where operator would verify the field and specify the particular object to observe. The two spectrographs: Short Wavelength Spectrograph (115 − 200nm) and Long Wavelength Spectrograph (185 − 330nm) were each equipped with two television cameras (hence a total of four) to perform UV observations. Each spectrograph had one camera designated as primary and another as redundant in case of failure in the primary. The UV gathered by the telescope and spectrographs was made detectable by the cameras through a UV-to-visible converter (caesium-tellurium cathode) that gave off electrons when struck by UV photons [31].

1.1.3 High Precision Parallax Collecting Satellite

The HIgh Precision PARallax COllecting Satellite (HIPPARCOS) was a European Space Agency (ESA) scientific mission launched in 1989 (operational until 1993) dedicated to astrometry, or precise measurement of the motion of stars. This mission led to composition of the highly precise Hipparcos Catalogue published in 1997 containing 1, 058, 332 stars [5]. With a launch weight of 1, 140kg, the Hipparcus spacecraft carried a single Schmidt telescope of 29cm diameter aperture. The payload utilized an image dissector tube (a video camera using phototube technology) for recording observations. It had two instruments, with the main one using a single, broad-band filter called Hp and with the second (Tycho) part of the satellite using two filters somewhat similar to (but not identical with) B and V bands in the (Johnson) UBV photometric system.

1.1.4 Hubble Space Telescope and Charge Coupled Devices Technology

Roughly eight months after the launch of HIPPARCOS was the launch of NASA and ESA’s Hubble Space Telescope in 1990. The telescope was carried into Low Earth Orbit (LEO) by Space Shuttle Discovery. With an impressive aperture of 2.4m diameter, Hubble houses a wide array of instruments that captures the visible light, ultraviolet, and near infrared wavelengths. Upon launch, the Hubble carried five initial instruments: Wide Field and Planetary Camera (WF/PC), Goddard High Resolution Spectrograph (GHRS), High Speed Photometer (HSP), Faint Object Camera (FOC) and the Faint Object Spectrograph (FOS) [4]. The WF/PC is the original main instrument onboard Hubble. It is a high resolution imaging device targeting bright astronomical objects and is particularly relevant to this thesis CHAPTER 1. INTRODUCTION 4 because its design spurred the development of CCD technology and pushed it towards maturity [19]. The LST project, a pre-incarnate of Hubble, was first proposed in 1965. The Jet Propulsion Lab (JPL) was commissioned with the design effort for the high resolution instrument. However, it was not until 1972 that JPL was introduced to CCD technology through Bell Laboratory. Prior to JPL’s development efforts, early companies such as Fairchild Semiconductor and RCA Corporation had already spent years in paving the road to major improvements in CCD technology. In 1974, Fairchild Semiconductor’s 1 × 500 linear array and 100 × 100-pixel CCD utilized buried channel architecture to achieve a high charge transfer efficiency (and reduced the information degradation due to charge transfer loss from pixel to pixel). The improved transfer efficiency meant large arrays of pixels could be built. Around the same time, RCA Corporation took a different approach to develop a surface channel, full-frame and backside-illuminated CCD. The combination of full-frame plus backside-illuminated architecture uses the entire pixel region for photon collection and charge generation. Backside illuminated sensors move the metal gate structure below the substrate layer to provide incident photons a direct path for charge generation. These features maximize the efficiency in which photons are recorded as charges by the CCD (quantum efficiency) [19]. JPL took on a custom R&D effort to combine the best attributes of all commercially available CCD technologies at the time. As a result, JPL (in collaboration with Texas Instruments) developed a scientific CCD sensor based on full-frame, backside-illuminated, buried channel architecture in an effort to achieve high charge transfer efficient and quantum efficiency. In 1975, JPL released a 400 x 400 pixel flight version CCD for the Mariner Jupiter-Urnanus mission. In 1976, JPL further announced that a buried channel, backside-illuminated, 800 × 800 pixel device was being developed for the Jupiter Or- biter Polar (JOP) mission (now known as Galileo). The same device eventually made its way to form the WF/PC in 1976 [19]. The WF/PC is two individual cameras, each composed of a 2 × 2 set of JPL’s 800×800 pixel CCD. Different optical arrangements were designed to suite the purpose of each camera. The Wide Field Camera (WFC) covered a large field of view (2.6 × 2.6 arcminutes [4]) at the expense of resolution. The Planetary Camera (PC), on the other hand, has only a 66 × 66 arcseconds [4] field of view but it provided images of distant planets at a much higher resolution.

1.1.5 Microvariability and Oscillations of Stars

The Microvariability and Oscillations of Stars (MOST) satellite is the next space optical telescope on the frontier launched in 2003. The MOST mission was designed by Dr. K. Carroll and Dr. S. Rucinski. The proposal was submitted to the Canadian Space Agency (CSA) which contracted Dynacon Enterprise as the main contractor. SFL was in turn, subcontracted by Dynacon with the design, construction and test of the MOST satellite bus. At time of writing, it is the first and only Canadian space telescope. As the name suggests, the goal of MOST is to observe and detect the microvariability and oscillations in stars’ brightness. Data collected by MOST is then used to carry out asteroseismology, which is the CHAPTER 1. INTRODUCTION 5 science that studies the internal structure of pulsating stars through interpreting their emitted spectral frequency. The primary goals of MOST are to provide clues that may help date the age of the universe, search for extrasolar planets and explain the origin of heavy elements (elements heavier than Helium) that compose the world and all life forms as we know it [6]. At just 60kg with a satellite bus form factor of 65 × 64 × 30cm, the MOST satellite belongs to the “Microsatellite” category and earned itself the nickname - “The Humble Telescope”. The instrument onboard MOST is an optical telescope with a 15cm diameter aperture. The telescope then projects the field of view on to two 1024 × 1024 frame-transfer CCD cameras arranged side-by-side. One camera is used to collect scientific measurements while the other is used as a star tracker for satellite attitude control. The instrument contains a single filter which selects lights in the wavelength range of 350nm to 700nm. There are not active moving parts in the entire instrument design (except for the actuated lens cap) and thermal control is achieved by passive radiation [6]. This “Humble Telescope” is ancestral to the BRITE mission in many different ways. From the perspectives of the scientific goals and mission objective, BRITE is very similar to MOST with the exception that MOST targets stars characterized by stellar oscillation periods from minutes to hours, while BRITE targets those with oscillation periods of hours to months. From a heritage point of view, the MOST design and development process provided much infrastructure from ground station to hardware designs that enabled many of SFL’s future missions. Programmatically speaking, MOST paved the way to prove that employing small satellites to tackle ambitious space mission goals is not only feasible but also cost effective.

1.1.6 Swift

Swift is a spacecraft launched into low earth orbit in 2004 for the Swift Gamma-Ray Burst Mission. The goal of the mission is to observe the afterglow of gamma-ray bursts in gamma-ray, X-ray, ultraviolet, and optical wavebands. The spacecraft is 1, 470kg and contains three instruments onboard. The Burst Alert Telescope (BAT), the X-Ray Telescope (XRT) and the Ultraviolet/Optical Telescope (UVOT). The BAT is used to continuously monitor the sky (field of view of 1.4 steradian) and detect gammar-ray bursts and accomplishes this by using a X-ray detector (cadmium zinc telluride semiconductor based detector tiles). When a burst is detected, BAT calculates an initial position and determines whether to send a slew command to the spacecraft or not [10]. The XRT is used to measure the fluxes, spectra, and light curves of gamma-ray bursts. Furthermore, the XRT can locate burst events to within 5-arcseconds accuracy. The XRT has a 23.6 × 23.6 arcminute field of view and uses a CCD based detector. The CCD has 600 × 602 402micron pixels and uses high resistivity silicon material to achieve a target passband of 0.2 − 10keV [12]. The UVOT also uses a 2048 × 2048 pixels CCD detector to examine the afterglow of gamma-ray bursts. With a 30cm diameter aperture and a 17 × 17arcminute field of view, it is the primary instrument to collect measurements for photometry in both ultra-violet and optical bands (170 CHAPTER 1. INTRODUCTION 6 to 650nm) on the gamma-ray burst after the spacecraft has slewed into position. The detector is a specially designed micro-channel plate intensified CCD that allowed detection of extremely low signal source. The UVOT has a moving filter wheel mechanism to provide pass bands on the desired spectral range [11].

1.1.7 Convection Rotation et Transits Planetaires´

Besides the MOST satellite, the Convection Rotation et Transits Planetaires´ (CoRoT) is perhaps the most relevant historical mission to BRITE. At 630kg and launched in 2006, the CoRoT mission is led by the French Space Agency (CNES) in conjunction with ESA. It shares similar mission goals with MOST and BRITE in searching for extrasolar planets and performing asteroseismology by measuring oscillations in the brightness of stars. The spacecraft is equipped with a 27cm diameter aperture telescope with four 2048 × 4096 pixels CCD cameras to detect tiny light variations of stars. The CCDs are based on thinned, back illuminated architecture to improve quantum efﬁciency and they are actively cooled down to −40◦C with temperature regulation better than 0.015◦C over the orbit. These stringent operating temperature requirements reduces the thermal noises (in the form of dark current) and random noise sources to a minimum [17].

1.2 Thesis Objectives, Contributions, and Signiﬁcance

Compared to CoRoT, BRITE is revolutionary in the sense that the same scientific objectives can be achieved smaller (20cm3 versus 4.3m high and 9m wide), lighter (6.5kg versus 630kg), and progra- matically less expensive satellite. Note the author simply states that similar mission objectives can be achieved more efficiently by employing small satellites and makes no claim that BRITE is superior to CoRoT, as the performance capabilities of a small satellite are obviously constrained by its size. Even compared to its ancestor MOST, the first microsatellite-class satellite within the list of past missions surveyed, BRITE is roughly one-fifteenth in size (20cm3 versus 65 × 64 × 30cm) and about a tenth in mass. These physical properties categorize BRITE under the nanosatellite class. Every aspects of the BRITE spacecraft design and every design resource is focused towards delivering the critical mission goal by trading off against secondary goals [26]. Combined with the mentality of actively adopting commercially off-the-shelf (COTS) components, this allows the tiny satellite to be developed with a small fraction of the cost of big satellites such as CoRoT. An example of where BRITE utilized COTS components, and the central theme of this thesis, is the adoption of a KODAK KAI-11002 burried-channel, interline CCD detector for the instrument (the supporting CCD driver electronics are custom designed and built by SFL). The usage of COTS components for instrument detector provides a significant programmatic cost saving. The tradeoff to bear is the shift in emphasis from custom component design to component testing to ensure reliability - and it is the main focus of this thesis. CHAPTER 1. INTRODUCTION 7

The BRITE instrument electronics and software driver designs were completed at the time when the author joined SFL. As a result, the author took on the role of carrying out the characterization tasks and tests necessary to qualify the instrument design. The main objective of these tests is to ensure the instrument’s performance matches the speciﬁcations required by the science team. Speciﬁcally, the author’s work and contributions to the BRITE mission contained herein include:

• The design of a long form functional test procedure (LFFT) for the instrument detector and driver electronics that veriﬁes system functionality.

• Execution of LFFT in conjunction with thermal vacuum conditions to qualify instrument electronics design under environmental extremes.

• Execution of the scientiﬁc acceptance test (originally designed by the BRITE Science Team) that characterizes the instrument at seven different system temperatures.

• Integration of the instrument detector with optical telescope to achieve the ideal focus.

• Preliminary testing of the completely integrated instrument under real sky conditions to observe faint stars and serve as ﬁeld veriﬁcation of proper optics-detector integration.

This thesis is written with three objectives in mind. One, to provide a background of the BRITE mission, an overview of the Generic Nanosatellite Bus (GNB) used for the mission, the mission goal, and mission operation (Chapter 2). Two, to act as a primer for charge coupled devices (CCD) and give a high level overview of the instrument design, covering the set of driver electronics that compose the BRITE instrument (Chapter 3). Three, to present the goals/requirements, methodologies, setups and results of each test carried out on the BRITE instrument (Chapter 4 through 7). Speciﬁc topics of interest covered are methods used to characterize the BRITE instrument CCD in terms of its bias level and stability, gain factor determination, saturation, dark current and readout noise level evaluation. The manner in which these characterizations are performed is not limited to CCDs and they provide the basis for anyone who wishes to characterize any type of imager for scientiﬁc applications. Furthermore, the high level programmatic test procedure and methodology presented herein are useful references for anyone who wishes to plan future missions involving space telescopes. Chapter 2

The CanX-3 BRIght Target Explorer (BRITE)

The CanX-3 BRITE is a space astronomy mission with the goal of measuring brightness oscillations of some of the most intrinsically luminous stars in our universe, specifically targeting those with apparent visual magnitude of +3.5 or brighter (m ≤ +3.5) [34]. The original mission concept for BRITE was developed by a Canadian astronomer, Dr. Slavek Rucinski, who proposed this specific science program for a nanosatellite, and this was not such an obvious thing at that time. The mission was envisioned to be a single nanosatellite mission following its predecessor, MOST. The mission concept has attracted wide interest in the science community causing the single satellite project to grow to two, then four and finally a constellation of six nanosatellites. The BRITE mission is similar to MOST with the exception that BRITE will target stars with stellar oscillation periods from hours up to months as opposed to minutes to hours. Judging from the success of MOST, which in its 8th year of continuous on-orbit operation has evolved from its original science objectives into an instrument for exoplanet exploration, there is simply no telling the extent of scientific applications that can be drawn from the data to be collected by BRITE. This chapter provides an overview of the satellite bus, scientific objective, mission operation and a brief chronology of how the lone nanosatellite mission grew into a constellation involving Austria, Poland and Canada to become a truly international collaboration effort.

2.1 The Generic Nanosatellite Bus (GNB)

A signiﬁcant advancement of SFL’s satellite bus technology was made after the success of early satellite missions due to increase in scope, complexity and the number of planned SFL missions. A new satellite bus platform design that can be easily conﬁgurable to suite various missions was necessary. This led to the development of the Generic Nanosatellite Bus (GNB) in preparation for the CanX-3 BRIght Target Explorer (BRITE) and the CanX-4/5 Formation Flying missions. The GNB leverages the various

8 CHAPTER 2. THE CANX-3 BRIGHT TARGET EXPLORER (BRITE) 9 technologies qualified through previous SFL missions. It has a 20cm3 form factor with the BRITE configuration of GNB weighing less than 6.5kg. Structurally, the satellite is composed of two primary trays that envelope a volume of approximately 17×13×8cm dedicated to payload and six surface panels that complete the bus enclosure. The GNB is generic in the sense that it is designed to be configurable to support multiple missions. This section will provide a brief overview of the major subsystems of the GNB, their main features and functionalities. Figure 2.1 shows the exploded view of the GNB satellite customized for the BRITE mission.

Figure 2.1: GNB in BRITE conﬁguration (courtesy of B. Johnston-Lemke - SFL)

2.1.1 Attitude Determination and Control Subsystem

The GNB houses a suite of attitude determination hardware including six sun sensors and one magne- tometer to provide measurements for attitude determination. In the case specific to BRITE, an additional star tracker unit is incorporated to resolve the fine arc-minute pointing requirement. The attitude measurements made by these hardwares feed into the dedicated attitude determination and control computer (ADCC) that processes these measurements through the on-orbit attitude system software (OASYS). The OASYS implements a standard Kalman filter and closed-loop inertial-pointing control algorithm to output a set of commands to the attitude control actuators. The system uses a primarily reaction-wheel CHAPTER 2. THE CANX-3 BRIGHT TARGET EXPLORER (BRITE) 10 based attitude control subsystem supplemented by magnetorquers. The three reaction wheels are arranged in orthogonal axes to provide full control coverage in the three rotational degrees of freedom and provide control torques to the spacecraft by spinning up to cause a net change in angular momentum, induce a reactive torque to the satellite body in the opposite direction. However, the wheel speed is limited and this sets a cap on the momentum that the reaction wheels can provide. For this reason, the GNB also houses three sets of magnetorquers that provide supplementary torques by interacting with Earth’s magnetic field. There is momentum build up in the wheels every time a torque command is issued and the magnetorquers act in a manner to transfer the momentum in the wheels to the Earth’s magnetic field, effectively dumping the momentum away from the spacecraft and preventing wheel speeds from reaching their saturation limit.

2.1.2 Power Subsystem

Common to most satellites, the GNB power subsystem uses photovoltaic (PV) or solar cell arrays for power generation. While the GNB is capable of connecting up to 22 pairs of triple junction solar cells the BRITE conﬁguration only needs a total of 17 pair to satisfy its energy budget (i.e. generated power less used power is greater than the prescribed design margin). Two sets of Lithium-Ion batteries are used to provide continuous power supply to the satellite during eclipses. The batteries are each equipped with a Battery Charge and Discharge Regulator (BCDR) that acts as the regulating interface between the batteries, the solar array and the GNB power board which connects with the rest of the spacecraft. A simpliﬁed topology of the GNB power subsystem is shown below:

Figure 2.2: Simpliﬁed GNB power subsystem topology (courtesy of G. Bonin - SFL) [9] CHAPTER 2. THE CANX-3 BRIGHT TARGET EXPLORER (BRITE) 11

Battery Charge and Discharge Regulator

The GNB power architecture implements a fully regulated, direct energy transfer bus with peak power tracking functionality [9]. The basic operation of the BCDR is to provide satellite bus voltage regulation. Figure 2.2 illustrates that the solar arrays are parallel connected to the bus voltage hence they are effectively clamped to the same level and are also effectively regulated by the BCDR. The solar array’s power output level is dependent on its voltage level as shown by the power-voltage curve in Figure 2.3. As a result, the peak power tracking functionality of the BCDR comes from the fact that it can actively adjust the satellite bus voltage to a level that draws the maximum power from the cells. Furthermore, the BCDR provides autonomous operation to draw the necessary amount of current from the battery to maintain the bus voltage at the desired set point as satellite loads are turned on or off. When excess power is present, the BCDR allows a stream of controlled current to charge the battery directly. If the battery’s state of charge is full and excess power is available, the BCDR will then raise the bus voltage beyond the peak power point in the solar cell power-voltage curve and effectively decrease the power output from the solar cells.

Figure 2.3: Theoretical power-voltage characteristics of triple junction solar cell pair on constant temperature lines (courtesy of G. Bonin - SFL) [9]

Power Board

A second major component of the power subsystem is the GNB power board that acts as the main power distribution and control device for the satellite. Every subsystem, with the exception of UHF uplink communication, the primary house keeping computer (HKC), attitude determination and control computer (ADCC) and S-band transmitter, can be turned on/off through commands addressed to the power board. The power board contains a field programmable gate array (FPGA) that implements a CHAPTER 2. THE CANX-3 BRIGHT TARGET EXPLORER (BRITE) 12 specific power controller code to interpret and execute commands from both the HKC and ADCC to turn subsystems on or off. Every switch has a dedicated regulation circuit that provides automatic over current protection by resetting the switch to the off state should there be a fault. Three sets of digital to analog converters (DAC) are implemented on the power board to individually control the current set point of the three magnetorquers (up to a maximum of 130mA each). In addition, the power board continuously monitors the uplink for firecode detection. A system reset firecode can be sent from the ground station to cause the power board to glitch the satellite main switch for a short period, triggering the power controller to return every resettable switch to the off position and returning the satellite to the most basic, post-deployment state. An additional safety feature of the GNB power board is the loadshed monitor system that triggers when the satellite battery voltage drops below approximately 3.49V. In the event of a loadshed, the satellite would return to the post-deployment state with additional constraint on every switch that restricts it from being turned on unless the loadshed latch is manually cleared by a ground operator.

2.1.3 On-Board Computer Subsystem

The GNB has two sets of on board computers (OBC) that are identical in terms of hardware components but differs in usage. One OBC is used for house keeping that handles general gathering of satellite telemetry and forwarding of commands/response packets addressed to and from various subsystems. The other OBC is used as the ADCC that acts as the brain of the attitude subsystem. It takes in attitude measurements and outputs controller commands to the reaction wheels and magnetorquers. In terms of hardware, each OBC contains a TMS470 ARM7 processor with bootloader code stored inside dedicated flash memory internal to the processor. This allows quick access and protects the code from corruption. Each instrument on-board computer (IOBC) also has a 256MB external flash memory that stores the application-level code which would be loaded into SRAM for execution during operation. Three 2MB SRAMs and a dedicated memory controller are populated on each OBC. Each of the three SRAMs are connected to a memory controller, which acts as a peripheral device to access the SRAM. In writing mode, the memory controller would write the same data to each SRAM. In reading mode, a triple- voting, majority wins, algorithm is implemented to mitigate errors due to bit flip. Each OBC is directly connected to the UHF uplink receiver and is able to decipher command packet from the High-level Data Link Control (HDLC) protocol sent from ground. Each OBC also has a firecode monitor circuit, similar to the power board that detects firecode commands to turn on, off, or reset the OBC.

2.1.4 Communication Subsystem

The standard GNB communication hardware consists of the UHF receiver for uplink and S-Band transmitter for downlink. The UHF receiver is always powered on in anticipation for commands from the CHAPTER 2. THE CANX-3 BRIGHT TARGET EXPLORER (BRITE) 13 ground station. The receiver receives at a ﬁxed data rate of 4 kbps (kilobits per second) with Gaussian Frequency-Shift Keying (GFSK) modulation. Data lines from the UHF connect directly to the power board, HKC and ADCC. The address byte in the uplink command packet would specify which on-board component it is intended for and only the designated target will take action. The UHF receiver utilizes four canted, monopole antenna elements arranged in a quad canted orientation on the exterior of the satellite bus in order to provide the highest spherical coverage to ensure commands from ground can be received regardless of satellite orientation. The UHF antennas can be seen on the +Y face of the satellite in Figure 2.1. Similarly, the downlink capability of the GNB is provided by the S-band transmitter and its two planar patch antennas mounted on the +Z and -Z faces of the satellite. Each planar patch antenna radiates power in a near hemispherical pattern therefore the combination of the two planar patch antennas arranged opposite to each other would transmit in a near omnidirectional manner. The S-band transmitter is capable of transmitting at variable data rates of 32, 64, 128, 256, 512 and 1028 kbps with two different phase modulation schemes (Binary Phase Shift Keying, BPSK, or Quadrature Phase Shift Keying, QPSK) to provide boosts in data transfer rate when the link condition is favorable.

2.2 Science Objective

The science objective of BRITE is to provide photometric time series measurements on stars with apparent visual magnitude of +3.5 or brighter (m ≤ +3.5). The collected data will then be used by the BRITE science consortium to perform precise differential stellar photometry over the span of the observation campaign in order to identify the oscillation pattern in the apparent brightness of the stars. Because internal seismic activities are directly reflected in the star’s luminosity, observing stellar oscillations allow scientists to extract information to validate existing theories and produce new models that would enhance our understanding of the internal composition, dynamics and life cycle of these stars. Specifically, it would allow them to deduce the size of convective cores, the radiative areas and the rotation of stars which is not possible with the level of measurement accuracy achievable from ground based observations due to atmospheric extinction [8]. In astronomy, a star’s brightness is described in units of magnitude. The apparent visual magnitude system [m] (different from absolute magnitude [M]) is the measure of brightness of a star as seen by an observer located on Earth, normalized to the value it would would have been in the absence of Earth’s atmosphere. The normalization process is accomplished by taking the flux of the star being observed as a ratio over a reference star and the magnitude is given by: m = −2.5log F , where F is the total 10 F0 flux of the target star and F0 is that of a target reference star, both observed within the same spectral band. Photometry, in the broad sense, describes the art of counting photons detected by the observation instrument and absolute photometry refers to the faithful determination of a star’s brightness based on the photometer measurement. Precise absolute photometry is generally very difficult to achieve because CHAPTER 2. THE CANX-3 BRIGHT TARGET EXPLORER (BRITE) 14 it is prone to variations due to atmosphere conditions and in the measuring instrument itself. As a result, the measurements made are subject to uncertainties introduced by the temporal state of the scenery and of the system itself. By using a space borne photometer, the atmospheric effects are eliminated but sensitivity variations in the measurement system still persist. Differential photometry is an alternative method of gaging the brightness of stars that mitigates these uncertainties by taking the ratio of target star’s apparent flux to the average level of a group of nearby stars and expressing it as a differential magnitude: FT m = −2.5log10 (2.1) Favg Each differential magnitude measurement made on the target star will represent its apparent brightness at that instant in time. The division taking place in (2.1) effectively normalizes the sensitivity variations due to spacial and temporal differences in the state of the photometer system. Hence the BRITE instrument is aimed to observe both the target star plus a group of reference stars with the goal of identifying the brightness variations of the target star over time with respect to the background brightness variations (which is safe to assume as being approximately constant). The target stars of interest are the apparently brightest stars as seen from Earth’s sky. These apparently bright stars are also intrinsically the brightest and most massive stars in the universe, which is an indication of their massive size and high surface temperature as can be seen from the Herzsprung-Russell (or H-R diagram) shown in Figure 2.4.

Figure 2.4: Modiﬁed version of the Herzsprung-Russell diagram from Richard Powell [25] showing the brightest stars only (courtesy of W. Bode [8])

These features of bright and massive size are consequently indications of the abundance of heavy ele- CHAPTER 2. THE CANX-3 BRIGHT TARGET EXPLORER (BRITE) 15 ments (elements heavier than hydrogen and helium) that are created through the active nuclear fusion processes taking place inside the star’s core. As a result, these stars are most inﬂuential in the creation and distribution of heavy elements either by surface eruption that ejects and form stellar winds or by the eventual supernova formed at the end of star’s life when all the heavy elements are spread over the interstellar medium to form the fundamental building blocks for all life forms on Earth. Ultimately, the science consortium expects that the data collected with BRITE will allow them to answer the few remaining questions regarding the life cycle of these giant stars and perhaps even provide clues to the grand mystery that is the origin of life.

2.3 Mission Operation

Each BRITE spacecraft is equipped with an optical telescope and a CCD imager that compose the science instrument to perform space astronomy. The primary mission objective is to provide data images that will support milli-magnitude accuracy differential photometry measurements on stars with apparent visual magnitude of +3.5 or brighter. The planned observation period is 15 minutes per orbit over an observation campaign that can last up to 6 months [34]. Milli-magnitude accuracy refers to the sampling spread in the differential magnitude of the observed star, specifying that the standard deviation needs to be on the order of 0.1% of the mean value. The mission operation is illustrated in Figure 2.5.

Figure 2.5: Top level BRITE mission operation sequence (courtesy of J. Lifshits [22]) CHAPTER 2. THE CANX-3 BRIGHT TARGET EXPLORER (BRITE) 16

The entire operation cycle will begin with the BRITE science consortium that publishes an observation plan specifying the star fields of interest. The plan is then passed into the BRITE Target ground software and split into an observation schedule and an observation setup file. The observation schedule file accounts for the time when the target star field will be within the field of view (FOV) of the spacecraft and provide the necessary attitude coordinates for the spacecraft attitude control system (ACS) to place the star field within FOV of the science instrument. This information is in turn passed to BRITE Schedule ground software which generates a set of time-tagged command scripts to be uploaded to the spacecraft. The commands will be stored in a queue and executed at the specified time, thus allowing autonomous imaging operation without ground station intervention. The observation setup file contains the set of exposure times to be use for the observations and is uploaded on to the dedicated instrument on-board computer (IOBC). In general, the exposure time chosen will take account of the star’s magnitude and be just long enough to allow the star images on the photometer to approach the imager’s saturation level (but of course under saturation) in order to sufficiently mitigate effects of readout noise and dark current that would dominate low signal regions. Each BRITE spacecraft will perform its intended task by pointing the science instrument’s optical boresight axis to the desired star field as specified by the time-tagged command script. Once the system achieves a lock on to the target, the instrument will begin to take measurements at the highest cadence possible over a planned 15-minute observation window per orbit (where the particular cadence level would depend on the integration time to be used for each exposure). An entire observation campaign on a particular star field can last up to six months in order to capture the long stellar oscillation periods of some of these bright stars. To optimize the usage of available onboard memory and processing power, only a predefined raster region surrounding a star of interest will be extracted from the full image and stored in memory. A particular feature of the BRITE mission is the strict pointing requirement that demands attitude control of the system to be accurate to within 1 arc-minute root-mean-square (RMS). This requirement is based on the strong desire to maintain the target star images to within 2-3 pixels of the same position throughout the observation campaign in order to minimize pixel-to-pixel sensitivity variations that would hurt differential photometry accuracy [34]. This operation strategy further allows rasters to be downloaded instead of the full frame image. This significantly reduces the download time, maximizes the usage of available on-board memory and minimizes the amount of data to be downlinked. Furthermore, software algorithms running on the IOBC will further co-add raster sets that capture the same star of interest in order to reduce the inherent noise level and improve the overall signal to noise ratio (S/N) [14]. CHAPTER 2. THE CANX-3 BRIGHT TARGET EXPLORER (BRITE) 17

2.4 Formation of the BRITE Constellation

Since its inception, the BRITE mission has drawn much attention amongst the scientific community. The early stage mission concept envisioned just two BRITEs, each identical to the other with the exception of carrying a different set of optics and color filter, one blue and one red tuned to the blue and red spectrum. The multi-spacecraft nature of the mission allows the mitigation of moving parts such as moving color wheels and provides multiple ”eyes” in the sky that increase observational duty cycle. The BRITE mission became a reality in 2005 when the University of Vienna took interest and spearheaded the funding for the first satellite known as UniBRITE. Although funded by the University of Vienna, UniBRITE was designed, built and tested entirely at SFL in Canada. Funding for the second satellite, known as BRITE-Austria, came through in January of 2006 by the Austrian Space Agency. BRITE- Austria was assembled and tested in Austria by engineers at the Technical University of Graz using a kit of parts with guidance from SFL. The UniBRITE spacecraft would carry a red optical telescope while BRITE-Austria would carry a blue version to target the red and blue spectral passbands respectively. Several years later into the development of the first two BRITE satellites, Poland took interest in the program and decided to fund two more BRITEs in 2010. These new additions, known as BRITE Poland 1 and BRITE Poland 2 are exact duplicates of the two Austrian BRITE satellites with the possibility that BRITE Poland 2 might carry an ultraviolet (UV) version of the instrument optics instead (should a feasibility study yield favorable results). The Canadian Space Agency came through with the deci- sion to fund two Canadian BRITE spacecrafts to be known as BRITE-Toronto and BRITE-Montreal in 2011. This ensured Canada’s full participation in a project that was originally a Canadian idea. The two Canadian BRITEs will also be duplicates of the two Austrian satellites carrying a Blue and Red version of the payload instrument optics. At the time when this thesis was written, the planned launch for the first two Austrian BRITE spacecrafts are scheduled for the fourth quarter of 2011 onboard the Indian Polar Star Launch Vehicle-C20 (PSLV-C20). The first Polish BRITE is to be launched in September 2012 with the second Polish BRITE, and the two Canadian BRITEs are to be launched in 2013. In retrospect, UniBRITE and BRITE-Austria bore all of the development efforts which allowed the Polish and Canadian development cycle to be accelerated. Furthermore, the advanced launch of the two Aus- trian BRITEs provides the lead time for on-orbit commissioning and calibration of the spacecraft and science instrument. Experiences gained through this process should significantly speed up the processes for the subsequent BRITEs. Most likely, the constellation of BRITE spacecraft will begin observations at roughly the same time with a slight stagger of operation life time. This also provides the benefit of ex- tending the mission life time (from the time the first satellite comes online until the last one dies) while also permitting highly configurable observation strategies whether the simultaneous observation of the same targets in multiple passbands or to maximize sky coverage by having different pairs of BRITEs observing different star fields [14]. Chapter 3

BRITE Instrument

The BRITE instrument is a photometer equipped with a customized optical lens system that will perform photometry on bright stars of interest. Charge-Coupled Device (CCD) based imaging technology was chosen for this purpose. In terms of hardware, the instrument consists of a dedicated Instrument OBC (IOBC), a CCD header board and a telescope assembly. The IOBC is a single printed circuit board (PCB) stored in the same stack with the HKC, ADCC and the GNB power board located on the +Z tray of the satellite bus as shown in Figure 3.1.

Figure 3.1: BRITE instrument hardware overview

18 CHAPTER 3. BRITE INSTRUMENT 19

The CCD header board is a separate PCB containing the CCD imager chip and is mounted to the electronics mounting tray at the back of the telescope assembly. The data transfer and power supply between the IOBC and the CCD header board is accomplished by a 28-pin micro-D wire harness and a dedicated coaxial cable for imager analog signal output. The entire payload instrument only takes up a space of 7.5cm×7.5cm×19cm of the allocated payload subsystem volume, and is located right in the middle of the satellite bus between the two trays as illustrated in Figure 2.1.

3.1 Detector Selection - From CMOS to CCD

The original mission concept proposed by Dr. Slavek Rucinski was to take advantage of the low cost, low power and ease of integration offered by Complimentary Metal-Oxide-Semiconductor (CMOS) imager technology. Unfortunately Fabry–Perot´ fringe patterns were identified in the images taken by the prototype camera using an IBIS4-14000 CMOS imager that was selected at the early stage of the BRITE design phase. Tests performed with an uniform light source and spectrograph revealed intensity variations up to 40% across the imager at all visible wavelengths. The cause of the fringing effect was eventually traced back to unevenness in the applied surface passivation layer on the CMOS imager chip and was deemed to be common to all CMOS imagers. As a result the IBIS4 imager was not appropriately suited to achieve the high level of accuracy required for BRITE’s scientific objective [14] [29]. In the search for an alternative candidate, the BRITE team came across the STL-11000M astronomy research CCD camera assembly offered by Santa Barbara Instrument Group (SBIG). [4] Spectrograph tests performed with this camera revealed no evidence of Fabry–Perot´ fringing and intensity variation over the imager was significantly lower at approximately 6%. The result of this test steered the choice of detector towards the Kodak KAI-11002 CCD used inside the STL-11000M SBIG camera. This CCD had relatively low power consumption and very good noise and dark current performance even at room temperature [14] [29]. The Kodak KAI-11002 CCD is an 11-MPixel interline, buried channel architecture CCD with 4008 x 2672 effective pixel dimensions. Each pixel is 9µm2 in size and individually fitted with micro lens in order to better focus light toward the photo-reactive region of the pixel. At only 37.25mm(H) x 25.7mm(V) in size, this detector also offers built-in electronic shutter and anti-blooming protection [2].

3.2 Charge Coupled Devices (CCD)

A Charge-Coupled Device (CCD) is essentially an arrangement of Metal-Oxide Semiconductor (MOS) capacitors that share the same substrate and oxide layers. Local charge accumulation in the substrate is induced when a bias voltage is applied to an individual electrode or gate, creating a potential difference relative to other gates. As a result, local potential wells in the substrate can be set up to trap free charges CHAPTER 3. BRITE INSTRUMENT 20 through proper biasing of a series of gates. Furthermore, these potential wells can be manipulated to control the movement of free charges when gate biasing is combined with proper timing and hence the name “charge-coupled”. Despite having originally been developed as an alternative memory technology, CCD was found to have greater potential in imaging application due to their light sensitive nature (through the photoelectric effect) and their ability to control the movement of electric charges. Since its conception, the CCD technology has evolved to this day as a particular type of imaging technology that has gained popularity in photography, medical, astronomy, and space imaging applications.

3.3 Fundamentals of CCD Operation

The topic regarding the intricate inner workings of the CCD deserves a book of its own and a good reference is Scientiﬁc Charge-Coupled Devices by J. R. Janesick. This section only aims to introduce the basic aspects of how CCD captures an image. The operation of a CCD imager is best pictured through the “bucket brigade” analogy originally devised by Jerome Kristian and Morley Blouke. In this analogy, an array of buckets capture rain drops (analogous to photons). These buckets are arranged on columns of conveyor belts that provide transportation in the vertical direction. A single row of conveyor belt positioned at the end of the columns provide transportation in the horizontal direction. During every cycle, one bucket from each column conveyor belt is shifted down on to the row conveyor belt to populate an entire row. Each bucket on the row is then shifted one by one sideways and emptied into a measurement cup that measures the amount of water collected. The cycle continues until the entire array of buckets have been emptied. The measurements taken from each bucket is then reconstructed to form the original array and an accurate depiction of the rain pattern. In an actual CCD imager, the rain drops are photons, the buckets are pixels, the columns are vertical CCDs (VCCD) and the row is a horizontal CCD (HCCD). The measurement cup is an analogy for an output ampliﬁer that converts the collected charge at each pixel into an output voltage level. A CCD imager, and as a matter of fact every electronic imager technology, must perform four distinct functionalities: charge generation, charge storage, charge transfer and charge readout.

Figure 3.2: Bucket Brigade Analogy for CCD Imager Operation CHAPTER 3. BRITE INSTRUMENT 21

3.3.1 Charge Generation

Charge generation is achieved through the photoelectric effect. An incident photon possessing sufficient energy can interact with a loosely bound valence electron in the substrate layer and create a free electron- hole pair in the material. The photoelectric effect is an all-or-nothing type of event. This means no electron-hole pair is generated if the energy level of the photon is insufficient. On the other hand, multiple electron-hole pairs can be created if the photon energy is high enough. As a result, CCD imagers exhibit different levels of sensitivity to different photon energy levels, or wavelengths. Quantum Efficiency (QE) is a measure of this sensitivity and it specifies the percentage of photons that will create an electron-hole pair at specific wavelengths. Figure 3.3 below illustrates the varying degree of QE to photon wavelength for the Kodak KAI-11002 CCD imager with the target spectral range intended for the BRITE Blue and Red optics highlighted.

Figure 3.3: Monochrome QE of Kodak KAI-11002 CCD imager ﬁtted with micro-lens [2]. Highlighted region represents the target spectral range that the BRITE Blue and Red optics are designed for.

3.3.2 Charge Storage

The charge storage functionality of the CCD concerns the amount of electrons that can be effectively confined within a single pixel region. Width-wise, each pixel column is defined by vertical channel stops made with opposite doping than the substrate layer. They are laid out equally spaced along the imager CHAPTER 3. BRITE INSTRUMENT 22 area and prevent charges from migrating from one column to another. Length-wise, it is defined by two or more gate structures (i.e. 2-phase, 3-phase, or n-phase CCDs) made of polysilicon material that are laid atop but isolated from the substrate layer, the channel stops and other gates by silicon oxide. These gates can induce large potential differences within the substrate when bias voltage is applied. Regional potential wells can be shaped with a combination of electrode biasing to confine free electrons and prevent recombination with the substrate material. Charge capacity is important since greater capacity means larger dynamic range (DR). The photodiode charge capacity level of the Kodak KAI-11002 CCD is approximately 60, 000e− [2] and is deemed by the Science Team to be more than sufficient for the purpose of the BRITE mission.

3.3.3 Charge Transfer

In the simplest example of a true two-phase CCD (i.e. the VCCD of the Kodak KAI-11002 imager), two gates define a single pixel as illustrated in Figure 3.4. A true two-phase CCD requires additional doping treatment to a strip region beneath the metal gates in order to create the step potential required for effective electron transfer. Suppose at time T1 (refer to Figure 3.4), a high-state potential is applied on Gate 1 to form a collector, and a low-state potential is applied on Gate 2 to form a barrier. When charge transfer initiates at time T2, Gate 2 is switched to high and causes a portion of the electrons to drift towards the substrate under Gate 2 due to electrostatic repulsion (self-induced drift). An instant later, Gate 1 is switched to a low state to become a barrier and forces the remaining electrons toward Gate 2. Fringe field effect is mostly responsible for charge transfers happening near the Gate 1 and 2 boundary while a small quantity of remaining charge at the center of Gate 1 would drift toward Gate 2 due to thermal diffusion (naturally tending toward). At time T3, the potentials applied on Gate 1 and Gate 2 are once again reversed to cause a further shift of the electrons. This process completes the transfer of electrons from one pixel to another. Inevitably, some of the electrons are lost during the transfer process due to recombination with the substrate as it moves from pixel to pixel. The percentage of electrons lost during the transfer process is measured by charge transfer efficient (CTE). CTE is one of the primary factors that limits the size of the CCD. In the case of Kodak KAI-11002 CCD, it has a CTE of 0.99999 [2] or a cumulative CTE of 0.99999(2672+4048) = 0.9350 that accounts for the furthest transfer from the far top right pixel to the readout site. This means an original full capacity of 60, 000e− will result in having 56, 100e− remaining when the charge packet arrives at the final readout site. CHAPTER 3. BRITE INSTRUMENT 23

Figure 3.4: Charge transfer process for a true 2–phase CCD

3.3.4 Charge Readout

The charge readout mechanism of the CCD consists of an output ampliﬁer that is responsible for con- verting from the charge domain to the analog signal (voltage) domain. The output ampliﬁer sensitivity of the Kodak KAI 11002 CCD is 13µV/e−.

3.4 A Two Board CCD Driver Design

CCD performance is extremely sensitive to temperature. Generally speaking, lower imager temperature brings higher signal-to-noise ratio (S/N). The original STL-11000M camera assembly contains a two CHAPTER 3. BRITE INSTRUMENT 24 stage thermal electric cooling unit with liquid assist that can bring the operating temperature down to 50◦C below ambient conditions [1]. Unfortunately active thermal cooling is not achievable on BRITE due to volume and power constraints, which drove the science instrument design towards a two-board system that would minimize the heat dissipation effect caused by the CCD driver electronics on the CCD itself. The ﬁrst board, known as the CCD header board, contains the CCD imager chip, heaters and control electronics. The second board, known as the instrument on-board computer (IOBC) contains all on-board computing, power regulation and CCD driver electronics. The IOBC board is an amalgamation of a GNB on-board computer (OBC) and the driver electronics required to operate the CCD imager. The two-board conﬁguration not only mitigates thermal issues, it also isolates the imager from the noise generated by supporting electronics located on the IOBC.

3.4.1 CCD Header Board

The CCD header board provides the mounting pins for the Kodak KAI-11002 CCD imager chip and has a 28-pin connector that connects to the IOBC for all the power and clock timing signals necessary to operate the imager. A dedicated SMA connector is also present to allow the transfer of CCD analog signal off the header board and to the IOBC through a coaxial cable that minimizes external noise interference. The header board houses an additional output ampliﬁer with an ampliﬁcation of 3dB (or 2 times) and a large inline resistor connected to the output SMA connection in series to match the impedance of the coaxial cable. An additional feature of the CCD header board is the active thermal control system based on four resistive heaters that run on conventional Proportional-Integral-Derivative (PID) control law [22]. CCD performance is greatly affected by temperature. The heater units are in place to provide active biasing of the CCD operating temperature to a level higher than the highest temperature variations that would be experienced as the satellite moves through its orbit.

Figure 3.5: Left: top view of CCD Header Board with a clear view of the Kodak KAI-11002 CCD imager chip. Right: bottom view of the CCD Header Board showing the SMA and Micro-D connector mounting points CHAPTER 3. BRITE INSTRUMENT 25

3.4.2 Instrument On-Board Computer (IOBC)

The IOBC is based on the GNB OBC design but contains many modifications to become a dedicated computer to operate the imager. It uses the same TMS470 ARM7 processor, three separate 2-MB SRAM chips and a memory controller for EDAC, 32-MB SDRAM used to temporarily store data after each exposure and a 256-MB flash chip for a more permanent storage. There is also a field-programmable gate array (FPGA) chip that is used for tasks of clock frequency division, signal and switch control. This section will focus on the CCD driver electronics aspect of the IOBC to provide a high level overview of the key component blocks that handle the CCD analog signal output.

Pixel Clock Driver

Two sets of clock drivers are used to separately control the VCCDs and the HCCD on the imager chip. The KODAK KAI-11002 is an interline CCD. This means each pixel is divided into a photoactive and a light shielded region. The photoactive region is where the photoelectric effect takes place and where free electrons are created. Underneath the light shielded region lay the VCCD structures. As a result, the VCCD clock driver must perform two distinct tasks: charge readout where electrons are transferred from the photoactive region to the VCCD region, and charge transfer where electrons are transferred from the VCCD region of one pixel to the next. The VCCD driver accomplishes this by using two bias signals, V1 and V2 that are applied to Gate 1 and Gate 2 of the VCCD respectively. The V2 signal has three states: V2low , V2mid and V2high as shown in Figure 3.6. The V2high state triggers the charge readout process. A large potential is applied to Gate 2 to induce a great potential well that causes the free electrons generated in the photoactive region to drift underneath Gate 2 of the VCCD. Following the readout process, the electron packets are transferred from one VCCD to the next at the completion of each clock cycle (as illustrated in Figure 3.4). Since the V1 and V2 signals are being shared amongst all the VCCDs, this results in charge transfer from all pixels of one row to the next row. At every VCCD clock cycle, the bottom row of pixels would transfer its electrons down one row to the HCCD. The HCCD is then responsible for shifting the entire row out of the CCD for analog ampliﬁcation after every VCCD cycle. As a result the HCCD clock driver works at a much higher frequency than that of the VCCD. The HCCD clock driver only needs two bi-state signals, H1 and H2 from the timing generator to alternatively pulse the gate structure to perform horizontal transfer. Figure 3.6 illustrates the combination of HCCD and VCCD timing. The Kodak KAI-11002 CCD is capable of both single and dual output mode. The dual output mode provides a faster way to transfer the electrons out of all the pixels on the CCD. The tradeoff is that a second set of output ampliﬁer and data transfer link (SMA connector and coaxial cable) is needed. As a result, the single output mode is selected in favor of reduced amount of electronics which would take additional volume and produce excess thermal dissipation. CHAPTER 3. BRITE INSTRUMENT 26

Figure 3.6: VCCD and HCCD clocking diagram illustrating single line readout [2].

Timing Generator

A Kodak KSC-1000 timing generator chip lies at the heart of the CCD driver electronics. It is designed specifically by Kodak to implement timing control on a wide range of their imager products and provides all the pixel clock signals, bias rails necessary to run the CCDs. It also has an internal Programmable Logic Device (PLD) to allow custom, programmable timing strategies. The timing generator takes in the system clock, produced by a 20MHz crystal oscillator, and outputs the pixel clock rate at 10MHz which results in an exposure time resolution of 100ns; significantly better than the fidelity required on even the shortest exposure (i.e. 0.01% of 0.1s, or 10µs).

Bias Power Supply

Two branches of power regulators are present on the IOBC to perform DC-to-DC conversions from the spacecraft bus voltage. Switching power supplies are adopted to maximize power conversion efficiency. The first branch operates at 1MHz to regulate the 1.8V and 3.0V rails that are used to power all the digital logic components. The second branch of the switching power regulator (using single-ended primary- inductor converter topology, SEPIC) operates at 300 KHz and produces six rails of ±6V , ±12V and ±18V that are used to supply substrate biasing and clock voltage generation. These two branches are synchronized with the main crystal oscillator to ensure the noise signatures produced by the switching power supplies, should there be any, are constant and deterministic [14] [29]. In order to be compliant with the built-in ElectroStatic Discharge (ESD) protection circuit in the CCD, the IOBC also implements a dedicated power up sequence circuit to divide the entire instrument CHAPTER 3. BRITE INSTRUMENT 27 power-up process into three stages. When the bias power is turned on, the ±6V , ±12V and ±18V bias rails become active first. The ESD circuit comes up 60ms later and a regulated precision rail is powered up at 120ms. The precision rail is used as a reference rail to a total of 18 high-speed voltage follower circuits that produce all the clock biases used for pixel readout. This highly stable precision rail is designed to prevent possible variations at the input of the voltage follower circuits, which would show up as fluctuations at the output and may produce noticeable effects on the quality of the resulting image.

CCD Analog to Digital Converter

The CCD analog to digital converter (ADC) chip populated on the IOBC is dedicated to sample and convert the analog data output from the Kodak KAI-11002 imager. It consists of several notable built-in features such as correlated double sampling (CDS), gain amplification, bias level offset, and the final ADC process to produce the resulting digital data that are stored into the SDRAM. The sampling process is initiated by signals supplied from the Kodak timing generator. The CDS samples the analog signal waveform coming from the imager at two instances for every pixel. It then takes the difference between the two samples as the pixel charge measurement. This approach eliminates uncertainties in photometric measurements due to possible variations in the DC offset level. Two sets of amplifier modules are present in the ADC: pixel gain and variable gain. The pixel gain is intended for color balance in color imaging applications. For BRITE’s monochrome application, this option is turned off but still provides a base 3.3dB signal amplification. The variable gain amplification is set to 3.53dB to sufficiently utilize the 2V input range of the 14-bit ADC while providing some headroom to prevent saturating the ADC. As a result, a total of 6.83dB amplification is applied to the signal. Before the amplified analog signal is converted into the digital domain, a 8-bit digital-to-analog (DAC) circuit superimposes an user programmable DC offset level for image biasing. A setting of 80 Analog to Digital Units (ADU) was chosen. This way the zero signal reference is offset to a sufficiently high level to ensure all negative noise fluctuations remain positive and can be captured in the resulting image for analysis. Finally, the biased signal is fed into the internal 14-bit ADC to become a stream of digital outputs.

3.4.3 Analog Front End (AFE) Chain

The tradeoff made in going for a two board design is the increased distance that the pixel charge measurement data, in analog form, must travel in order to arrive at the analog to digital converter (ADC). A coaxial cable is used in order to guard against external signal interference. Because of the inherent impedance in the coaxial cable, an inline resistor with large resistance is used to offset the inductance CHAPTER 3. BRITE INSTRUMENT 28 and capacitance effects of the coaxial cable. This results in the voltage divider (that reduces the signal by a factor of 2) and coaxial arrangement as illustrated in Figure 3.7, which prompted the installation of an output ampliﬁer on the CCD header board to boost the signal by a factor of 2 to counteract signal degradation.

Figure 3.7: Analog Front End (AFE) chain design of CCD output signal.

3.5 Instrument Telescope Assembly

The instrument telescope assembly was designed to be simple and modular for ease of assembly and integration with the CCD imager. It consists of primarily three modules: bafﬂe, optical cell, and header tray that houses the CCD header board as shown in Figure 3.8.

Figure 3.8: Component diagram of the BRITE payload instrument (courtesy of C.Grant) [14].

3.5.1 Stray Light Suppression

Stray light was found to be a big inconvenience for the MOST mission and hence extra effort was made during the design phase to make sure the entire telescope assembly would be light tight. Any surface CHAPTER 3. BRITE INSTRUMENT 29 within the instrument that might possibly reflect light directly into the optics is anodized matte black. A baffle module is incorporated to block out the majority of the stray light. All the housing rings in the telescope and baffle apertures are also machined knife edge sharp to minimize gap between telescope housing and the optical elements. Finally, fine threadings are engraved into the inside wall of the optical cell to minimize shallow angle reflections [14].

3.5.2 Instrument Optics and Point Spread Function

The BRITE instrument optics is designed to be a wide field of view (24◦ x 19◦) telescope with the opposite goal as most conventional optical lens systems. Instead of aiming for a sharp focus where the stars would appear as a single bright pixel, the BRITE Science Team requires the instrument to achieve a suitably defocussed Point Spread Function (PSF) to overcome the problem of under sampling. Under sampling refers to the limitation in the absolute knowledge of flux of the stars as analogous to Shannon’s theorem but in terms of the spatial bandwidth sampled. In the context of photometry, the ultimate goal is to count the number of photons detected from the stars, but the finite, pixelated structure of the imager poses a limitation on the available spatial resolution. As a result, many photons could be lost in the gaps between the pixels and true knowledge of the star’s luminosity could be limited. PSF refers to the particular pattern that appears on the imager plane from a point source. A defocussed PSF overcomes the problem of under sampling. By designing the instrument optics to achieve a larger PSF, a single point source is essentially split into smaller groups and are sampled by multiple pixels instead of one. This increases sampling efficiency and reduces loss of signal. This approach is especially suitable for differential photometry where the goal is to observe the relative luminosity fluctuation of the stars over time. The science team came up with the specific goal to achieve a target PSF that is Gaussian in shape with Full Width at Half Maximum (FWHM) of 5-7 pixels and 99% of the energy enclosed in a 12-pixel diameter. However, such a PSF was found difficult to achieve due to the lack of commercially available optimization tools. After numerous design iterations, a double Gauss lens system with an external aperture stop was deemed by the BRITE Science Team to have a PSF that was likely sufficient and the optics design was settled. Figure 3.9 shows the resulting BRITE Blue and Red instrument optics and the simulated spot diagrams illustrating the respective theoretical PSF achieved at boresight [14]. CHAPTER 3. BRITE INSTRUMENT 30

Figure 3.9: BRITE Blue and Red instrument optics design and their respective theoretical PSF at boresight (courtesy of C. Grant) [14].

3.5.3 Header Tray and Focusing Mechanism

The CCD Header Board is mounted inside the header tray at the back of the telescope assembly during the instrument assembly and integration process. In addition, the header tray preloads the lenses in the optical cell segment with approximately 200N of force using a rubber o-ring sandwiched between the fifth lens and the tray [14]. This preload ensures that there is no chatter of the lenses against the housing due to vibration during the launch process. In addition to the already defocused optics design, the BRITE Science Team wished to explore the option of further defocusing the instrument by shifting the imager plane with respect to the optical cell. Due to this desire, the header tray design adopted a simple spring loaded mechanism that allowed fine adjustment of imager position through rotating the four sets of focusing screws with a socket wrench fitted with angular scale as illustrated in Figure 3.10. A total of four focusing springs and screws are used instead of the typical three in order to avoid unnecessary flexing of the CCD Header Board. The accuracy of turning the socket wrench by eye is conservatively estimated to be ±5◦, combined with the focusing nuts threading of 0.5mm per 360◦ turn, provides the ability to adjust the imager position to an accuracy on the order of ±0.007mm. More on the topic of imager focusing will be discussed in Chapter 6, Section 6.2. CHAPTER 3. BRITE INSTRUMENT 31

Figure 3.10: Header tray focusing mechanism where ﬁne adjustment of imager plane position can be adjusted with the socket wrench (courtesy of C. Grant) [14]. Chapter 4

Instrument Hardware Qualiﬁcation Tests

In the hardware testing aspect, the author was responsible for putting together a comprehensive test procedure that assesses the functionality of the IOBC. This test procedure serves as the Long Form Function Test (LFFT) for the unit. The LFFT serves as a component of the overall larger test procedure for a va- riety of environmental tests that need to be conducted on the unit and it must pass these tests in order to be deemed as “flight worthy”. The author was involved in both the protoflight testing of the UniB- RITE instrument, acceptance testing of the BRITE-Austria instrument, as well as the room temperature portion of the acceptance testing for all of the Polish and one of the Canadian BRITE instruments (the thermal portion of the acceptance test was to be completed by the Polish and Canadian teams, based on the procedures and methodologies established by the author). These terms will be defined shortly. The discussion herein will focus on the UniBRITE protoflight testing, which encompasses all the acceptance tests performed on the rest of the BRITE instruments at acceptance levels. It should be noted that all tests involving SFL hardware were performed in accordance with the electrostatic discharge (ESD) protection measures as stated in [33]. Testing is an important part of the spacecraft development process to assess the functionality and performance of the unit, subsystem or system of interest. A two level test classification system is implemented in SFL as defined in [27] with the distinction being qualification and acceptance . The two classes of test dictate the severity and duration of the test. The aforementioned protoflight approach essentially follows the qualification process except the severity and duration of the tests are adjusted to prevent possible damage being done to precious flight hardware. For each unit under test, the LFFT es- tablishes the pass or fail criteria for the unit’s functionality and performance. A Short Form Functional Test (SFFT) is also defined that is typically a subset of the LFFT procedure with the intention to provide quick diagnostics of system functionality without going into as much detail as the LFFT. Besides the typical bench top ambient temperature test, the unit, subsystem or system must also go through environmental tests to gain full confidence in its capability to survive the harsh space environment and perform its intended job once in orbit. These environmental tests are subsequently distinguished into mechanical,

32 CHAPTER 4. INSTRUMENT HARDWARE QUALIFICATION TESTS 33 thermal, vacuum and anechoic chamber (for communication system) testing on the unit and subsystem level with the addition of electromagnetic compatibility (EMC) test on the system level. [26].

4.1 Environmental Tests

Mechanical testing at the system level is done in the form of vibration tests. All units, subsystems or systems are required to pass the prescribed vibration load level specified by the launch provider to guarantee the integrity of the mechanical design and assembly such that no parts would come loose during launch that would result in satellite malfunction and worse yet, potentially damage other payloads or even the launch vehicle itself. An acceptance test with specified load and duration is provided in the launch provider’s manual with the qualification load levels and test durations being twice the levels and durations used unless otherwise specified by the launch provider. [27] At the unit level, mechanical workmanship testing is accomplished by thermally stressing the unit of interest by rapidly altering the temperature it is exposed to. This also known as a thermal shock test (T- shock). Different from the thermal testing, T-shock testing is only performed on the unit level to check whether the unit design has sufficient mechanical integrity to overcome sudden exposures to extreme temperatures and the effect of alternating these extreme exposures. Poor component assembly such as sufficiently weak solder joints will be exposed as failure(s) in overall unit functionality and performance by the rapid thermal expansion and contraction process. Hardware inspection under microscope and LFFT are performed before and after a unit undergoes T-shock to assess whether the unit passes or fails [16] [26]. Thermal testing evaluates the unit, subsystem or system functionality and performance under thermal variations. Temperature range definitions differ in the qualification level from the acceptance level. At the qualification level, typical thermal tests are done at the expected maximum hot and maximum cold temperatures that the unit is expected to see in orbit with an additional 5◦C to 10◦C margin; provided the resulting temperature range falls within the operating limits of the most limiting component on the unit under test (typically −30◦C to +70◦C for commercial electronics rated for industrial applications). In the protoflight approach, the unit under test is treated more conservatively to mitigate chances of damage. The temperature range can be set 5◦C to 10◦C less than the operational limits of the most limiting component. At the acceptance level, temperature range limits are based on the expected maximum and minimum operating temperatures for the unit under test and this temperature range is typically −20◦C to +60◦C. The duration of test is specified by the test procedure for the unit, subsystem or system under test [27]. Vacuum testing is performed in conjunction with thermal testing on new unit designs or at the satellite level in order to emulate a space-like environment. Vacuum alone can expose flaws in the mechanical aspects of the design and assembly process as trapped air that ordinarily remains idle under CHAPTER 4. INSTRUMENT HARDWARE QUALIFICATION TESTS 34 normal atmospheric pressure can cause damage and lead to failures. Vacuum combined with thermal (T-Vac) testing removes convection and is therefore a more realistic test to assess the functionality and performance over varying temperature ranges in space. The temperature range used follows the definition of thermal tests and the duration is specified by test procedure. T-Vac test is only performed at the qualification level for a new design and is not done at the acceptance level for units, although there is a spacecraft level T-Vac test.

4.2 Hardware Model Deﬁnitions

The hardware model classification system at SFL distinguishes each build of an unit, subsystem or system design as either: breadboard model, engineering model, qualification model, protoflight model and flight model with the definitions as follow [27]:

• Breadboard model is the earliest stage of a design prototype for the purpose of demonstrating the feasibility of the intended hardware functionality and performance where the parts used need not be the same as the actual components used in the final product. It is not representative of the actual build and is therefore not used for qualification, protoflight, or acceptance testing.

• Engineering model is built with components selected to be used for ﬂight with the intention of it being a prototype unit to demonstrate the functionality of the selected parts for the design but are typically not built for environmental testing or detailed evaluation of the design performance.

• Qualification model is flight representative and built to flight standards with all the intended flight parts for the purpose of qualification level testing. This model is not intended for flight after testing.

• Protoflight model is flight representative and is built to flight standards. Testing of a protoflight model involves qualification levels over acceptance durations and the model may be upgraded to flight status after testing.

• Flight model is built to flight standards where assembly follows the Joint-Industry Standard with space addendum (J-STD-001DS). Flight models are tested with acceptance levels over acceptance durations under the assumption that the design has been previously qualified using either a qualification or protoflight model.

In the context of the BRITE mission, the payload instrument design process followed the evolution from breadboard model development to the final flight model. Due to combined limiting factors in human resources, schedule and monetary budget, the protoflight model was promoted to flight status and became the payload instrument onboard the UniBRITE spacecraft of the BRITE Constellation. CHAPTER 4. INSTRUMENT HARDWARE QUALIFICATION TESTS 35

4.3 Qualiﬁcation Testing

The sequence of inspections, rework and tests performed on the protoﬂight model that eventually became the ﬂight instrument onboard the UniBRITE are listed below in logical order. The description of each test is provided in subsequent subsections. In contrast, the acceptance level testing consists of just Test 1 to Test 7 and Test 7 is done strictly as a thermal test (without vacuum).

1. Type I inspection, rework and delta inspection

2. IOBC unit level functional test

3. Instrument room temperature LFFT

4. Payload instrument scientiﬁc acceptance test (PISAT)

5. Type II inspection, rework and delta inspection

6. T-shock, post T-shock inspection and LFFT

7. T-Vac test at qualiﬁcation level (over acceptance duration to be consistent with the protoﬂight model approach).

8. Vibration testing of instrument telescope assembly

4.3.1 Type I Inspection, Rework and Delta Inspection

All SFL flight hardware is to be manufactured by a J-STD qualified technician. The type I inspection is intended to check the component assembly of CCD header board and IOBC electronics. The goal of the inspection is to identify human errors made during the manufacturing process such as shorts, wrong parts, incorrect placement or orientation of parts and unsoldered connections. This mitigates the risk of damaging electronics when powering up for the first time. The inspection is required to be done by a J-STD qualified inspector, who also needs to be a different person from the one who performed the assembly work. The inspector will perform a thorough inspection to highlight part errors or suspected errors. The unit is then handed back to the technician for rework. A post rework, delta inspection is then performed with focus placed on just the reworked parts to ensure they have been corrected for before being passed on to the next stage of testing.

4.3.2 IOBC Unit Level Functional Test

The IOBC unit level functional test not only places additional scrutiny on the IOBC in addition to the Type I inspection, but it also provides veriﬁcations for all the key functionalities of the IOBC. The IOBC is ﬁrst checked for shorts through direct probing at several key components before its circuit components CHAPTER 4. INSTRUMENT HARDWARE QUALIFICATION TESTS 36 are powered up progressively. This procedure further mitigates risks of damaging the circuit should any shorts go through Type I inspection unchecked. The author was responsible for the design and composition of this procedure in collaboration with the IOBC electronic designer Mihail Barbu, who provided insights to the crucial tests that need to be performed. The distinction to be made here is that the IOBC is not yet connected to the CCD header board to form the complete instrument, hence this test is designated as a unit level test. The unit level test procedure designed and conducted by the author on all BRITE instruments is described in the following sections.

Short Veriﬁcation

This test is a ﬁrst round of test to verify there are no short connections on the major logic, bias and power rails on the IOBC through direct resistance measurements using a digital multimeter (DMM). At this moment, no software has been written to the FPGA or ﬂash memory as these processes require the IOBC to be powered up. Nothing is connected on to the IOBC at this stage. The resistances are measured between the ground point of the IOBC (located at the SMA connector port) and the power rails listed in Table 4.1. Once it has been established that all the rails are free of shorts, the IOBC is then ready to be connected to an external power supply. At this point, the IOBC FPGA and bootloader codes are ready to be uploaded on to the hardware.

Name Description +1.5V Power for Vcc of the FPGA. +1.8V Power for high state of the oscillator clock input to the IOBC processor. +3.0V Power for logic high of processor, EDAC memory, FPGA, and regulators. +3.3V Power for Vcc of the 20.0MHz crystal oscillator. HeaterP ower Power rail to heaters located on the CCD header board.

Vref Reference voltage for analog to digital converter devices. +2.8V Supply voltage for the CCD A/D video converter. +5.0V Supply voltage for the Kodak timing generator.

±5VB Supply voltage for the low noise ampliﬁer on CCD header board. ±6V Two of the bias rails used to power the horizontal pixel clock driver. ±12V Two of the bias rails used to power the vertical pixel clock driver. +15V Supply voltage for the CCD imager on CCD header board. ESD Supply voltage for ESD protection circuit of CCD imager on CCD header board.

+2.048VPREC Precision rail used to power horizontal and vertical pixel clock drivers as well as the control signals for the CCD imager on CCD header board. Table 4.1: IOBC electronics shortage check points CHAPTER 4. INSTRUMENT HARDWARE QUALIFICATION TESTS 37

Hardware Setup

A high degree of conﬁdence can be placed in the hardware after checking for shorts. The IOBC must now be powered on and programmed in order to proceed with the rest of the functional test. The test setup used to power the IOBC is illustrated in Figure 4.1. The rest of the satellite hardware is absent since this is a unit level test. As a result, a set of protocol conversion hardware is needed to interface the IOBC data lines (which follows the RS485 protocol) with the USB port on the workstation used to carry out the functional test. This interface is carried out in two stages, ﬁrst from RS485 to TTL and then from TTL to USB. The second set of power supply shown in Figure 4.1 is used to power the RS485 to TTL converter. Even though the IOBC is now deemed to be free of short circuits, but caution is still exercised through setting current limits. The voltage level is set to the nominal GNB bus voltage of 4.4V with the current limit set to a conservative setting of 150mA before powering up.

Figure 4.1: IOBC hardware test setup

Voltage Rail Veriﬁcation - Basic State

After double checking the connections, the IOBC power supply is turned on. Under the powered-up state, some of the power rails listed in Table 4.1 are active while most remain off. The voltage level of the telemetry points listed in Table 4.1 are checked again via DMM to ensure the values agree with expectation (given in [13]) before proceeding to the next step.

Software Setup

As it stands, the IOBC has no software uploaded on to it and cannot carry out any of the functionalities being tested. The FPGA code is a set of hardware instructions that specify the output states of some of the pins on the FPGA chip. It also specifies how these output states would react in response to signals received on the input pins. Therefore, the FPGA code needs to be programmed on to the IOBC first before any further functional test can be performed. After the FPGA code has been successfully uploaded, CHAPTER 4. INSTRUMENT HARDWARE QUALIFICATION TESTS 38 the bootloader code is the second piece of software to be programmed. The bootloader, as the name suggests, provides a set of basic functionalities such as peek, poke, read, write, initialize and etc. based on which the operating software may be uploaded on-the-fly. As designed, the application-level software, known as the Canadian Nanosatellite Operating Environment (CANOE) needs to be running in order for the IOBC to perform any of the useful functionalities. The IOBC CANOE provides the operating platform upon which the Science Data Generation Code (SDGC) application code is run. SDGC is developed specifically for the BRITE mission. It defines all the software functions to operate the CCD imager, parse and store the image data. As well, it can perform some rudimentary image processing such as co-addition of two image frames, and perform mean and standard deviation calculations on an image.

There are also three pieces of ground station software that are needed to interact with the flight software. These are: Terminal Interface Protocol (TIP), GNB Control and BRITE Payload Ground Control (BRITE PGC). TIP serves as the interface between GNB Control and BRITE PGC to the IOBC in the sense that all command/response packets are sent/received through it before reaching the destination. GNB Control provides a platform to issue all the necessary commands to operate the GNB satellite. In the scope of this test, GNB Control simply provides the means to upload SDGC code on to the IOBC. BRITE PGC is designed to operate the BRITE instrument. Operationally, three switches must be turned on sequentially before all the circuits necessary to take an image are powered on. These switches are: bias, CCD, and output amplifier. The bias switch controls the ON/OFF state of a set of bias rails that supply the Kodak KSC-1000 timing generator chip with reference voltage levels for output clock signals, which are required to run the pixel clock drivers. The CCD switch sends an ON/OFF signal to the timing generator chip which then acts accordingly to power the Kodak KAI-11002 CCD imager chip ON/OFF. The amplifier switch enables the power rail that supplies power to the output amplifier located on the CCD Header Board. All these commands are issued through BRITE PGC and forwarded to TIP before they are received by the IOBC.

Voltage Rail Veriﬁcation - Bias On, CCD On, Ampliﬁer On

In this test, the IOBC power supply current limit is increased to 200mA. The application code is uploaded and initialized via GNB Control following the software setup provided in [13]. The power level being drawn from the power supply is checked to be nominal before proceeding. Next, the power supply current limit is increased to 1.0A, the bias power rails are turned on via BRITE PGC software and the power drawn is noted and checked to be normal. The CCD switch is then turned on, followed by the output ampliﬁer switch. All the power rails listed in Table 4.1 are measured again with DMM to conﬁrm conformity with expectations under the “everything on” state of the IOBC as stated in [13]. CHAPTER 4. INSTRUMENT HARDWARE QUALIFICATION TESTS 39

Substrate Voltage Level Conﬁrmation

The substrate voltage level (Vsub) setting on the IOBC determines the average charge capacity of the pixels on the CCD imager and should therefore match the prescribed level as provided by the manufacturer.

The Vsub level on the UniBRITE IOBC was indicated to be around 10.55V and a simple measurement veriﬁcation made with DMM with respect to IOBC ground sufﬁced.

Power Sequencer Timing Conﬁrmation

The CCD imager has a specific bias and clocking voltage startup sequence that must be adhered to in order to prevent damage to the built-in ESD protection circuitry. As a result, a power sequencer circuit was implemented to divide the CCD bias power rail startup sequence into 3 stages as controlled by 3 sequencer output signal flags: flag1 − BIASRUN , flag2 − ESDEN and flag3 − BIASON . This portion of the hardware test confirms that the time delay between the 3 output flags are timed 60ms apart when the “bias ON” command is issued (as illustrated in Figure 4.2) and the reverse sequence, also 60ms apart, is observed when the ”bias OFF” command is issued.

Figure 4.2: IOBC bias power-up sequence diagram.

An oscilloscope was employed for this test. The measurement setup involved connecting channel-1 probe to the enable signal (EN) line with rising edge set as trigger and channel-2 probe to flag-1 output. The “bias ON” command was sent and the resulting waveforms were analyzed to verify the first flag was triggered 60ms after. The process was repeated for flag-2 and flag-3 to confirm their delays were 120ms and 180ms respectively. The reverse power-down sequence verification was then performed in a similar manner to verify flag-3 signaled low 60ms after the ”bias OFF” command, flag-2 120ms and flag-1 180ms. CHAPTER 4. INSTRUMENT HARDWARE QUALIFICATION TESTS 40

Oscillator Frequency Conﬁrmation

The IOBC uses a single 20MHz crystal oscillator to drive all the clocks on the IOBC. The 20MHz clock is used both for the ARM7 processor and the FPGA. The FPGA provides further clock division by a factor of 2 to 10MHz that feeds into the Kodak timing generator to produce all the subsequent clock signals required for the CCD operation. The purpose of this test is to confirm that the crystal oscillator is running at 20MHz. A DMM or oscillator frequency measurement verification suffices in this case.

Heater Current Limit Conﬁrmation

The rail providing power to the 4 resistive heaters located on the CCD header board contains a current limit integrated circuit (IC) component that provides a current limit of approximately 330mA. The purpose of this test is therefore to confirm the current limit is working correctly. This test is performed using a variable load box (i.e. the GNB load box) to connect to the heater power lines in series with a DMM in the loop to implement current measurement as illustrated in Figure 4.3. The resistance dial is set to the maximum before turning on the heaters. Next, the “heater ON” command is issued and the resistance dial is steadily lowered to enable current flow. Eventually there comes a point when the current flow observed by the DMM drops suddenly indicating the current limit functionality kicking into play. The level at which the current flow is cut off is confirmed to be approximately around 330mA as expected by spec.

Figure 4.3: Heater current limit test setup.

Exposure Time Setting Conﬁrmation

The IOBC software implementations allow user to set the exposure period during which the CCD pixels are left to collect photons and integrate charge accumulation. This test is intended to conﬁrm the software command translates into the desired exposure time being set on the CCD. Although requirements demand that “exposure time shall be accurate to 0.01%” and “exposure time shall be variable between CHAPTER 4. INSTRUMENT HARDWARE QUALIFICATION TESTS 41

0.1 − 100s” (Requirement M-B.19 [34]), the resolution of the oscilloscopes available at SFL do not support measurements down to such a level. In terms of hardware capability, because the base clock rate used by the Kodak timing generator is 10MHz, the finest resolution achievable is 100ns, which is 100 times finer than the most demanding accuracy of 0.01% of 0.1s, or 10µs. As a result, the intent here is to simply confirm the software setting indeed implements the change in exposure time with a crude ±2% accuracy. Confirmation of the exposure settings are made via connecting the oscilloscope channel-1 to the ”AmpEn” pin on the Kodak timing generator. This pin is programmed via software for the purpose of checking exposure period setting. The signal is toggled from its idle state when an exposure starts and returns to idle at the end of exposure. Verifications are performed at exposure time settings of 60ms, 100ms, 500ms, 1s, 3s, 5s and 10s to obtain general confidence that the exposure time setting functionality worked.

4.3.3 Instrument Room Temperature LFFT

Once the IOBC unit level functional test is completed, the IOBC is ready to be integrated with the CCD header board and tested as the complete instrument electronics through the instrument LFFT procedures. At this stage, the CCD header board electronics has gone through its own unit-level functional testing as designed by M. Barbu and is deemed to be safe to connect to the IOBC (the CCD header board unit level functional test procedure is omitted here but can be found in [23]). The CCD header board electronics has also gone through T-shock testing and post T-shock functional test to validate the workmanship of the electronics assembly before the CCD imager is assembled (it is best not to expose the CCD imager to the T-shock process in order to prevent unnecessary thermal stress that could damage the CCD). Only after the header board has gone through its sequence of tests and checks is the CCD imager chip then soldered on and a thorough visual inspection under the microscope is performed. The instrument LFFT procedure is designed as a complete payload system-level functional test to evaluate whether the instrument is working correctly. This test is designed as a black-box test such that all the procedures could be implemented as digital inputs to the payload and the assessment of the test done based on its digital output. It is necessary to design the test in such a manner because the set of procedures needed to be carried out repeatedly, throughout the instrument qualiﬁcation test, as well as during the spacecraft system level test where it would be impossible to access the hardware in most of these tests (i.e. T-Vac tests). The room temperature LFFT discussed here is the initial LFFT performed on the bench in an open environment to assess that the instrument is fully functional.

IOBC Automated OBC Test

The ﬁrst test of the instrument LFFT is to run the standard set of automated OBC test designed to evaluate the hardware functionality of all SFL OBCs. It is a subset of the automated hardware test CHAPTER 4. INSTRUMENT HARDWARE QUALIFICATION TESTS 42 software developed by J. Lifshits [21]. The intent of this test is to verify the hardware functionality of IOBC’s external RAM, external FLASH and the two serial communication ports (UART1 and UART2). The IOBC is powered up in its bootloader state. The test script is set up in accordance with the structure provided in [21] with OBC ping, OBC ram, OBC ﬂash tests to be performed through UART1 channel and OBC ping, OBC ram2 through UART2. The test script is then executed on the IOBC through the terminal application of the workstation. Table 4.2 provides descriptions of these tests [21].

LFFT test Description OBC ping Intended to verify communications with the satellite by sending a ping command to the specified destination address and provides a record of the ping response. OBC ram Intended to verify functionality of the external RAM by writing a test pattern into RAM and reading it back to ensure the values read match those written. OBC flash Intended to verify functionality of the external flash by performing format, write, read on the first block of the flash chip and verify each operation is successful. OBC ram2 An extended version of the OBC ram test that fully exercises every available memory location of the external RAM. Table 4.2: LFFT test description.

As a result, this section of the LFFT fully tests that bootloader has been uploaded on to the IOBC correctly through assessing the Ping command response as well as proper handling of the Peek, Poke, Read and Write commands on top of checking the computing and memory functionalities.

Peripheral Device Communication Test

Communication between the IOBC processor and its peripheral devices (i.e. Kodak timing generator and CCD A/D video converter) are checked. This is designed to be a simple test by sending commands to check the initial register values of these peripheral devices, changing the registers to a different value and reading them back to verify their values have been changed (details of this test can be found in [13]).

I2C Communication Test

The communication protocol implemented between the IOBC and the CCD header board is the I2C protocol. This test is designed to ensure proper I2C communication functionality by issuing an I2C Ping command. The response to this command is a “Pong” response from the CCD header board stating: ”BRITE CCD Header Board, SFL, Compiled hdatei htimei”, thus provided veriﬁcation that communication to the CCD header board works. CHAPTER 4. INSTRUMENT HARDWARE QUALIFICATION TESTS 43

Telemetry Test

The telemetry of the IOBC provides a convenient way to assess health of the system. As designed, there are two telemetry sources that combine to monitor all the necessary telemetry points on the IOBC: the TMS470 ADC internal to the processor and the MAX1231 ADC [7]. The TMS470 ADC monitors the voltage level of +3.0V logic rail, +6V , +12V , +18V rails and the heater power rail. It also monitors the temperature near the bias power supplies and the temperature of the CCD A/D converter. The MAX1231 ADC provides telemetry points over all the other bias power rails listed in Table 4.1. The telemetry test is designed to be a straightforward check by progressively turning on the power switches available to the IOBC in the order of (1) bias power, (2) CCD, (3) ampliﬁer, (4) heater. The telemetry reported by the IOBC is checked at each stage. All the power switches are then turned off and another round of telemetry checks is performed to ensure all the bias and heater power rails have returned to their off state. Furthermore, because all logic, bias and heater power supply voltage rails are derived from the spacecraft bus voltage, which is simulated by the IOBC power supply voltage in this test setup, it is necessary to perform the telemetry test a total of three times: one at a power supply voltage level of 3.6V , another at 4.4V , and one at 5.5V to capture the minimal, nominal and maximum spacecraft bus voltage scenarios, respectively. The telemetry test in all three cases should report correct and stable voltage level on all the rails. This is then sufﬁcient to conclude the IOBC can generate all the correct voltage rails off the spacecraft bus voltage over the range of 3.6V to 5.5V .

Exposure Time Setting Test

The exposure time setting test performed here is different from the ones done in the IOBC unit level functional test as this test aims to examine the effect in the resulting images due to changes in exposure time. This test is performed simply by placing the imager toward scenery with distinguishable features (i.e. an LED was placed in front of the CCD imager at reasonable distances) and taking two exposures with the second exposure integrated over twice the exposure period of the first. The exposure times are chosen to be sufficiently low as not to saturate the CCD. The resulting two images were then examined to be valid pictures of the scenery and compared to confirm that the second image registered intensity values approximately twice of those of the first image. An additional intention combined with this test is to validate access and functionality of both the upper and lower block of the SDRAM. When the first exposure is taken, it is stored into the lower block of the SDRAM by default. Explicit commands are sent before taking the second exposure to specify the IOBC to store the image into the upper block of the SDRAM. By the virtue of confirming both resulting images reported valid capture of the intended scenery serves as confirmation for full SDRAM access. CHAPTER 4. INSTRUMENT HARDWARE QUALIFICATION TESTS 44

FPGA Control Test

Common to the standard SFL on-board computer, a FPGA chip is utilized to implement additional control functionalities. Specific to the IOBC, the FPGA provides end of exposure signal reset command, power switch reset command and memory pointer setting functionalities. One of the general purpose input/output (GPIO) pins on the processor flags whenever an exposure was taken and a flag reset command can be issued through the FPGA to clear the flag. As a result, this test consists of a simple check on the exposure flag before and after taking an exposure to verify the flag has been set and once again after issuing a flag reset command to validate the flag has been reset. Next, one of the FPGA registers should indicate whether the bias power rails and/or the amplifier power switches are on or off. The test begins with both the bias and amplifier switches off. The register values are peeked and validated to correspond to the state of the switches. The bias and amplifier power are then turned on sequentially with the register value checked after each action. Finally, a power switch reset command is issued and the status register checked again to confirm both the bias and amplifier switches returned to their off state. The last FGPA test involves checking that the data reset pointer and data read pointer are able to be set to the desired location. The distinction between the two is that the data reset pointer specifies the default location in the SDRAM where the image data will be stored, while the data read pointer specifies the memory location from where the image data will be read. The procedure involves poking the pointer set register with the desired address to change the default data reset pointer location. The bias, CCD and amplifier are then powered up to take an exposure. After the exposure, the data reset pointer address values are peeked to confirm the setting took effect before initializing the image download process. After the image download completes, the data read pointer is set to the address of the default data reset pointer and another image download is performed with the intent of re-downloading the same image. Since the second image is specified to be downloaded from the same starting point in memory, the data read pointer set functionality is checked by confirming the two images are identical.

Heater Control Test

The purpose of the heater control test is to verify the four set of resistive heaters installed on the CCD header board function. The test involves switching the heater control to manual control mode and altering the pulse width modulation (PWM) setting of the four heaters to 25%, 50%, 75% and 100% of a one second PWM period. The validation is done by visually observing the change in current level drawn from the IOBC power supply. At low PWM settings, the increase in current draw is barely visible and if so only lasts for an instant. As the PWM settings are increased the period of higher current draw is also conﬁrmed to increase in proportion. This test demonstrates the hardware can control the heaters to the desired set level. CHAPTER 4. INSTRUMENT HARDWARE QUALIFICATION TESTS 45

Readout Noise Level Test

The readout noise level test is the same as the one performed in the Payload Instrument Scientiﬁc Ac- ceptance Test (PISAT) discussed in Section 5.9. The readout noise level is one of the most important parameters to the BRITE Science Team. This must be kept as low as possible. It is also the only test in PISAT that can be performed relative quickly by gathering two images without light in very short exposures. The exact procedure is presented in full detail in Section 5.9. The readout noise level determined here in the room temperature LFFT provides a baseline for later tests.

4.3.4 Payload Instrument Scientiﬁc Acceptance Test (PISAT)

The PISAT is a very involved process and requires much more background discussion of the inner workings of the CCD. Hence it is delayed until the next chapter for a dedicated overview.

4.3.5 Type II Inspection, Rework and Delta Inspection

In contrast to Type I inspection, Type II inspection aims to conﬁrm that soldering workmanship con- forms to the rules of Joint Industry Standard rather than just check for shorts, unsoldered joints, missing or wrong parts. Type II inspection required much closer attention to identify parts that do not or are suspected of not meeting the required standards. It is usually carried out by a more experienced inspector. The inspection, rework and delta inspection process is otherwise the same as Type I inspection.

4.3.6 Thermal Shock, Post Thermal Shock Inspection and LFFT

The purpose of the thermal shock (T-shock) process is to evaluate, on the unit-level, whether there is sufficient resilience to sudden exposure of thermal extremes. Sudden swings in temperature from extreme cold to extreme hot and vice versa can occur in the space environment due to tumbling and transition between sun light and eclipse. A second purpose, as stated in Section 4.1 and [16] is to uncover any defect or weakness in the electronics assembly. The room temperature LFFT described in Section 4.3.3 provides a pre-T-shock functional verification of the payload instrument electronics to establish that the system worked prior to T-shock. The actual T-shock process is outsourced to Celestica Inc. and is done only on the IOBC at the unit-level. The CCD header board electronics has been T- shocked prior and it now contains the CCD imager chip at this stage (which did not undergo T-shock to avoid possible damage). A post T-shock inspection is then conducted to identify parts and joints that might have been damaged or removed by the rapid thermal expansion and contraction during the T-shock process. Any necessary rework is then conducted to rectify any damage identified, and another round of LFFT as outlined in Section 4.3.3 is performed at room temperature. SFL quality assurance policy then dictates that the T-shock process must be repeated again with reduced number of cycles, followed by inspection, rework and room temperature LFFT until the unit passes the process without CHAPTER 4. INSTRUMENT HARDWARE QUALIFICATION TESTS 46 requiring any further rework or modifications [33]. However, inspection and room temperature LFFT alone may not reveal cold joints (broken, but still intact) until the IOBC is functionally tested under thermal test conditions.

4.3.7 Thermal Vacuum Test

Normally, the purpose of performing the thermal vacuum test is to fully assess the functionality of the IOBC under a space-like environment at different temperatures. In the case of qualiﬁcation testing for the UniBRITE science instrument, the thermal vacuum test is also to provide full assessment of the IOBC after the T-shock process.

Thermal Vacuum Test Setup

The first step of the thermal vacuum (T-Vac) preparation is to perform a thorough cleaning of the payload instrument electronics assembly and the interior portion of the vacuum chamber with isopropyl alcohol as required by the standard thermal and thermal vacuum test procedure [32]. The IOBC board is soaked in an alcohol bath and gently brushed around all major pins and cavities to eliminate debris, excess solder flux and human grease. The alcohol in this first bath is now tainted and must be disposed of, while the IOBC is placed in a second alcohol bath for another round of soaking. After two rounds of bathing, the IOBC is blown dry to remove surface debris with clean pressurized air. The same cleaning procedure should not be applied to the CCD header board because of the delicate electronics housed within the CCD imager chip. Instead, a clean swab dipped with alcohol is gently stroked along the surface of the CCD imager and header board electronics to remove surface grease. Clean pressurized air is then gently blown to dry the electronics and to remove surface debris. The IOBC and CCD header board assembly is then mounted onto an aluminum plate that contains the screw housings which match the mounting holes on the IOBC and CCD header board as well as the vacuum chamber plate. The entire payload and plate assembly is then mounted onto the vacuum chamber plate and wiring harnesses are connected on to the IOBC. The wiring harness is specifically made for the instrument T-Vac test. It consists of the normal set of micro-D connectors that attach to the IOBC to provide power, ground and data lines. Furthermore, these wires need to have teflon wire jackets instead of the standard PVC to prevent outgassing of the material. A single LED mounted inside a cover box is installed over the CCD header board housing to provide illuminated scenery for the Exposure Setting Test in the LFFT procedure. In addition, seven thermistors are attached to the SDRAM, IOBC power rail regulator, CCD A/D video converter, CCD header board, CCD header board housing, aluminum plate near the header board housing and aluminum plate near IOBC power rail regulator (the last to provide additional temperature measurement to judge when the system temperature has stabilized to the desired levels throughout the test). All the power, ground and data lines from this setup are then CHAPTER 4. INSTRUMENT HARDWARE QUALIFICATION TESTS 47 wired to the interface connector located on the interior side of the vacuum chamber lid that is exposed to vacuum. On the exterior side of the lid, a mating interface connector allows the IOBC power supply, RS485-TTL power supply and TTL-USB converter to be hooked up to the system in the interior of the vacuum chamber. A LFFT is performed to make sure the system is ready before sealing the vacuum chamber. Once the payload instrument is confirmed to be fully functional, the vacuum chamber is sealed and placed inside the large thermal chamber at SFL. The pump hose is inserted into the vacuum chamber to initiate the pump down process until pressure inside the vacuum chamber drops to approximately 2.5 × 10−5 Torr while the thermal chamber is turned on and set to 25◦C in preparation for the start of the T-Vac test.

T-Vac Test Process

In order to begin the T-Vac test process, the vacuum chamber must reach high vacuum (speciﬁcally at approximately 2.5 × 10−5 Torr). At this point the science instrument is turned on. The T-Vac test procedure follows the standard SFL procedure described in [32] and summarized in Figure 4.4. When the test was performed on UniBRITE instrument, the temperature reading shifted from 25◦C when the instrument was powered up for Phase I LFFT (refer to Figure 4.4) and eventually settled at approximately 30 ± 1◦C at the end of phase one testing. This drift in temperature was not observed from tests conducted at other temperature levels. Hence it was most likely to be the case that instrument temperature had settled at 25◦C in the original bias OFF, CCD OFF, and ampliﬁer OFF state it started off with, but started to self heat once the circuits associated with these switches were powered ON. Therefore causing the gradual rise in temperature. The instrument passed all the tests conducted during LFFT. Even though the temperature was higher than intended, it did not have too great of a consequence on the interpretation of the results. If the unit had passed the tests at 30◦C then it must also have passed at 25◦C.

Phase II of T-Vac testing involves the transition to bring the instrument temperature to Thot,survival ◦ for the hot soak. Ordinarily, Thot,survival is deﬁned as 70 C but worries of damaging the CCD prompted the level to be lowered to 65◦C, which is just 5◦C above the hottest operating temperature of 60◦C. The instrument is powered off for the ramp up to Thot,survival. The thermal chamber temperature setting is gradually turned up to 70◦C to bring up the system temperature at a pace of approximately 2◦C/minute and turned back down to 65◦C once the temperature had been brought up to close proximity of 65◦C to avoid overshooting Thot,survival. Once the various temperature sensor readings attached to the instrument indicate the system temperature has settled at the target 65 ± 1◦C, it is left to soak for at least

60 minutes (tsoak) as recommended by [32]. After the soak time elapses, the thermal chamber is then ◦ turned down to Thot of 60 C, and the instrument then cools to Thot, at which point the instrument is then turned on. The warm start LFFT is conducted and the system functionality after the soak under survival temperature is veriﬁed. CHAPTER 4. INSTRUMENT HARDWARE QUALIFICATION TESTS 48 Figure 4.4: Thermal vacuum test temperature proﬁle [32]. CHAPTER 4. INSTRUMENT HARDWARE QUALIFICATION TESTS 49

Phase III of T-Vac testing is similar to Phase II except the instrument is powered off and the tem- ◦ perature is transitioned down to Tcold,survival of −25 C for a cold soak to verify the system can sur- ◦ vive (powered off) at cold temperature extreme. Note that Tcold,survival is set higher than the −30 C prescribed by [32] due to concern over the CCD. A minimum one-hour soak is provided before the ◦ temperature is ramped up to minimum operating temperature, Tcold = −20 C, for the cold start LFFT. The cold start LFFT is then conducted and all system functionalities are veriﬁed.

Phase IV of T-Vac testing involves two rounds of temperature slew transitions from Thot to Tcold. Similar temperature controls are made to the thermal chamber to attempt to stabilize the instrument temperatures to the desired level with the exception that the system remains powered on throughout the temperature transitions between Thot and Tcold (the operating temperature range). During each temperature slew, a SFFT in the form of automated OBC ping, OBC ram tests are performed on UART1 and LFFT OBC ping OBC ram2 tests on UART2 in a looping sequence to provide a constant communication check with the IOBC throughout the slews. A shortened form of the LFFT procedure (without the readout noise level test) is performed at each of the Tsoak points marked in Figure 4.4. This choice was made in order to speed up the entire test process and is acceptable since the readout noise level is fully characterized during the PISAT (hence its omission does not mean there is less information). Finally, at Phase V of the T-Vac test, the instrument system is returned to the standard temperature of 25◦C for a ﬁnal, full LFFT. For UniBRITE, all tests indicated the instrument was functioning properly. The readout noise level results indicated the instrument was comparable to the level determined by the LFFT performed in the beginning at Phase I. At the completion of the T-Vac test, the instrument system is powered off and pressure inside the thermal vacuum chamber is restored to atmospheric pressure before the entire assembly is disassembled and the instrument is removed.

4.3.8 Vibration Testing

Vibration testing is only performed on the mechanical assembly of the instrument. This includes the baffle, optics cell, and the CCD header tray. The vibration testing for UniBRITE was handled by the BRITE manager C. Grant and performed at Celestica Inc. Preliminary testing revealed that the optical filter had come loose after vibration and was both rattling inside the housing and free to rotate. A simple solution to add a generous amount of room temperature vulcanizing (RTV) sealant around the optical filter ring was taken. This essentially filled the tiny gap between the optical filter and its housing while also adding extra shear strength that prevents the filter from rotating. A follow-up vibration test was then conducted on the RTV fixed instrument mechanical assembly and no further rattling or loose parts were observed. Chapter 5

BRITE Payload Instrument Scientiﬁc Acceptance Test

The BRITE Payload Instrument Scientific Acceptance Test (PISAT) is a set of test procedures designed by the BRITE Science Team to assess the performance of the instrument and validate each requirement. It is one of the tests to be completed for the qualification and acceptance testing of the instrument. The set of performance expectations are derived based on the main scientific objective of achieving milli-magnitude accuracy in differential photometry on stars with apparent visual magnitude of +3.5. Although PISAT does not directly verify this mission level requirement (as this needs to be performed on real stars), it provides a set of provisional requirement verifications to ensure the instrument design is able to do its intended job. Furthermore, validation of these performance requirements also serves as a baseline characterization of the instrument.

5.1 Test Overview

PISAT targets aspects of the imager performance relate to bias level stability, effective system gain, system readout noise, saturation and dark current levels at temperatures of −20◦C, 0◦C, 10◦C, 20◦C, ambient, 30◦C and 60◦C. The bounds at −20◦C and 60◦C are intended for verification of unit survival rather than for performance. In other words, the imager only needs to function at these temperature extremes but no expectations are placed upon its performance, especially at 60◦C where noise and dark current level of the CCD become overwhelming. Testing at other temperatures provide indications to the payload subsystem performance over the 0◦C to 30◦C range. The PISAT test also takes on a ”black box” test approach in the sense that evaluations are conducted solely based on the quality of the output data images. The instrument’s data handling software provides outputs in the Flexible Image Transport System (FITS) image format. FITS is designed specifically to store, transmit, and manipulate scientific images and is a commonly used digital file format in astronomy. Visualization of the data

50 CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 51 images are all accomplished through using FITS ﬁle viewing software “SAOImage DS9” and data post processing tasks were done through the open source Image Reduction and Analysis Facility (IRAF) software package, both of which are standard tools used in Astronomy.

5.2 Test Setup

The test setup for PISAT is designed in such a way to produce all the required data images without requiring further hardware adjustments once testing begins. In preparation for PISAT, the CCD Header Board is mounted to the header tray ﬁtted with a neutral density ﬁlter and a single 1.5× lens element over the header tray aperture for attenuation and focusing respectively. A box with a single LED is mounted over the layers of optics on top of the header tray. This arrangement is captured in Figure 5.1 and this scenery setup is required in order to perform the gain determination test to be discussed in Section 5.5.

Figure 5.1: Optical Element Setup for PISAT

The same hardware setup used for the IOBC hardware tests discussed in the prior chapter (as illustrated in Figure 4.1) is used for PISAT. In addition, this test requires complete hardware and software integration in order for the results to be representative (the payload subsystem application software implements speciﬁc CCD pixel timing strategies that affect its performance). The entire hardware setup is sealed inside an ESD-safe bag and placed inside the thermal chamber at SFL, with only power and signal cables running out of the chamber to connect to external power supplies and workstation. Also of note is that up to ﬁve desiccant bags are placed inside an ESD bag, close to the CCD Header Board CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 52 assembly, to absorb and remove any moisture in the air. It is important to take precaution in moisture control since the test runs over a large temperature range during which condensation could form on top of delicate electronics such as the CCD imager (a single patch of condensation can easily span multiple 9µm2 pixels and create shorts). Note that all the tests at ambient temperature are not done with this test setup - they are simple bench top tests without any active temperature control. As a result the system temperature may differ up to ±5◦C through an ambient test that lasts for days. For tests at other temperatures, the system temperature is monitored with the use of three external thermistors interfaced with Maxim’s 1-Wire Device Graphical User Interface (GUI). One thermistor is attached to the bottom of the CCD Header Board to provide temperature for the imager chip, another is placed on top of the CCD ADC located on the IOBC, and a third is placed on the large aluminum plate to which the entire payload system assembly is mounted to provide ambient temperature measurements. Each thermistor is bonded to the target surface with a thin layer of double sided adhesive to ensure contact. An additional layer of Kapton tape is applied over the thermistors to ensure they do not detach in the middle of the test. Despite effort, temperature gradients across the system are inevitable. The temperature reported by the three installed thermistors has shown a maximum difference of 3◦C at all temperature levels tested, which is not a crucial spread for the purpose of the test. The CCD temperature reported by a thermistor attached to the back of the CCD Header Board is used as the primary reference temperature since a large portion of this test is to evaluate the CCD’s performance in response to different temperatures. As a result, the thermal chamber temperature is adjusted accordingly throughout the test in order to maintain the CCD Header Board temperature within ±0.5◦C.

5.3 Data Types

Three types of image were gathered for PISAT: bias, dark, and gradient. Bias images capture the bias information of the system and can be obtained by taking an extremely short exposure. The ideal exposure time would be 0s taken in the absence of any external light source, but the version of the payload application software being used restricts the exposure time setting such that the minimal programmable period is 59ms. As a result, subsequent PISAT bias images are all taken at an exposure period of 60ms, to be consistent in case comparisons need to be made between instrument builds. Dark images, on the other hand, are exposures taken over longer exposure times in the absence of any external light source. It captures dark current information in the form of additional charge counts, a byproduct of thermal effects within the CCD photodiode substrate that increase with exposure time and/or temperature. Gra- dient images capture the particular scenery produced by the set of optics and LED arrangement shown in Figure 5.1. These images are to be used for determining the effective system gain. The scenery is designed to capture a smooth, linear photon transition from the center of the LED, where the signal is strong enough to saturate the CCD, to the fringe region where it is sufﬁciently dark to provide bias level CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 53 information.

Figure 5.2: Gradient image produced with the PISAT optical setup when LED is turned on.

5.4 Bias Level and Stability Test

The nature of random noise produces both positive and negative variations about the mean signal measurement. The purpose of the bias level is to sufficiently offset the mean such that any negative variations can be fully captured when converted to the digital domain through the ADC, which utilizes a strictly binary output coding (no negative representations). Hence the bias level needs to be high enough to provide the necessary offset but sufficiently low in order not to reduce the effective dynamic range. Recall in Section 3.4.2 that the bias level is set to 80 Analog to Digital Unit (ADU) and this setting is implemented through the 8-bit DAC module of the CCD ADC on the IOBC. The bias pattern is dependent on the CCD clock timing. The particular timing strategy employed by the IOBC application software at the time of writing produces a ramp pattern where bias level remains relatively constant along the horizontal axis but increases along the vertical axis as can be seen in the contrasted example shown in Figure 5.3. Although not ideal, such bias characteristic is acceptable as long as it remains stable, but this would also imply a reference bias frame will be needed for post processing the flight observation images to remove the uneven background features upon which signals are recorded. Otherwise some measurements will sit on higher bias levels relative to others and introduce skews in the differential photometry results. As a result, the goal of this test is to verify the bias level setting provides the necessary offset and the resulting bias pattern is sufficiently stable. The following subsections will discuss the criteria, procedure and results of these verifications. CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 54

Figure 5.3: Sample bias image proﬁle. The bias ramp pattern is greatly exaggerated visually in this case as the image is a capture from DS9’s Z-scale display with units in ADU.

5.4.1 Requirements

In the following, usage of should implies a desire but not a requirement and shall implies a strict requirement. Note that most of the bias level-related requirements (except BLS − 3) are written in terms of “should” rather than “shall” because the bias level setting is “arbitrary” [28]. The set of requirements posed on Bias Level and Stability (BLS) by the BRITE Science Team are:

BLS − 1 The bias level should be set to 100±40ADU (with a desire to be within 100±30ADU). BLS − 2 The bias level should remain stable to a level of ±4ADU (with a desire to be within ±2ADU) averaged over 1, 000 neighboring pixels at constant temperature (±0.5◦C). BLS − 3 The average bias shall not drop below 40ADU at any operating temperature. BLS − 4 Any 4σ outliers should not fall below 0ADU. BLS − 5 The deviation of each bias frame from the average bias should be calculated and analyzed. A Gaussian will be ﬁtted to the histogram of deviations. The ﬁtted curve and the data should be consistent to within 10% within 3σ of the peak and there should be one obvious peak. Table 5.1: Bias level and stability test requirements [28]. CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 55

5.4.2 Data Gathering

The data gathering process involves taking five full frame exposures to examine requirements BLS − 1, 3, 4, and 5. To verify BLS−2, a strip of the bias data approximately 500 rows tall is gathered repeatedly for both the bottom and top portions of a complete image frame, roughly 20 times for each. This test configuration is chosen since the time to download 500 rows of pixels translated to approximately 2 minutes of download. This way, the same region of the bias frame can be examined at 2 minute intervals over the course of an hour (plus time for operation overhead). Furthermore, it is expected that bias level variations near the top rows would be the largest because those pixels would take the longest time to be transferred out of the CCD. Random charge generation due to thermal effects will occur constantly at each pixel during the readout operation and these ”noisy” charges are picked up at every pixel transfer. As a result it is reasonable to assume that the minimum and maximum bias level variation bounds can be established by examining the bottom and top portion of the image alone, respectively. Lastly, 5 additional full-frame bias images are gathered after the partial image acquisitions to examine whether any of the characteristics have changed and requirements BLS − 1, 3, 4, 5 are still fulfilled.

5.4.3 Data Analysis and Results for UniBRITE Instrument

Requirement BLS-1

An average bias frame was ﬁrst constructed from the 5 full-frame raw bias data images gathered at the beginning of the test. Next, the bias values of all pixels were output in a histogram format with bin width of 1ADU. The histogram information was then examined to determine how many pixels within a full frame bias image conformed to BLS − 1. This same procedure was repeated on the 5 full-frame raw bias data images gathered at the end of the test to ensure consistency. Table 5.2 summarizes the results.

Temp. Pass/Fail Observation −20◦C PASS* 0.36% of the pixels exhibited bias below 70ADU with a minimum of 64ADU but none exceed 130ADU. 0◦C PASS 0.41% of the pixels exhibited bias below 70ADU with a minimum of 64ADU but none exceed 130ADU. 10◦C PASS 0.55% of the pixels exhibited bias below 70ADU with a minimum of 64ADU but none exceed 130ADU. 20◦C PASS 0.17% of the pixels exhibited bias below 70ADU with a minimum of 65ADU but none exceed 130ADU. Ambient PASS 0.18% of the pixels exhibited bias below 70ADU with a minimum of 65ADU. 0.02% of the pixels showed bias above 130ADU. CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 56

Temp. Pass/Fail Observation 30◦C PASS 0.06% of the pixels exhibited bias below 70ADU with minimum of 65ADU. 3% of the pixels showed bias above 130ADU. 60◦C PASS* None of the pixels exhibited bias below 70ADU. More than 97% of the pixels had bias above 130ADU. Table 5.2: BLS-1 veriﬁcation results

The maximum bias level in many of the higher temperature images are not stated because they are mostly likely a result of “hot” pixels (extra temperature sensitive pixels) instead of being actually due to bias setting. A small percentage of the pixels did exhibit bias levels outside the desired 100±30ADU range. It is apparent that slight improvement could be made by shifting the bias setting up to 90ADU. However, through the author’s conversation with the Dr. S. Mochnacki, a member of the BRITE Science Team responsible for coming up the PISAT procedure, it was understood that BLS − 1 was put in place after observing the ramp pattern in the bias images (refer to Figure 5.3) and its sole purpose was to restrict the bias level difference between the lower and upper portion of the ramp; and the 100±30ADU range was arbitrary. An additional point of note is that thermal effects in the form of dark charge build up on top of the bias level is evident in the data as the mean bias level was observed to increase with temperature. Theoretically this would not be the case had the exposure been set to 0 seconds (or much shorter than the 60ms exposure period used here). Dark current effects most likely contributed to cause some of the more sensitive pixels to exhibit the higher bias levels observed at ambient (close to 25◦C), 30◦C and 60◦C. This effect is especially apparent at 60◦C. Note that a “*” indication is appended to the “PASS” designation given to the −20◦C and 60◦C results for the aforementioned reason that these temperatures test the survivability of the hardware rather than its performances.

Requirement BLS-2

The series of partial frame images capturing the top and bottom portions of the full bias image were individually smoothed by averaging over 1,000 pixels horizontally using the boxcar command with “xwindow” set to “1000” and “ywindow” set to ”1”. Variation level in the bias pattern was determined by sequentially comparing each pixel’s bias value (after the smoothing, in accordance with BLS-2 specifications) throughout the series using the imarith command to perform a “max” operation. This produces a resultant image where each pixel within the frame is the greatest value of all corresponding pixels in the series of partial frame images. The significance of this resultant image is that it contains the maximum bias recorded for each pixel (after smoothing) throughout the 1-hour period of continuous imager operation. The same process was then repeated using the “min” operation to produce a “minimum” image. A mean bias frame was then constructed through using the “average” operation. The CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 57 maximum/minimum variations above/below the mean bias level were then computed by subtracting the maximum and minimum from the mean bias frame respectively. This data processing sequence is illustrated below in Figure 5.4. This same procedure was carried out to find the maximum and minimum bias variation about the mean on the set of partial bias images capturing the top and bottom portion of the full frame bias pattern. The observations are presented in Table 5.3 and Table 5.4.

Figure 5.4: BLS-2 data process sequence illustration

Temp. Observation (500 lines of pixels, bottom section of bias) −20◦C 0.27% showed variation more than 2ADU above the mean, maximal: 2.15ADU. 0.20% showed variation more than 2ADU below the mean, minimal: −2.25ADU. 0◦C 0.46% showed variation more than 2ADU above the mean, maximal: 2.25ADU. 0.28% showed variation more than 2ADU below the mean, minimal: −2.27ADU. 10◦C 0.07% showed variation more than 2ADU above the mean, maximal: 2.21ADU. 0.02% showed variation more than 2ADU below the mean, minimal: −2.05ADU. 20◦C 0.10% showed variation more than 2ADU above the mean, maximal: 2.35ADU. 0.04% showed variation more than 2ADU below the mean, minimal: −2.25ADU. Ambient 0% showed variation more than 2ADU above the mean. 0% showed variation more than 2ADU below the mean. 30◦C 0.16% showed variation more than 2ADU above the mean, maximal: 2.35ADU. 0.06% showed variation more than 2ADU below the mean, minimal: −2.15ADU. 60◦C 1.44% showed variation more than 2ADU above the mean, maximal: 2.85ADU. 1.93% showed variation more than 2ADU below the mean, minimal: −2.95ADU. CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 58

Temp. Observation (500 lines of pixels, bottom section of bias)

Table 5.3: BLS − 2 veriﬁcation results for the 500 lines of pixels capturing the bottom section of the full frame bias image

Temp. Observation (500 lines of pixels, top section of bias) −20◦C 0.01% showed variation more than 2ADU above the mean, maximal: 2.35ADU. 0.04% showed variation more than 2ADU below the mean, minimal: −2.45ADU. 0◦C 0.01% showed variation more than 2ADU above the mean, maximal: 2.25ADU. 0% showed variation more than 2ADU below the mean. 10◦C 0.05% showed variation more than 2ADU above the mean, maximal: 2.25ADU. 0% showed variation more than 2ADU below the mean. 20◦C 1.19% showed variation more than 2ADU above the mean, maximal: 2.7ADU. 0.17% showed variation more than 2ADU below the mean, minimal: −2.4ADU. Ambient 4.18% showed variation more than 2ADU above the mean, maximal: 2.95ADU. 0.67% showed variation more than 2ADU below the mean, minimal: −2.95ADU. 30◦C 48.25% showed variation more than 2ADU above the mean, maximal: 3.6ADU. 30.36% showed variation more than 2ADU below the mean, minimal: −3.4ADU. 60◦C 97.56% showed variation more than 2ADU above the mean, maximal: 7.7ADU. 89.85% showed variation more than 2ADU below the mean, minimal: −6.55ADU. Table 5.4: BLS − 2 veriﬁcation results for the 500 lines of pixels capturing the top section of the full frame bias image

From Table 5.3, the bias stability of the bottom portion of the full image was very good with more than 99.5% of the pixels exhibiting bias level stability within ±2ADU about the mean level for tested temperatures up to and including 30◦C and 100% of the pixels were bounded within ±3ADU about the mean, even at 60◦C. The top portion showed a relatively higher level of bias variation as expected. Table 5.4 revealed that up to 98.8% of the pixels located at the top portion of the full image were stable to the level specified by BLS-2 but only up to 20◦C. Bias level variations were bounded within ±3ADU about the mean at 25◦C, within ±4ADU at 30◦C, and ±8ADU at 60◦C. From examining the top and bottom portion of the bias frame, it can be concluded that the bias level variation of the entire image frame is bounded within ±4ADU for CCD operating temperatures up to and including 30◦C since the variability of the middle section of the full image frame is expected (and verified) to be less than that of the top section (again, minor thermal noise effect was present and most likely contributed CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 59 to an additional level of variability). PASS/FAIL designations have been given to the instrument performance based on the observations described in Table 5.3 and Table 5.4. The “*” indicates verification of instrument survivability rather than performance at the corresponding temperature.

Temp. −20◦C 0◦C 10◦C 20◦C Ambient 30◦C 60◦C Pass/Fail PASS* PASS PASS PASS PASS PASS PASS* Table 5.5: BLS − 2 veriﬁcation results.

Requirement BLS-3

The average value of the bias frame was determined through the imstat command. This command was used on the 10 full frame bias images taken at the beginning and end of the data gathering process. The average bias levels at different temperatures are presented in Table 5.6, where all sample image ﬁles gathered showed consistent statistics.

Temp. Pass/Fail Avg. Bias Level (before) Avg. Bias Level (after) −20◦C PASS 78.6ADU 78.5ADU 0◦C PASS 79.5ADU 79.5ADU 10◦C PASS 81.5ADU 81.5ADU 20◦C PASS 88.0ADU 88.2ADU Ambient PASS 98.9ADU 96.5ADU 30◦C PASS 104ADU 104ADU 60◦C PASS 585ADU 584ADU Table 5.6: BLS − 3 veriﬁcation results, ADU values indicated are based on the mean of the average bias levels at each temperature.

Table 5.6 shows that the average bias levels are above the 40ADU level required by BLS − 3 at all test temperatures. Furthermore, the average bias levels are 78.6ADU and 79.5ADU at −20◦C and 0◦C respectively, which are very close to the 80ADU value set into the 8-bit DAC register within the CCD analog-to-digital converter as described in Section 3.4.2 and illustrated in Figure 3.7. Evidence of thermal effects is also evident here as the average bias level is seen to increase with temperature.

Requirement BLS-4

The 4σ outlier level specification stated in BLS−4 is equivalent to saying 99.99% of the pixel bias level needs to be above 0ADU. However, the raw data images would never report negative values because CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 60 the CCD ADC utilizes strictly unsigned binary representation. The objective of BLS-4 is to verify that the bias level setting provides sufficient offset to capture the full effect of negative noise variation below the mean. The mean bias levels are given in Table 5.6 but the appropriate σ value is not simply the standard deviation associated with the bias frame. This is due to the vertical ramp feature in the bias pattern as was shown in Figure 5.3. The ramp introduces additional variations in the bias level, which would contribute to a higher variance level, hence σ determined from a single bias frame would be over estimated. In order to obtain the proper σ, two samples of bias frames taken at the same temperature were co-substracted to remove the common bias pattern (i.e. the ramp). The resulting image would ideally be 0ADU at every pixel, but instead one would get a noise image where the pixel measurements fluctuate about 0ADU due to random noise. σ of this noise image was obtained and had to be reduced √ by a factor of 2 because the variance level (σ2) was doubled during the co-subtraction process. The final σ value of the noise image was then multiplied by a factor of 4 and subtracted from the average bias levels listed in Table 5.6. The results of this process are presented in Table 5.7, where 4σ below the average bias level is verified to be above 0ADU for all the temperatures. This means that the 80ADU DAC bias setting is sufficiently high.

Figure 5.5: BLS-4 data process sequence illustration. Note that the contrast ratio has been exaggerated to illustrate the ramp pattern in the bias frames.

Temp. Pass/Fail Avg. Bias Level (from Table 5.6) 4σ 4σ below avg. bias −20◦C PASS 78.6ADU 14.8ADU 63.8ADU 0◦C PASS 79.5ADU 15.2ADU 64.3ADU 10◦C PASS 81.5ADU 16ADU 65.5ADU 20◦C PASS 88.0ADU 18.4ADU 69.6ADU Ambient PASS 98.9ADU 21.2ADU 77.7ADU 30◦C PASS 104ADU 22.4ADU 81.2ADU 60◦C PASS 585ADU 70ADU 515.1ADU Table 5.7: BLS − 4 veriﬁcation results. CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 61

Requirement BLS-5

The average bias frames at every temperature produced in the BLS − 1 test were used in this analysis. The average bias was subtracted from each raw bias frames to reveal only the random variations (similar concept as illustrated in Figure 5.5). The mean and standard deviation of the resulting difference frames were examined to ensure they exhibited similar statistics. Figure 5.6 illustrates the pixel distribution of a typical difference frame in which a clear Gaussian feature can be identiﬁed.

Figure 5.6: Visualization of a typical difference image. Top left: difference frame. Top right: pixel distribution of difference frame.

In order to assess how close of a Gaussian distribution the pixel histogram came to be, the histogram data associated with each difference image was output into a text ﬁle. The histogram data was then normalized with respect to the total number of pixels in each frame and entered into MATLAB. Using MATLAB, a Gaussian curve constructed based on the corresponding mean and standard deviation values of the same difference frame was superimposed on the histogram for comparison. Figure 5.7 shows an example of the comparison between a Gaussian curve and the pixel distribution of a difference image taken at −20◦C. As indicated in Table 5.8, pixel distribution of the difference images obtained at all tested temperatures exhibit similar Gaussian distribution nature as illustrated in Figure 5.6.

Temp. −20◦C 0◦C 10◦C 20◦C Ambient 30◦C 60◦C Pass/Fail PASS PASS PASS PASS PASS PASS PASS Table 5.8: BLS − 5 veriﬁcation results. CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 62

Figure 5.7: The normalized pixel distribution of the difference image showed close match to a Gaussian ﬁt (data shown here was based on −20◦C bias data).

5.5 Gain Determination

The gain of the CCD imager is a conversion factor that specifies the number of electrons required in order to register 1ADU in the CCD ADC output and has an unit of [e−/ADU]. There are four stages of transformation that an incident photon would go through to be registered as one ADU in the output as illustrated in Figure 5.8. The first stage describes the ratio of incident photons to the interacting photons (subset of incident photons that makes their way to interact with the photo-active region of each pixel). The second stage describes the conversion efficiency that interacting photons are translated to electrons collected in the potential well region of each pixel (as some induced electrons are lost due to recombination with holes in the substrate). The combination of the first and second stage describes the overall quantum efficiency (QE) of the system, or the ratio of incident photons that are registered as an electron-hole pair in the CCD. Photon noise is introduced during this stage of the photometric measurement. The statistics of the photon collection process is described by the Poisson relation which states the noise associated with the photon measurement is square root of the signal (i.e. √ σ = µ). This type of noise results from discrete arrival of photons when they are measured at very low illumination levels. For large signals, the photon statistics changes into the Gaussian statistics. The third transformation is implemented by the CCD chip’s readout amplifier that gives the pixel sensitivity CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 63 of the imager and converts the signal from photon domain to the analog domain. The operation of the typical CCD imager chip can be summarized by these first three stages. The fourth and last stage, is finally the conversion from analog to digital data performed by the ADC. The gain to be determined, therefore, represents the overall transfer function from Stage 2 to Stage 4.

Figure 5.8: Photon conversion stages of a typical CCD imager [20].

The gain allows the digital output to be related back to physical photon quantity (because the photoelectric effect is an all or nothing event, hence the photon-induced electron charge is directly proportional to the incident photon). In the context of PISAT, knowledge of gain is needed in later analysis of dark current (Section 5.8) and readout noise measurements (Section 5.9) where requirements are posed in terms of electrons. Dark current and noise levels in units of ADU can be extracted from the gathered image data, and knowledge of imager gain allows these ADU quantities to be converted into electrons.

5.6 Gain Measurement

A direct approach to gain measurement would require a precise photon source and other elaborate equip- ments to create the precise number of electrons in the imager. The resulting digital measurement could then be read off directly to calculate gain through simple division. Unfortunately such approach is out of the question in terms of the resources available to the BRITE mission hence another approach is needed. An alternative approach is the variance method that evaluates the gain through mean and variance of the signal. The derivation starts with the deﬁnition of gain where SADU is the measured signal in ADU, − Se− is the electrons generated per pixel and K is the gain in [e /ADU].

S − S = e (5.1) ADU K In [20], Janesick used the propagation of errors formula to expand (5.1) into:

∂S 2 ∂S 2 σ2 = ADU σ2 + ADU δK2 + σ2 T otalADU S − 0ADU (5.2) ∂Se− e ∂K CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 64 where σ is the total noise in the measurement, σ is the photon noise associated with the T otalADU Se− signal in electrons and σ0ADU is the zero signal noise ﬂoor associated with the system, contributed primarily by the signal ampliﬁer. Substituting the proper values of the partial derivatives into (5.2) and assuming the gain variation to be negligible (δK2 = 0) yields:

2 2 1 2 2 σT otal = σS + σ0 (5.3) ADU K e− ADU

2 Further substituting σ by Se− (Poissonian statistics) and using the definition of K (5.1), (5.3) can be Se− reduced to the final form in (5.4) that reveals a linear relationship between total measurement variance, σ2 , and measured signal, S , with the square of noise floor per pixel, σ2 , as the y- T otalADU ADU 0ADU intercept.

1 σ2 = S + σ2 (5.4) T otalADU K ADU 0ADU The variance method reveals the nature of the gain parameter, K, to be inverse of the slope that relates measured signal level and total variance associated with the measurement. It is the linear form of the relationship that invites gain measurement to be done through constructing a photon transfer curve with S on the x-axis and σ2 on the y-axis plotted in an (x, y) relation from which a linear trend can ADU SADU be extracted to obtain estimates for both K and σ0ADU .

5.6.1 Requirement

“The gain setting for the BRITE payload imager should be set to 3.0 to 3.5[e−/ADU]” [28].

5.6.2 Data Gathering

The typical approach taken to construct the photon transfer curve consists of gathering a series of images of an uniformly illuminated scene taken at different exposure periods in order to cover the entire dynamic range of the imager (i.e. from bias to saturation).

Figure 5.9: Typical process of constructing a photon transfer curve for gain measurement. CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 65

At each exposure setting, an average dark frame (image of a completely dark scenery taken over a certain exposure time) is subtracted from the mean illuminated frame in order to remove the dark count level (artificial counts created due to thermal effects, which becomes significant for tests at high temperatures). The (S , σ2 ) is then extracted from the resulting dark calibrated image to form a ADU SADU single data point on the photon transfer curve as illustrated by Figure 5.9. This process however, means additional dark frames need to be gathered at each exposure level, thus significantly increasing the data gathering time. Furthermore, the art of removing dark current from an image can be a trickier feat than simply subtracting the dark frame from the image since residual noise can be introduced into the data during the subtraction process and distort the resulting photon transfer curve. An alternative approach was recommended by Prof. S. Mochnacki, on the BRITE Science Team. This process utilizes the gradient image as introduced in Section 5.3 to capture the entire dynamic range of the CCD in a single frame rather than varying the exposure period. The PISAT optical setup shown in Figure 5.1 was originally developed by the BRITE instrument designer M. Barbu empirically to support this method. The resulting raw gradient image produced with the LED installation turned on is shown in Figure 5.2. A specific exposure period setting (750ms) was chosen so the brightest region of the gradient image would be saturated. The position of the LED was biased toward one side such that the darkest region of the image would remain sufficiently black. Eight LED images were gathered for the sake of speeding up the test process and was deemed to be sufficient in constructing a reasonable photon transfer curve. It was crucial that the physical test setup remained constant during the data gathering process. Next, the LED was turned off and another set of eight dark images were gathered for dark current removal from the raw gradient image.

5.6.3 Data Analysis and Results

The first step of post processing is to create an average dark image frame from the set of dark images gathered. Next, the average dark image is removed from each raw gradient image via a simple arithmetic image subtraction process. This creates a set of dark removed gradient images. Second, the set of dark removed images are examined to determine the position of a raster that contained an approximately constant vertical profile while having a horizontal profile that captured the entire dynamic range as illustrated by Figure 5.10. The raster information are extracted and saved as a set of new raster files. Third, statistics of resulting raster images are examined to ensure similarity. Typically, the mean value of each raster fluctuates due to unstable light intensity level in the LED (especially over the different temperature ranges since the current passing through the LED tends to decrease at high temperatures and cause the output intensity to decrease) and this needs to be adjusted by normalizing each image such that every raster region is brought to the same mean signal level. Normalization is performed by dividing each raster by the average of the mean value of each raster such that the mean of all rasters would CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 66 become equal. A mean image is then created by combining the set of normalized gradient rasters.

Figure 5.10: An ideal gradient image raster example showing smooth, linear intensity transition from saturation to dark.

Fourth, a sigma image is created by calculating the standard deviation associated with each pixel based on the list of raster inputs. This way, value of each pixel in the sigma image is the standard deviation of the corresponding pixels from each raster within the input set. Fifth, boxcar averaging with a boxcar of 1 × 100 pixels dimension is applied to both mean and sigma images. This captures the value spread in the images along the vertical direction and reveals a clear trend in the data. Sixth, a single row of data along the middle of mean and sigma images is output into a text ﬁle. This gives the numerical mean and sigma values. Finally, the photon transfer curve can be constructed by plotting the the mean versus variance (i.e. σ2) as seen in Figure 5.11.

Figure 5.11: Photon transfer curve constructed based on ambient temperature (25 ± 2◦C) data images. CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 67

Note that “ambient temperature” refers to the fact that test is conducted in a bench top setup format with no active temperature control. Telemetry records indicate the temperature fluctuation ranges from 23◦C to 27◦C. The trend in the data can be observed as being slightly nonlinear towards the high signal regime (as shown by the quadratic fit imposed on data from 200ADU up to roughly 1, 100ADU), and eventually drops sharply past a certain signal level. The question is then which region of the data should be used to perform the linear fit (this is the form given by (5.4)) in order to extract the slope value (which 1 is K , from which gain (K) can be determined). The answer is that gain should be determined from the region of the photon transfer curve that exhibits the highest degree of photon noise (also known as shot 2 noise). Since Poissonian statistics dictates σe− = Se− , where σe− and Se− are the standard deviation and mean in units of electrons, then the following is true:

log σe− log σe− 1 = 2 = (5.5) logSe− logσe− 2 This suggests the photon noise dominated region of the photon transfer curve can be identiﬁed by plotting the mean versus standard deviation (i.e. σ) in a log-log format. The anatomy of the photon transfer curve expressed in log-log format is explained in [20] and shown in Figure 5.12 as having four distinct regions: read noise, shot noise, ﬁxed pattern noise, and full well saturation.

Figure 5.12: Three noise regimes and full well saturation on a signal vs. noise log-log plot [20].

Figure 5.13 shows the data in Figure 5.11 plotted in the log-log format. Comparison of Figure 5.13 against Figure 5.12 reveals clear features of the shot noise and full well saturation regime. The read noise regime characterized by zero slope is not present, which suggests the darkest region of the gradient image data is not sufficiently free of additional noise to be called the noise floor. The fixed pattern noise refers to flat field effects that are completely absent from the plot. However, this does not suggest the CCD is free of flat field variations, but rather hints the flat field regime is not being captured due to the choice of using a gradient image based methodology instead of the typical approach as illustrated in

Figure 5.9. Regardless, the distinct shot noise region (linear region from logSe− = 2.5 to 4 where the CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 68 slope is approximately 0.5) is sufﬁcient to establish the appropriate signal range that should be used to estimate gain.

Figure 5.13: Signal vs. noise data from Figure 5.11 plotted in log-log format.

In order to obtain gain, a line of best ﬁt was imposed on signal region from 316ADU up to

10, 000ADU of the data in Figure 5.11 that corresponds to the logSe− = 2.5 to 4 shot noise region identified. The original photon transfer curve is imposed with a linear fit over the identified shot noise region. This linear fit is further extended beyond the saturation signal level by extrapolation as shown in Figure 5.14. The slope of the linear fit was extracted and inverted to obtain the payload system gain, K.

Figure 5.14: Photon transfer curve constructed based on ambient temperature (25 ± 2◦C) data images.

The results of gain measurement determined based on this analysis methodology are shown in Table 5.9 CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 69

Temp. −20◦C 0◦C 10◦C 20◦C Ambient 30◦C 60◦C Gain[e−/ADU] 3.54 3.57 3.67 3.64 3.58 3.58 3.82 Table 5.9: UniBRITE payload instrument gain test results.

The resulting gain determined from this methodology was higher than the expected level of 3[e−/ADU] to 3.5[e−/ADU] at all temperatures. However, there are two shortcomings of this methodology that affects the accuracy of the results. First, the standard deviation σ value from which variance is calculated is limited to the number of gradient images gathered (recall that σ associated with each pixel was calculated based on sampling the set of gradient image inputs), which was only eight. This is clearly not enough to provide a satisfactory sample space. Second, six steps of data manipulation are required to obtain the results. In general, every step performed on the data either introduces additional noise or reduces the inherent noise. The effect of this is not simple and is not intuitive. In general, it is not an easy task to construct a working gradient image that exhibits the high degree of linearity in the transition region from dark to saturation from which a decent photon transfer curve can be extracted. Furthermore, one does not know whether the test setup for the gradient scenery is good enough without having gone through the entire data gathering and analysis process which renders the entire process of acquiring the correct test setup to be very time consuming. All the shortcomings suggest an alternative approach should be used and to provide a check on the results.

Fast Gain Measurement through Co-Subtraction Method

In order to overcome the ﬂaws in the previous methodology, the author explored alternative methods and discovered the “Fast Gain Measurement” procedure documented in [15]. Fundamentally, the principle behind it is also based on the variance method, however this method only needs two gradient images instead of relying on a whole series. Two rasters of the same region capturing the transition from dark to saturation with constant vertical proﬁle are used from the original data set. The two rasters are averaged to create a average raster frame, each raster is then normalized against the average such that the adjusted rasters have the same mean value. The difference between this method and the previous lies in the step where the two adjusted raster frames are co-subtracted to yield a difference frame. This is similar to bias frame co-subtraction to reveal the Gaussian noise pattern discussed earlier in Section 5.4. The difference frame eliminates the gradient pattern and reveals the discrepancy in measurement between Frame 1 and Frame 2 at each signal level due to noise. The mean and difference frames are then sliced into 1 × 100 pixel slivers. The mean signal level is calculated by taking the average of each sliver from the mean frame. The σ value is obtained by calculating the standard deviation of each sliver from the difference frame, which now offers 100 sample points rather than 8 so it is much more accurate CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 70

√ in terms of statistical sampling. As before, the resulting σ values need to be reduced by a factor of 2 to obtain the standard deviation of a single raster frame. The set of mean signal and σ values are then plotted in X-Y plot format to reveal the photon transfer curve as shown in Figure 5.15. The large spread in variance is apparent in the plot which conceals the trend. As a result, a moving average is applied to the data to reduce this spread and identify the actual photon transfer curve as shown in Figure 5.16. The results of gain measurement using this co-subtraction methodology are given in Table 5.10

Figure 5.15: Photon transfer curve at ambient temperature (25 ± 2◦C) using the gradient co-subtraction method.

Figure 5.16: Figure 5.15 processed with moving average to reduce the variance spread.

A signal versus noise log-log plot is constructed based on the data obtained through the gradient image CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 71 co-subtraction method and is given in Figure 5.17. The linear ﬁt shown in Figure 5.16 is based on signal regions corresponding to the photon noise dominant regime of the signal versus noise log-log plot shown in Figure 5.17 (approximately from 500ADU to 10, 000ADU). The gain results obtained through the co-subtraction method are listed in Table 5.10.

Figure 5.17: Signal vs. noise data represented in log-log format.

Temp. −20◦C 0◦C 10◦C 20◦C Ambient∗ 30◦C 60◦C Gain[e−/ADU] 3.37 3.34 3.38 3.42 3.43 3.42 3.45 Pass/Fail PASS PASS PASS PASS PASS PASS PASS Table 5.10: UniBRITE payload instrument gain determination test results using gradient co-subtraction method.

*Ambient refers to bench top testing with no active temperature control. Telemetry records indicate temperature variation was within 23◦C to 27◦C.

The resulting gain values are lower than the results obtained through the original method given in Table 5.9 and can be seen to pass at all temperatures. Overall, more confidence should be placed in the results obtained through the gradient co-subtraction method because it involves less data processing steps hence reduces the chance of artificial noise injection or reduction. In this case, the higher gain values obtained using the original method suggest that the noise level had been reduced during data processing (i.e. shallower slope in the linear fit). Furthermore, the gradient co-subtraction method calculates the noise level based on a much wider sample space (100 data points vs. only 8), which implies it should be more representative. In addition, the method has been compared to other gain measurement methods in [15] and was experimentally proven to be consistent. Finally, the gradient CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 72 co-subtraction method relies on only two gradient images as inputs rather than eight (or more). This means there is less opportunity for the physical setup to fluctuate (such as LED intensity, temperature, etc.) during the data gathering process which renders the whole process much more robust and time efficient.

5.7 Saturation Test

The purpose of this test is to determine the saturation level that determines the upper limit on the dynamic range of the CCD as well as to verify that it is sufﬁciently high compared to the level expected by the BRITE Science Team. In operation, knowledge of saturation level would identify limitations on the minimum apparent magnitude of star (lower the magnitude, the brighter the star) and the maximum exposure time that can be used before saturating the imager.

5.7.1 Requirement

The nominal charge capacity level of the KAI-11002 chip is 60, 000e− per pixel [2] but for the purpose of BRITE, the Science Team had decided that a full-well saturation level of 30, 000e− was sufﬁcient. Therefore the full-well saturation level of the CCD imager must exceed 30, 000e− [28].

5.7.2 Data Gathering

Information regarding full well saturation level of the CCD can be extracted from the photon transfer curve constructed in Section 5.5. As a result, no additional data gathering was required for this test.

5.7.3 Data Analysis & Results

From the log SADU versus log σ plot shown in Figure 5.12, the full well saturation level could be distinctly observed at the signal level where a rapid vertical drop from the linear trend occurred and it was observed to be above the 10, 000ADU level at all the tested temperatures. Furthermore, because the gain value was determined to be above 3e−/ADU in Section 5.5, the full well saturation level is therefore above the required 30, 000e− level at all the tested temperatures.

Temp. −20◦C 0◦C 10◦C 20◦C Ambient 30◦C 60◦C Full well sat. level [ADU] 13, 000 12, 600 11, 900 11, 200 11, 600 10, 800 10, 000 Full well sat. level [e−] 43, 810 42, 084 40, 222 38, 304 39, 788 36, 939 34, 500 Pass/Fail PASS PASS PASS PASS PASS PASS PASS Table 5.11: UniBRITE instrument full well saturation level (approximate) CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 73

5.8 Dark Current Test

Dark current is the rate at which thermally induced charges, or dark charges, are generated within the CCD system. These charges are independent of any photon source and can therefore greatly distort the true signal that is being measured. Typically, CCD systems are thermally cooled (i.e. the original SBIG STL-11000M camera assembly) in order to alleviate dark current effects. However, active cooling is not present on BRITE, and the temperature of the CCD is made constant by biasing to a higher temperature instead. As a result, dark current level needs to be veriﬁed to be acceptable.

5.8.1 Requirements

The requirements imposed on the dark current level of the CCD are listed in Table 5.12.

DC − 1 Dark current should not exceed 5e−/s/pixel for integrations up to 10s at +20◦C. DC − 2 Not more than 0.5% of pixels should exceed a dark current of 50e−/s. DC − 3 Not more than 0.1% of pixels should exceed 100e−/s in a 10s exposure (with a desire that not more than 0.05% of pixels should exceed 100e−/s in a 10s exposure). DC − 4 At +20◦C, the CCD must make a 60s exposure with no saturated rows or columns. DC − 5 At +20◦C, the CCD must produce exposures with no more than 5% of the detector area saturated at 90s. Table 5.12: Dark current requirements [28].

5.8.2 Data Gathering

A series of dark images are gathered for the purpose of this test. Dark images are essentially images taken without any light source, over certain exposure periods. During dark exposures, dark charges accumulate, resulting in an image that registers higher ADU values than a bias frame. As a result, dark images taken at different exposure times must be gathered in order to examine how the dark charge build up varies with exposure time and extract the dark current [e−/s] information. For this purpose, sets of three dark images are gathered at exposure time settings of 1s, 3s, 10s, 30s, 60s and 90s at ambient temperature (i.e. 25 ± 2◦C). Multiple dark frames are necessary at each exposure setting in order to create an average dark frame so that the impact of random noise variation may be reduced. Images are gathered at fewer exposure time settings (i.e. only three or four exposure times as opposed to six) at temperatures of −20◦C, 0◦C, 10◦C, 20◦C, 30◦C and 60◦C for the sake of shortening test length in order to meet the overall project timeline. In addition, the average bias frame constructed in the Bias Level and Stability test is used for removal of bias from the dark images. CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 74

5.8.3 Data Analysis and Results

The general approach taken here to estimate the dark current level is to first examine how the dark counts vary with increasing exposure time. A correlation on the dark count versus time is formed to provide a model of the temporal relationship from which the rate of dark count accumulation can be extracted. From there, the dark count growth rate is converted to the electron domain through knowledge of the gain parameter determined in Section 5.6. The first step taken to process the raw dark images is to remove bias in order to obtain only the dark count. Second, bias removed dark frames taken with the same exposure time setting are combined to obtain an average dark frame that has a reduced level of random noise. Third, each processed average dark frame is exported as histograms with a bin size of 1ADU. The histograms are input into a spread- sheet program and sorted in ascending order by dark count. Threshold dark count levels that mark the top 0.01%, 0.05%, 0.06%, 0.1%, 0.5%, 1%, 5%, 10%, 25% and 50% most dark current sensitive pixel groups are identified at each exposure time setting. These threshold values will increase with longer exposure times and provide a trace on the dark count accumulation rate. The assumption used here is that pixels which registered high levels of dark count at one exposure setting should continue to register higher dark counts at longer exposures. This dark count evolution information is presented in Table 5.13 and plotted for the top 0.01%, 0.05%, 0.06% and 0.1% groups of most dark current sensitive pixels in Figure 5.18 for the UniBRITE instrument.

Exposure Time 1s 3s 10s 30s 60s 90s Top % Pixel Groups Dark Count Threshold Levels [ADU] 0.01% 178 577 2002 5628 10405 11342 0.05% 24 81 293 889 1851 2754 0.06% 20 74 272 813 1659 2421 0.1% 13 46 176 535 1121 1673 0.5% 6 6 11 37 84 128 1% 5 5 7 8 14 19 5% 3 3 4 4 5 6 10% 2 2 3 3 3 3 25% 0 0 0 0 0 0 50% 0 0 0 0 0 0 Table 5.13: Dark count threshold levels that categorize each dark current sensitive pixel groups at different exposure times, at ambient temperature (25◦C ± 2◦C). CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 75

Figure 5.18: Dark count evolution over exposure time for the top percentage of most dark current sensitive pixels at ambient temperature (25◦C ± 2◦C).

The rate at which dark count level increases for the different pixel groups can be visualized from Fig- ure 5.18. A quadratic polynomial was employed to ﬁt each set of data points in order to capture the nonlinear trend. The trend lines for the 0.05%, 0.06% and 0.1% top sensitive group of pixels actually exhibit minor quadratic features. This is not apparent in Figure 5.18 due to scale distortion by the 0.01% pixel group which shows a clear downward opening parabola, suggesting the rate of dark count accumulation eventually subsides as the level approaches the full well saturation of the pixel at approximately 11, 600ADU determined in Section 5.7. The behavior seen here makes sense intuitively because the electrostatic repulsive forces between the dark charges held within the CCD potential well would increase as the pixel charge capacity is approached and hence impede further accumulation while increasing the likelihood of charge recombination. In order to determine the dark current level, equations of the quadratic ﬁts were obtained and dif- ferentiated with respect to time and evaluated at each exposure time. The dark currents corresponding to the dark counts given in Table 5.13 are determined and converted to the electron domain using the previously calculated gain. The results are presented in Table 5.14 below:

Exposure Time 1s 3s 10s 30s 60s 90s Top % Dark Current [e−/s] 0.01% 844 826 763 582 312 41 0.05% 105 105 105 105 106 106 0.06% 98 98 97 95 91 88 0.1% 63 63 64 64 65 65 0.5% 4 4 4 5 5 6 CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 76

Exposure Time 1s 3s 10s 30s 60s 90s Top % Dark Current [e−/s] 1% 0 0 0 0 1 1 5% 0 0 0 0 0 0 10% 0 0 0 0 0 0 25% 0 0 0 0 0 0 50% 0 0 0 0 0 0 Table 5.14: Dark current level of the most dark current sensitive groups of pixels at different exposure times at ambient temperature (25◦C ± 2◦C).

The effects of dark current are made visible by plotting a pixel histogram and comparing the distribution at each exposure time as illustrated in Figure 5.19.

Figure 5.19: Pixel dark count distribution at different exposure times, at ambient (25◦C ± 2◦C). CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 77

Similar trends in dark count evolution were observed at other temperatures. Dark current was observed to increase with temperature as expected as it is predominantly a thermal effect. Note that the vertical scale is plotted as log of the pixel count. The dark current behavior of the UniBRITE instrument at ambient temperature (25◦C ± 2◦C) was mostly contained and only minor effects could be observed as small groups of pixels “creep” toward the right of the horizontal scale over increased exposure times. This general behavior was observed at other temperatures as well and became much more drastic at 60◦C as shown in Figure 5.20, where data were collected at exposure time settings of 1s, 3s, 10s and 30s.

Figure 5.20: Pixel dark count distribution at different exposure times at 60◦C.

A large group of pixels can be seen to reach super saturation at a dark count level of approximately 16, 000ADU which is above the corresponding full-well saturation level of 10, 000 determined in the Saturation Test. The conclusion is that excess charges spill over to adjacent pixel rows and result in columns of saturated pixels as demonstrated by a dark image taken over 10s exposure at 60◦C (Fig- ure 5.21). Figure 5.22 further shows a dark image taken over 30s exposure at 60◦C at which point the image can be seen to be completely over saturated. CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 78

Figure 5.21: Dark image taken over 10s exposure at 60◦C. Saturated columns and hot pixels are clearly visible.

Figure 5.22: Dark image taken over 30s exposure at 60◦C. Left shows original contrast and right shows the enhanced contrast version for better visualization. CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 79

The dark current results in response to the set of dark current requirements are described in Table 5.15.

Requirement Pass/Fail Comments DC − 1 Pass For integrations up to 10s: • 99.95% of pixels have dark current less than 5[e−/s] at −20◦C. • 99.9% of pixels have dark current less than 5[e−/s] at 0◦C. • 99.5% of pixels have dark current less than 5[e−/s] at +10◦C. • 99.5% of pixels have dark current less than 5[e−/s] at +20◦C. • 99.5% of pixels have dark current less than 5[e−/s] at ambient. • 99% of pixels have dark current less than 5[e−/s] at +30◦C. • Less than 50% of pixels have dark current less than 5[e−/s] at +60◦C. DC − 2 Pass No more than 0.5% of the pixels, for exposures up to 30s integration time, have dark current levels that exceed 50e−/s at all temperatures except 60◦C. DC − 3 Pass No more than 0.1% of the pixels have dark current levels that exceed 100e−/s in a 10 s exposure at all temperatures except 60◦C DC − 4 Pass No saturated rows or columns were observed in 60s exposures at +20◦C. DC − 5 Pass 99.99% of the pixels were not saturated in a 90s exposure at +20◦C, assuming a saturation level of 11, 200ADU. Table 5.15: Dark current test results in response to the requirements stated in Table 5.12.

5.9 Readout Noise Level Test

The readout noise level identifies the noise floor in the imager system. The noise floor is the intrinsic noise of the system electronics without contribution from sources such as dark current noise or shot noise. Typically, readout noise could also be extracted from the photon transfer curve illustrated in Figure 5.12 as the low signal region with a distinct slope of 0. However, such characteristic was not observed in the signal vs. noise log-log relation shown in Figure 5.17 where a gradual transition towards the photon shot noise dominated region was instead observed at the low signal region. This indicates additional source(s) of noise is present in the data. As a result, extrapolating readout noise from the photon transfer curve is not the best approach in this case. Instead, an alternate method that follows the procedure suggested in [28] utilizes bias data frames collected in Section 5.4. CHAPTER 5. BRITE PAYLOAD INSTRUMENT SCIENTIFIC ACCEPTANCE TEST 80

5.9.1 Requirement

The readout noise level should be below 30[e−/pixel] at a readout clock rate of approximately 10MHz although a lower level would be extremely desirable with the target level being 15[e−/pixel] [28].

5.9.2 Data Gathering

All analysis done in this section utilizes the full bias data frames collected in Section 5.4 and no additional data collection is necessary.

5.9.3 Data Analysis and Results

The method of analysis here is based on the assumption that the bias frames contain no traces of signal due to photons or dark current and hence the signal registered by every pixel within the bias frame should be ideally constant. However, electronic noise adds a degree of variation in the measurements registered and it is this variation that provides a way to quantify the electronics noise ﬂoor. The bias pattern, although stable with time, is not uniform throughout the full image frame as illustrated in Figure 5.3. A co-subtraction process is therefore required to eliminate the diversity in the bias pattern in order to obtain the true readout noise level. This process is performed by choosing two bias frames with similar statistics (examined and veriﬁed in Section 5.4) and co-subtracting the two frames. The resulting difference image contained a mean of approximately 0ADU and the non-zero measurement registered by each pixel was a result of random variation due to electronics noise. The pixel distribution of the difference image was then examined to ensure it exhibited a Gaussian distribution as the one shown in Figure 5.6 and Figure 5.7. The standard deviation, σ, of the difference image was calculated. A subtle point to be noted is that the σ2 value gives the variance level within the difference image, which was doubled during the co-subtraction process of the two bias frames. Therefore, the 2 p 2 correct variance level is σ /2 and the readout noise level is hence σ0 = σ /2 or simply divide σ by a √ factor of 2. The result is then converted into units of [e−/pixel] through the gain values determined in Section 5.6. The results for readout noise are summarized in Table 5.16. The readout noise of the system is below the required level of 30[e−/pixel] for temperatures up to and including 30◦C. Note a “*” is appended to the “PASS” at 60◦C to indicate instrument survivability rather than performance.

Temp. −20◦C 0◦C 10◦C 20◦C Ambient 30◦C 60◦C

σ0[ADU] 3.7 3.8 4.0 4.6 5.2 5.6 17.5 − σ0[e ] 12.5 12.8 13.7 15.6 18.0 19.1 60.3 Pass/Fail PASS PASS PASS PASS PASS PASS PASS* Table 5.16: UniBRITE payload instrument readout noise results. Chapter 6

Instrument Integration

After the hardware qualification test of the UniBRITE IOBC and CCD header board hardware was completed, it was now ready to be integrated together to form the complete BRITE instrument as shown in Figure 3.8. The baffle and optical cell modules of the telescope were prepared, cleaned and assembled by BRITE Manager C. Grant inside SFL’s Class 10, 000 cleamroom with Class 1, 000 work area in order to mitigate having dirt and debris trapped inside the telescope assembly, as well as for any flight-intended assemblies. The primary involvement of the author was taking part in the instrument focusing process that involved the fine tuning of the CCD Header Board position with respect to the instrument telescope in order to achieve the desired point spread function (PSF) as described in Section 3.5.2. In addition to the already defocused optical design, the BRITE Science Team decided that the focal position of the imager should be explored as an additional means of defocusing to further mitigate the problem of under sampling.

6.1 Creating an Artiﬁcial Star Field

Star field scenery is needed for the purpose of instrument focus. Focusing of the instrument on real stars is ideal, but this would subject the whole process to the mercy of weather. In addition, it is extremely undesirable to expose the delicate electronics to the open environment such that dust, moisture and even insects may enter the instrument assembly. As a result, it is necessary to create an artificial star field to perform the focus activity. A star emits photons that travel through a distance of practically infinity to reach an observer located on (or near) Earth and the incident photons arrive in parallel rays of light. As a result, an artificial star would have to do just that - be a point source that emits parallel rays of light. LED light fitted with neutral density filter and pin-hole filter is used to create point sources of light. In order to collimate the light, BRITE Science Team member Dr. S. Mochnacki devised a clever scheme to utilize a commercial telescope, focused at infinity, in reverse, as a collimator.

81 CHAPTER 6. INSTRUMENT INTEGRATION 82

6.1.1 Collimator Setup

The work of setting up the collimator was conducted by Dr. S. Mochnacki in collaboration with Dr. R. Kuschnig and is included here for the sake of completeness of the instrument focusing task. A small, ancillary telescope was brought to the outdoor ﬁeld at UTIAS/SFL and mounted on top of a paramount tracking platform to align and track stars in the sky as shown in Figure 6.1. In addition, a small commercial CCD imager was mounted on to the eyepiece of the ancillary telescope and connected to a monitor to provide a live view of the ﬁeld of view (FOV) but also to observe the PSF achieved by the telescope at the CCD.

Figure 6.1: Focusing the ancillary telescope using a commercial CCD (not BRITE instrument) and paramount tracking platform.

A moon of Jupiter was used as a target star for the observation (but any apparent celestial object can be used for this purpose) and the focal length of the ancillary telescope was iteratively adjusted until the PSF of the star came into sharp focus on the monitor (refer to Figure 6.2). At this point, the ancillary telescope had been properly focused to a distance of inﬁnity and light entering the length of the telescope would emerge as parallel rays. The ancillary telescope was then brought indoors and placed to stare down the length of a larger primary telescope that would act as the collimator (see Figure 6.3). The pin hole light was turned on and placed at the eye piece end of the primary telescope while the ancillary telescope mounted with the CCD served as an observer. The settings of the primary telescope are then adjusted until the pin hole light appeared in sharp focus on the CCD which gave an indication that the pin hole and collimator assembly are now suitably calibrated to produce point sources that emitted parallel rays of light just as real stars do. The collimator setup was then thoroughly cleaned and CHAPTER 6. INSTRUMENT INTEGRATION 83 transported inside the SFL clean room work bench in preparation for instrument focusing.

Figure 6.2: Left: unfocused pin-hole lights as observed by the commercial CCD mounted on the ancillary telescope. Right: calibrated collimator setup that produced sharp points of collimated light.

Figure 6.3: The primary telescope being used as a collimator with the BRITE instrument placed at the observing end for focusing.

6.2 Instrument Focusing

The four focusing springs on the header tray module of the instrument assembly provide the mounting points for the CCD Header Board. Figure 6.4 (left) shows the backside interior of the header tray module where the four focusing springs, optics and wire connector mounting points can be identiﬁed. Figure 6.4 (right) shows the CCD Header Board assembled on to the header tray with the focusing nuts in place. CHAPTER 6. INSTRUMENT INTEGRATION 84

The IOBC was then connected with the instrument assembly and placed in front of the collimator to stare straight along the boresight. The focusing task was ready to commence.

Figure 6.4: Left: backside interior view of the instrument header tray module. Right: backside interior view with CCD Header Board installed (Photo credit to C. Grant).

An earlier study of defocusing examined the simulated structure of the PSFs with respect to extra- focal positioning and intra-focal (with respect to the sharpest possible image) of the CCD through a custom photometric simulation tool developed at the University of Vienna. A comparison between the extra-focal and intra-focal PSFs showed that the extra-focal ones generally bore more resemblance to a Gaussian shape and the intra-focal ones exhibited more of a toroidal spread as can be seen in Figure 6.5.

Figure 6.5: Simulated PSFs examining intra-focal vs. extra-focal options for the BRITE Red Instrument (courtesy of Dr. R. Kuschnig). CHAPTER 6. INSTRUMENT INTEGRATION 85

The conclusion of that study was to select an intra-focal CCD position producing PSFs that kept the vast majority of the light contained within 8 to 10 pixels, which ensured that crowding of stars would not become a major issue for photometry [14]. The instrument focusing process began by taking a first image of the artificial star field to examine the resulting PSF of the initial CCD Header Board placement. Next, a series of positional adjustments were made through turning each focusing screw by the same angle (usually by a quarter or one-eighth of a full turn that is equivalent to approximately 0.125mm and 0.0625mm movements respectively) using a custom torque wrench fitted with angular scale as was shown in Figure 3.10. A boresight image was taken at each position. Careful notes were taken to match the turn angle of the focusing screws with each image such that the desired PSF could be reproduced. The series of PSF images were analyzed by Dr. R. Kuschnig. It was up to Dr. Kuschnig to decide which PSF would be best for the mission. Figure 6.6 shows the PSF variations with extra-focal and intra-focal positioning of the CCD gathered during UniBRITE payload instrument focusing. A second round of fine focus adjustments was made for the UniBRITE payload instrument, where one-sixteenth of turn adjustments were made to further improve the Science Team’s satisfaction with the PSF. The adjustment process as well as the final PSF chosen is shown in Figure 6.7.

Figure 6.6: Initial PSF exploration of CCD placement.

Figure 6.7: Fine focus adjustment made on the set point (Figure 6.6) of initial PSF exploration for the UniBRITE instrument ﬂight assembly. CHAPTER 6. INSTRUMENT INTEGRATION 86

Once the desired CCD Header Board position had been chosen, the instrument was tilted ±5◦ and ±10◦ in both pitch and yaw directions with respect to the collimator boresight axis. Images were taken with each axis movement to examine the difference in PSF size due to possible tip and/or tilt of the CCD imager plane. Iterative adjustments were then made to the focusing nuts and the images re-examined per adjustment until the PSF sizes were brought to be roughly symmetrical about the center in each axis. Figure 6.8 shows the PSFs before and after tilt adjustment of the UniBRITE payload instrument where PSF size asymmetry was apparent before tilt adjustment.

Figure 6.8: PSF contrast before and after CCD imager plane tilt adjustment.

After imager plane tip/tilt was corrected, individual images were taken along boresight and assorted combinations of ±5◦, ±10◦ offsets about the yaw and pitch axes. The set of images collected was forwarded to Dr. R. Kuschnig, who examined each resulting PSF and constructed a collage of PSFs to visualize the degree of PSF variation that should be expected outside of the main ﬁeld of view about the boresight direction. The resulting PSF map is shown in Figure 6.9. After receiving conﬁrmation from the BRITE Science Team, the CCD header board, as well as the entire instrument assembly, was staked down with RTV glue to prevent further movement of parts. This then, concluded the UniBRITE payload instrument focus and integration process. CHAPTER 6. INSTRUMENT INTEGRATION 87

Figure 6.9: PSF map of UniBRITE payload instrument. Chapter 7

Field Testing with Real Stars

At the last stage of the instrument assembly, integration and testing, the CCD imager and its optical telescope had been properly integrated and focused to produce a desirable PSF based on an artiﬁcial star ﬁeld. The results of various performance characteristic tests of the subsystem suggested the hardware had largely met the requirements posed by the BRITE Science team. Aspects that did not strictly meet the requirements (i.e. bias level stability requirements BSL-1 and BSL-2) would certainly not jeopar- dize the primary science goal of achieving milli-magnitude differential photometry accuracy on stars of apparent visual magnitude of +3.5 or brighter. However, up to this point, the instrument had not been tested on real stars to verify its capability for real astronomy. This chapter provides the description of the author’s effort to conduct preliminary observations of actual stars using the BRITE science instrument.

7.1 Paramount Tracking Platform

A Paramount tracking platform was installed in the field outside of UTIAS-SFL specifically for the purpose of conducting night sky observation. Its location was chosen to be at the north corner of UTIAS in order to face a ravine to minimize the influence of light pollution produced by the city. The orientation of the mount was adjusted such that the parked position (i.e. default position) of the tracking platform would point towards the Polaris (North Star), which is the only star that remains fixed in the sky as viewed by ground observers. The tracking platform is actuated by two individual stepper motors to provide two-degrees of freedom. When integrated with a computer workstation installed with star tracker application software such as “The Sky 6” used here, the platform is able to align and perform continuous slews to track celestial objects as it moves across the sky during the course of the night. The top of the tracking platform is a flat panel with threaded holes to provide a mounting point for the BRITE payload instrument. These features provided all the functionalities required to conduct a proper ground based star observation.

88 CHAPTER 7. FIELD TESTINGWITH REAL STARS 89

7.2 Mobile Instrument Assembly

An engineering model of the BRITE instrument was chosen for the outdoor observation to verify the instrument’s capability in achieving mili-magnitude differential photometry measurements. The engineering model is sufficiently representative in this case because it consists of the same set of electronics hardware that had been tested completely through the acceptance test process. Furthermore, the instrument had also underwent the same focusing routine as described in the previous chapter and had been verified to produce satisfactory PSF by Dr. R. Kuschnig. The engineering model is hence equivalent to the UniBRITE flight instrument with the exception that it carries the blue version of the optical telescope instead of the red, hence it produces a different set of PSF patterns. The hardware was installed onto a steel mounting plate inside a plastic box and enclosed by an ESD bag to protect delicate electronics. An additional Telrad device with two illuminated concentric centering crosshairs inside its view finder was mounted on top of the mobile instrument assembly to provide guidance in initial manual alignment with target stars during the tracking platform alignment process. Figure 7.1 shows the final assembly of the mobile instrument assembly as ready to be mounted on to the paramount tracking platform.

Figure 7.1: The mobile instrument assembly composed of the BRITE prototype camera hardware for real star observations. CHAPTER 7. FIELD TESTINGWITH REAL STARS 90

7.3 Star Observation Setup

The hardware setup for the sky observation is illustrated in Figure 7.2 and the paramount tracking platform was connected to a dedicated workstation running the star tracking algorithm, The Sky 6.

Figure 7.2: Hardware setup for star observation (picture taken during the daylight).

The first step to readying the tracking platform for operation was to synchronize it with The Sky 6. Hardware-wise, this process involved configuring the software application to recognize the tracking platform by selecting the correct communication port. Next, because the software had no way of knowing where the tracking platform was pointing at initially, it was necessary to implement manual slew maneuvers to orient the payload instrument with a known celestial object. Vega was chosen for the alignment purposes as it has an apparent visual magnitude of 0 and is one of the brightest objects in the sky, thus making it easily identifiable especially on a humid or cloudy night as it is likely the only star visible. The Telrad device was especially useful for aligning the science instrument with Vega correctly and was implemented strictly for the purpose of synchronization and re-synchronization between observations to correct for unwanted drift in tracking. Once alignment with Vega was accomplished, the star was located in The Sky 6 and was selected as the target object to synchronize the tracking platform. The CHAPTER 7. FIELD TESTINGWITH REAL STARS 91 software now knew that the tracking platform was pointed at Vega, together with time, the longitude and latitude of UTIAS, hence provided information regarding the initial states. Once synchronized, subsequent slews could be commanded through the internal sky atlas provided by the The Sky 6 to target stars or constellations. In addition, the tracking option was selected in the software, which meant that the tracking platform would be continuously tracking the targeted star as it moved across the sky with time.

7.4 Observing the Stars

A field of view centered about the HD176051 star of 5.22 magnitude was chosen as the target star field of interest by Dr. R. Kuschnig. Three stars: Beta Lyr (m = 3.45), Gamma Lyr (m = 3.24) and Lambda Lyr (m = 4.9) of the Lyra constellation all should be visible within the instrument’s field of view. In addition, the constellation contains Vega (m = 0.03), or Alpha Lyr, which is used for tracking platform synchronization and is very easy to locate in the image (relative to other faint stars). As a result the paramount tracking platform was commanded to align and track the HD176051 star. The payload instrument was operating with the original SDGC 8.srec application software at the time in order for the performance to be consistent with the results obtained from PISAT. Generally, due to the presence of Earth’s atmosphere, cloud and moisture in the air, the level of photon attenuation and diffraction is significantly more severe than observations in space. As a result, a 10 second exposure on Earth could be equivalent to a 1 second exposure in space as suggested by Dr. R. Kuschinig.

7.5 Image Processing

A total of 10 stellar observation frames and 10 dark frames were gathered during an entire night’s observation campaign, with re-synchronization of the tracking platform to Vega performed in between, after every 2 − 3 observation. As part of standard photometry processing, the 10 dark frames were averaged via IRAF’s imcombine command to produce a mean dark frame which was then removed from each of the raw stellar observation frames. This process removed both dark counts and bias from the images. Figure 7.3 illustrates a snapshot taken from DS9 of a single dark removed observation frame of the Lyra constellation where the contrast had been adjusted to expose the many faint stars and it is zoomed-in to center on the Lyra constellation. Having identified the Lyra constellation is important as it allows the apparent visual magnitude associated with each star to be identified. In addition, having seen that Beta Lyr (Sheliak, m = 3.45) and even HD176051 (m = 5.2) were registered on the image frames served as direct verification that the mission level requirement M-B.1 - “the satellite instrument shall be capable of observing stars with apparent visual Magnitude +3.5 and brighter” [34] has been met. This feat, in fact, proved the instrument is more than capable of observing stars of magnitudes much fainter than +3.5. Furthermore, the resulting PSFs in the observation frames verified that imager focusing work CHAPTER 7. FIELD TESTINGWITH REAL STARS 92 done on the artificial star field was performed correctly and similar PSFs could be achieved on real stars.

Figure 7.3: The Lyra constellation as imaged by the engineering model instrument.

Figure 7.4: Close examination of the PSF achieved with the various stars of the Lyra constellation.

For analysis purposes, the full frame data images gathered were cropped into 1, 000 by 1, 000 pixel rasters with the new pixel coordinates centered about the mid point of Vega’s PSF. Figure 7.5 shows the CHAPTER 7. FIELD TESTINGWITH REAL STARS 93

10 stars of interest to be used for analysis. These stars were chosen because of their relative appearance in comparison to other fainter stars, and because they exhibit well structured, mostly symmetrical PSFs. Note that Vega is not included in the seven chosen stars since its pixels are registering > 15, 000ADU which means it has induced saturation.

Figure 7.5: Raster frame showing the seven stars to be used for differential photometry analysis.

Identiﬁer Name Magnitude Coordinates with respect to Vega Star 1 6-Zeta 1 Lyrae 4.34 (1367, 1225) Star 2 18-Lota Lyrae 5.25 (1555, 1835) Star 3 17 Lyrae 5.2 (2037, 1865) Star 4 Sulafat 3.25 (2022, 1626) Star 5 HD176051 5.2 (1995, 1571) Star 6 Sheliak 3.52 (1936, 1374) Star 7 8-Nu Lyrae 5.93 (2010, 1368) Star 8 9-Nu 2 Lyrae 5.22 (2046, 1368) Star 9 5-Lambda Lyrae 4.94 (2094, 1658) Star 10 HD174179 6.06 (2154, 1314) Table 7.1: Star identiﬁer, name, magnitude and coordinates with respect to Vega for each of the 10 stars of interest. CHAPTER 7. FIELD TESTINGWITH REAL STARS 94

7.6 Differential Photometry

A simple differential photometry analysis attempt is performed by the author in this section to examine the basic level of photometric accuracy that can be achieved with data of the 10 chosen stars obtained from the Lyra constellation observations. Aperture photometry simply refers to summing up the photon counts within a defined aperture region encompassing the star PSF (i.e. flux) and subtracting off the sky background level, where the aperture can be of any shape or size. As a result, the task of aperture photometry is to obtain the aperture flux for each star of interest and obtain their differential magnitude by dividing them over the mean flux level of a selected group of reference stars. This analysis had previously been performed by W. Bode [8] using observations obtained from an early prototype BRITE instrument which was not entirely representative of the flight instrument hence the results were not entirely representative.

7.6.1 Performing Aperture Photometry with IRAF

The shape of the aperture chosen for analysis was circular in order to take advantage of the existing photometry analysis algorithm phot in IRAF. The phot command provided calculation of flux and instrumental magnitude (the magnitude measurement of flux relative to the sky background) associated with each star of interest but required several input configurations. First, the algorithm required a list of center coordinates for each star to be examined. The coordinates of each of the 10 chosen stars were determined by inspecting a single raster and this set of coordinates was used as input. Note that only a single raster was examined because the set of observation frames were each cropped into 1, 000 by 1, 000 pixel rasters with the center of Vega as the new origin. The position of each star would remain fixed from raster to raster since the relative position of each star to Vega from frame to frame would remain constant. The coordinate list allowed the algorithm to find where each of the stars was located. Second, the phot algorithm required specification of an aperture radius to use for calculating the star’s PSF flux level within the aperture. Fortunately, the algorithm also allowed for a range of aperture radii to be specified such that the resulting error in differential magnitude could be examined to determine which aperture radius allowed for best performance. An aperture radii range of 5 to 9 pixels was specified as input parameter and the results indicated an aperture size of 5 pixels consistently provided the minimal magnitude error in the resulting calculations. Third, the night sky, despite being apparently dark, also registered a certain degree of brightness by the CCD. As a result, the star PSF would sit on top of an elevated ADU count level if the sky background was not removed and the flux value would be overestimated. In order to estimate the sky background level, the phot algorithm also required the dimensions of an annulus centered about each star with its dimensions to be defined. An inner annulus dimension of 15 pixels radius and an outer annulus dimension of 25 pixels radius were chosen for each star (with the exception of Star 5 that was located too close to a nearby star hence a smaller set of annu- CHAPTER 7. FIELD TESTINGWITH REAL STARS 95 lus dimensions of 10 and 15 pixels radii were chosen in this case). The phot algorithm then calculated the median of all the pixel values within this annulus to come up with an estimate for the background sky level. Note that median calculation was chosen instead of mean to prevent skew effects caused by cosmic rays and hot pixels. Manual inspection was performed to make sure the chosen annulus dimension for each star would not including pixels from the PSFs of neighboring stars. Figure 7.6 provides a visualization of the aperture and annulus chosen for each star of interest. Finally, the phot command was executed on the list of raster inputs targeting the 10 chosen stars within each raster. The output was a list of flux, instrumental magnitude, and associated error corresponding to each star within each raster using the defined set of aperture and annulus dimensions.

Figure 7.6: Visualization of the aperture and annulus chosen for each star.

7.6.2 Calculating the Differential Magnitudes

The differential magnitude of each star in each raster was now able to be calculated based on the obtained set of ﬂux levels. Only Stars 1 to 6 were chosen for analysis to remove those with fainter magnitudes.

For each of the six stars, their ﬂux level was taken as fi and the average of the other ﬁve stars was taken CHAPTER 7. FIELD TESTINGWITH REAL STARS 96

as favg as background comparison. The differential magnitude of the target star was then calculated by −2.5 log fi . This calculation was performed for each of six stars 10 times, once for each 10 favg raster frame gathered. The standard deviation value was then calculated based on the 10 differential magnitudes evaluated from the rasters for each star. A summary of the results is shown in Table 7.2 below.

Star 1 Star 2 Star 3 Star 4 Star 5 Star 6 Average Differential Magnitude -0.99 -0.69 0.20 -0.68 0.51 0.59 Differential Magnitude Error (σ) 0.052 0.058 0.049 0.056 0.084 0.056 Table 7.2: Differential magnitudes and associated errors.

As can be seen, the σ error associated with the differential magnitudes are all on the order of tens of milli-magnitudes, larger than the required level. However, this is not a bad indication of the science instrument’s performance since the calculation is based on a set of single raster frames with no image co-addition to improve the S/N and, furthermore Earth’s atmospheric attenuation effect is still present. In order to account for these factors it will require many more rasters than just 10. It will demand image post processing calibrations to remove atmospheric effects [8] that are beyond the scope of this thesis. As a result, the author is unable to provide conclusive evidence to prove whether or not the science instrument is able to achieve milli-magnitude-error differential photometric measurements or not. Unfortunately, this will remain as future work to be conducted. Chapter 8

Future Work, Summary and Conclusions

8.1 Future Work

Although a preliminary set of observations has been conducted, the amount of data collected was insufficient to provide conclusive evidence that milli-magnitude level differential photometry can be achieved. In order to verify this, substantially more data images (on the order of 50+ image rasters) are needed in order to examine the degree of accuracy that can be achieved by combining two, three, four, five and more images to improve the signal-to-noise ratio of the point spread function. An extension to this investigation will be to utilize the “BRITE Target” software, incorporating it into the data gathering operations chain. At the time this thesis was written, the BRITE Target software had been completed but had not been tested in a fully representative manner involving all the instrument hardware performing actual observations. This will present a excellent opportunity to both utilize this software to enable more rapid data gathering as well as to test the functionality of the software to the fullest extent. Other future work to be conducted also involves uniformity calibration to gather an initial flat field image for each instrument integrated with satellite to complete the system-level test portion of payload instrument scientific acceptance test (PISAT). Although listed as part of the acceptance test, this was later decided by the BRITE Science Team to be a before-launch calibration procedure instead. This process will be simple as it will involve taking an image of a uniformly illuminated flat field for reference purposes. Discussion is underway to possibly obtain some Lambertian paint to create a uniformly illuminated flat field.

8.2 Summary and Conclusions

The BRITE Constellation consists of six individual satellites: UniBRITE, BRITE-Austria, BRITE Poland 1, BRITE Poland 2, BRITE Toronto and BRITE Montreal. Each BRITE is based on Space

97 CHAPTER 8. FUTURE WORK,SUMMARY AND CONCLUSIONS 98

Flight Laboratory’s Generic Nanosatellite Bus design and equipped with an optical telescope instrument of different filter bandwidth to target either the blue or red spectrum of stars. The objective of the BRITE Constellation mission is to provide photometric time series measurements on stars with apparent visual magnitude of ≤ +3.5 or brighter. The collected data will be used to perform asteroseismology analysis and provide scientists with information and clues to validate existing theories, or even to develop new models that will enhance our understanding of the internal composition, dynamics and life cycle of bright stars. The target stars of the BRITE mission are the most influential stars in the creation and distribution of heavy elements into the interstellar medium by methods of either stellar winds or supernova explosions. These heavy elements are the fundamental building blocks for all raw material and life forms found on Earth. As a result, the BRITE science consortium expects that this mission will help answer the few remaining questions regarding the life cycle of these giant stars and with luck, even reveal clues to the grand mystery that is the origin of life. This thesis has provided an overview of past missions related to space optical telescopes to contrast the size and weight of BRITE spacecraft against traditional large spacecrafts such as HIPPARCOS, Hubble, MOST, Swift and CoRoT. The importance and uniqueness of the BRITE mission will be to demonstrate to the world that science on a grand scale does not need to come in a large package. The paradigm shift that enables the microsatellite and nanosatellite technology lies in breaking the mold of traditional mentality in using large, expensive space grade hardwares and embrace the use of modern, commercially off-the-shelf (COTS) components for space missions. The risk of using non- space heritage components is instead compensated through better design and extensive testing of the design under harsh space-like environmental conditions. The author is in charge of the various tests that need to be performed in order to qualify the BRITE instrument design. Thus the objective of this thesis is to present the goals/requirements, methodologies and results for the various tests conducted on the instrument. Specifically, the author’s contributions to the BRITE mission include design of a long form functional test procedure for the instrument detector and driver electronics and execution of thermal vacuum test to qualify instrument electronics design under environmental extremes. In this process, the instrument is subjected to high vacuum (2.5 × 10−5 Torr) to verify its ability to withstand temperatures at 75◦C and −30◦C in a power-off state. Full aspects of the instrument’s proper functionalities are further tested by conducting the LFFT at each temperature extreme of 60◦C and −20◦C in a power-on state. This test ensures the instrument is able to survive and operate in the temperature range it expects to see in orbit. The execution of the scientific acceptance test provides characterization of the CCD detector performance at −20◦C, 0◦C, 10◦C, 20◦C, ambient (approximately 25 ± 2◦C), 30◦C and 60◦C. Specific topics covered in this section are methods used to characterize the BRITE instrument CCD in terms of its bias level and stability, gain factor determination, saturation, dark current and readout noise level. CHAPTER 8. FUTURE WORK,SUMMARY AND CONCLUSIONS 99

The main objective of these tests is to ensure the instrument’s performance matches the specifications re- quested by the science team. Results of the scientific acceptance test has been presented for UniBRITE’s instrument. In the bias level and stability test, the instrument has been verified to produce bias levels within 100 ± 40ADU. The bias pattern has been verified to be stable within ±4ADU (averaged over 1000 pixels, at CCD operating temperatures up to 30◦C). The average bias value of the frame do not drop below 40ADU and the 4σ value is above 0ADU. These verifications ensure the bias level setting is sufficiently high to capture the entire range of noise fluctuation. The noise signature is then further verified to exhibit a highly Gaussian distribution. In the gain factor determination, the author introduced a fast gain measurement methodology through the co-subtraction method. The gain of the instrument has been verified to be within 3.3 to 3.5[e−/ADU] and the full well saturation level has been determined to be above 10, 000 ADU at all temperatures tested. In terms of dark current, the CCD’s performance has been determined to satisfy all the requirements posed by the science team. Most importantly, the readout noise has also been verified to be well under the required 30e− level. The various characterization processes presented herein are performed purely based on image data. As a result the methodologies are not limited to just CCDs and they provide the basis for anyone who wishes to characterize any type of imager for scientific applications. One of the major major tasks performed by the author is the assembly and integration of the CCD imager with the instrument optical telescope. An artificial star field has been setup inside the SFL clean room to allow the instrument focusing task to take place inside a sanitized and controlled environment that is independent of weather, and yet still able to provide scenery comparable to real stars. Fine focus adjustments have been made iteratively for the UniBRITE, BRITE-Austria and the prototype science instruments until each achieved a star point spread function that was deemed acceptable by the BRITE science team to mitigate the problem of under-sampling. Finally, the author has prepared the mobile instrument assembly unit and conducted a preliminary field test attempt on the capability of the instrument and proved it is more than capable of registering stars much fainter than the level specified by requirement (greater than +3.5 magnitude). Moreover, a close examination of the point spread function observed on real stars confirmed the validity of the focusing process conducted with the artificial star field. In conclusion, the results of all the tests carried out by the author indicate the BRITE instrument will be able to perform astronomy measurements on stars of +3.5 magnitude or brighter to provide useful data to the scientific community. Furthermore, the high level programmatic test procedure and methodology presented in this thesis will serve as useful references for anyone who wishes to plan future missions involving space telescopes on a nanosatellite scale. Judging from the past success of its predecessor, the MOST satellite, there is no doubt that the constellation of BRITE nanosatellites will be able to provide a vast number of opportunities for new science discoveries for many years to come. Bibliography

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