MASTER'S THESIS Design and Testing of the Star Trackers for the SEAM Nanosatellite Nikola Shterev 2015 Master of Science (120 credits) Spacecraft Design Luleå University of Technology Department of Computer Science, Electrical and Space Engineering Design and Testing of the Star Trackers for the SEAM Nanosatellite Nikola Shterev Stockholm 2014 Department of Computer Science, Electrical and Space Engineering Space Technology Division Abstract Star trackers are instruments for attitude determination and are a must for spacecrafts where precise attitude knowledge is needed. Star trackers take accurate images of the stars and then perform image processing and pattern recognition in order to accurately determine the attitude. This requires high processing power, size and mass of the instrument, which are limited on cubesats. In this thesis work the sensor of a star tracker has been characterized, and calibration and image correction procedures that require low processing capabilities have been developed. The thesis work also includes the design of the star tracker in CAD software, with special attention being payed to the baffle due to the limited size available and the importance of stray radiation suppression. Stray radiation suppression is especially important due to limited ability for cor- rections on image level. Furthermore, a method for processing the used star catalog has been developed so the stellar brightness in the catalog corresponds to the brightness detected by the star tracker. List of Figures 1.1 SEAM solid model (boom stowed). Taken from [1] . .2 3.1 Master bias frame . 10 3.2 Columns of the master bias frame with their elements averaged . 11 3.3 Thermal frame . 12 3.4 Histogram of the hot spots . 13 3.5 Histogram when hot pixels are removed . 14 3.6 Flat frame taken by pointing at a monitor . 15 3.7 Middle columns and row radial values and their curve fit . 16 3.8 Middle columns and row radial values and their curve fit for the lens assembly to beused ......................................... 16 4.1 Johnson's visual passband and visual flux density of a G5V star with magnitude 1 18 4.2 Flux density of a G5 star with magnitude 2 . 19 4.3 Flux density and the corresponding number of photons of G5 star with magnitude 2 20 4.4 Quantum efficiency of the sensor . 21 4.5 Photons and generated photoelectrons . 21 4.6 Read intensities vs predicted photoelctron count . 22 5.1 Histogram of RMSE of each pixel for 49 bias frames . 25 5.2 Box in which the sensor was mounted . 28 5.3 Box and light source used in the setup . 29 5.4 Mean value of the gain 1 image set for different exposure time . 30 5.5 PTC determined by both methods and linear fit to the data from the first method 30 5.6 σ for different mean signal values per pixel. The plot is for analog gain of 1 . 31 5.7 SNR for different mean signal values per pixel. The plot is for analog gain of 1 . 31 6.1 Placing vanes in a baffle . 35 6.2 Derivation of the equations for determining vane position and height . 36 6.3 Baffle vanes designed with tolerance margins (n=2 on the drawing) . 38 6.4 Bevel edge on the object side (a) and on the objective side (b) . 39 6.5 Rectangle and entrance pupil circle used to find the area viewed by the system at certain distance . 40 6.6 The star tracker viewed from the front, showing the area viewed by the system at the distance of the vanes . 40 6.7 Design of the star tracker . 41 6.8 Design of the star tracker. Viewed from the rear . 42 6.9 V shape of the lens and mount support . 42 i Contents 1 Background 1 1.1 SEAM . .1 1.2 Star Trackers . .2 1.2.1 First-Generation Star Trackers . .2 1.2.2 Second-Generation Star Trackers . .3 1.2.3 Modes of Operation . .3 1.2.4 Field of View, Resolution, Update Rate . .3 1.2.5 Typical Star Tracker and the One for SEAM . .4 1.3 Stars, stellar spectra and stellar classes . .4 2 Imaging 6 2.1 Different Signals and Noise . .6 2.2 Calibration . .7 3 Calibration 9 3.1 Initial Calibration . .9 3.1.1 Bias Frames . .9 3.1.2 Dark Frames . 10 3.1.3 Flat Frames . 12 3.2 Final Calibration . 14 4 Estimating the Received Photoelectrons 17 4.1 Estimating the Intensity to Be Read by the Sensor . 17 4.2 Comparison of Predicted and Read Intensities . 20 5 Sensor Characterization 23 5.1 Determining the Threshold . 23 5.2 Noise . 24 5.3 Photon Transfer Curve . 25 5.4 Constructing the PTC and Results . 27 5.4.1 Test Setup . 27 5.4.2 Creation of the PTC . 28 6 Optomechanical Design 32 6.1 Definitions . 32 6.2 Baffle . 33 6.2.1 Critical Surfaces . 33 6.2.2 Power Transferred . 33 ii 6.2.3 Vane Placement . 34 6.2.4 Vane Edges . 37 6.3 Body Star Tracker . 38 6.3.1 Star Tracker Configuration . 40 7 Conclusion 43 7.1 Work Done, Conclusions and Recommendations for Future Work . 43 7.2 Environmental, Social and Ethical Aspects . 44 iii Abbreviations AC Alternating Current ADU Analog-to-Digital Units AP S Active Pixel Sensor BF L Back Focal Length CAD Computer-Aided Design CCD Charge-Coupled Device CMOS Complementary Metal-Oxide Semiconductor DC Direct Current EF L Effective Focal Length ELF Extremely Low Frequency ET Edge Thickness F P GA Field Programmable Gate Array FPS Frames per Second F OV Field of View KTH Kungliga Tekniska Hgskolan - Royal Institute of Technology LED Light Emitting Diode OD Outer Diameter PCB Printed Circuit Board PTC Photon Transfer Curve QE Quantum Efficiency SEAM Small Explorer for Advanced Missions SNR Signal to Noise Ratio S=C Spacecraft V LF Very Low Frequency iv Chapter 1 Background The background includes short description of the SEAM (Small Explorer for Advanced Mis- sions) nanosatellite, description of star trackers, stars and spectral classes, and the theoretical knowledge needed for understanding most of the work described in this document. 1.1 SEAM SEAM is a project that aims to design, build and operate a nanosatellite for observation of the magnetic field around Earth and to improve the understanding of the geospace environment. The project is part of the 7th Framework Programme funded by the European Union, involves a consortium of 8 partners and is coordinated by the Royal Institute of Technology (KTH) in Sweden. Solid model of the SEAM satellite can be seen in figure 1.1. The SEAM satellite is to contribute to the research in three areas: the auroral current system, natural ELF/VLF waves in the ionosphere, and man-made ELF/VLF emissions. This is accomplished by performing high-quality measurements of the three-axis DC magnetic field, the three-axis AC magnetic field and a single component of the AC electric field. These high-quality measurements require attitude knowledge of 1 arcminute which neces- sitates the use of star tracker. The attitude sensors to be used on the SEAM satellite are sun sensors located on the exterior solar panels, a 3-axis Honeywell HMC5843 magnetometer, a gyro- scope and two star trackers. Attitude determination will be performed by an unscented Kalman filter which uses the information from all sensors. [1] One of the star trackers will be placed on the tip of a boom carrying a fluxgate sensor and the other one will be in the main body. These star trackers and the preliminary work on them are the focus of this thesis. The star trackers are being developed by KTH and are based on 5-mexapixel CMOS monochrome image sensor operated at multiple frames per second (FPS). Both star trackers are functionally the same but the on-board one will have larger optics aperture and have all of its electronics integrated in a single unit, while the boom-mounted one will only have its optics and pre-processing hardware on the boom and the rest of the electronics in the satellite. The optics have to be mounted on the boom in order to accurately determine the pointing of the boom and the rest of the electronics are kept inside the satellite to reduce the magnetic disturbance caused by them to the magnetometer mounted on the boom. The boom- mounted star tracker will have smaller optical aperture so it will require longer exposure times and will thus have smaller FPS rate. 1 Figure 1.1: SEAM solid model (boom stowed). Taken from [1] 1.2 Star Trackers The most accurate instruments for attitude determination are the star trackers. Star trackers take images of the sky and compare them to star catalogs. The position of the stars in the catalog is well known and the angles of ration needed to match the observed stars with those in the catalog give information about the attitude of the spacecraft. Star trackers are most suitable for three-axis stabilized spacecrafts. A star tracker is basically a digital camera where the sensor at the focal plane is either a CCD (charge-coupled device) or CMOS (complementary metal-oxide semiconductor) one. CCD sensors usually have lower noise but CMOS sensors are more resistant to radiation and can read different pixels at different rates. Sensors with pixels that have data processing capabilities are called active pixel sensors (APS). A star image usually covers several pixels and the location of the centroid of a star image is the "center of mass" of the photoelectrons collected by each pixels in an n by n block.