ABSOLUTE RADIOMETRIC CALIBRATION OF AN ON-ORBIT INFRARED SENSOR USING STARS Thesis Submitted to The School of Engineering of the UNIVERSITY OF DAYTON In Partial Fulfillment of the Requirements for The Degree Master of Science in Electro-Optics by Mark Donofrio UNIVERSITY OF DAYTON Dayton, Ohio December 1996 UNIVERSITY OF DAYTON ROESCH LIBRARY ABSOLUTE RADIOMETRIC CALIBRATION OF AN ON-ORBIT INFRARED SENSOR USING STARS APPROVED BY ABSTRACT ABSOLUTE RADIOMETRIC CALIBRATION OF AN ON-ORBIT INFRARED SENSOR USING STARS Name: Donofrio, Mark University of Dayton, 1996 Advisor: Dr. G. R. Little This thesis addresses the absolute radiometric calibration of an infrared sensor that is already on orbit. Stars located in the Southern sky are approximated as blackbody sources in the near infrared portion of the electromagnetic spectrum (i.e. 2.0-3.5pm) and used to measure the absolute response of each pixel on a 16 x 16 Mercury-Cadmium-Telluride (HgCdTe) focal plane array. Since the sensor is equipped with a six-position filter wheel (five spectral bands and one opaque position), the pixels are also spectrally dependent. Stars of different spectral classes (i.e. temperature) and magnitudes (i.e. radiant flux) are used to create a broad selection of data. In turn the data is used to validate the conversion constants that relate the sensor count value to a 2 radiant flux (W/cm /pm) for each of the four primary bands. Predictions are based on the Engelke normalized blackbody function and converted to sensor counting units for direct comparison with actual measurements [1, 2], A sum-of- nine pixel centroid is calculated for each frame of measured data and an attempt ii is made to assess the accuracy and consistency of the absolute response correction factors for all pixels in the four spectral bands. Conclusions on the current uniformity of the focal plane array, accuracy of the voltage to flux conversion constants, and recommendations for future radiometric calibrations of similar on-orbit sensors are included as well. ACKNOWLEDGMENTS I would like to thank several key individuals who made this thesis work possible. In particular, Dr. Gordon Little, my advisor, for providing the necessary oversight of my work and taking the extra time to learn about the sensor calibrated.. In addition my sincerest thanks to Mr. James Lisowski for his endless enthusiasm and dedication to this project. I would also like to express my appreciation to several other people who contributed to this effort. Mr. Brett Borowski provided the computer programming expertise for the data processing software. Mr. Robert Cody offered guidance with the sensor performance and anomalies. Mr. James Engel advised on the technical aspects of the sensor. Mr. Wilbur Dong provided personal contacts and unpublished reports for star radiant flux density predictions. Mr. William Parker tasked the sensor for each collection and resolved any scheduling conflicts. Finally, the time required to complete this thesis would not have been possible without the support and patience of both my wife, Andrea, and my daughter, Megan. This report is first and foremost dedicated to them. iv PREFACE Space based sensors are normally calibrated in a laboratory environment prior to deployment to orbit. In the case of this sensor, aggressive launch schedules and other priority items severely restricted the amount of absolute calibration performed on the ground. Coupled with the detection of new and unique targets over the past few years, the need arose for an absolutely calibrated focal plane. This thesis is an attempt to satisfy that need and identify improvements for future calibrations. v TABLE OF CONTENTS ABSTRACT..............................................................................................................ii ACKNOWLEDGMENTS........................................................................................iv PREFACE................................................................................................................v LIST OF FIGURES.............................................................................................. viii LIST OF TABLES.............................................................................................. ix LIST OF SYMBOLS/ABBREVIATIONS..................................................................x CHAPTER I. INTRODUCTION.......................................................................................1 II. BACKGROUND.........................................................................................7 III. EXPERIMENT.........................................................................................20 IV. OBSERVATIONS AND DATA ANALYSIS..............................................27 V. CONCLUSIONS......................................................................................33 APPENDICIES A. Sensor Description..................................................................................37 B. Blackbody Approximation for Stars........................................................ 39 C. Q Calculation for Lambda Velorum........................................................ 41 D. Collection Files Used for Calibration...................................................... 42 E. Processing Program Source Code......................................................... 44 F. Focal Plane Uniformity and Pixel Gap Corrections................................56 vi G. Star Radiance Predictions (LDs)............................................................59 H. Star Measurements (LDs)...................................................................... 76 I. Measured versus Predicted Comparisons...............................................92 BIBLIOGRAPHY.................................................................................................107 vii LIST OF FIGURES 1. Schematic drawing of the sensor signal processing......................................17 2. Location of active and inactive pixels on sensor focal plane array.............. 17 3. Notional scan pattern for star measurements...............................................21 4. Notional scan pattern for four pixels.............................................................21 5. Plot of raw sensor data................................................................................. 23 6. Plot of preprocessed sensor data.................................................................24 7. Geometry for centroid calculation.................................................................25 viii LIST OF TABLES 1. Initial stars for sensor calibration................................................................... 3 2. Zero magnitude spectral flux density for standard astronomical bands........ 9 3. Published solid angle and effective temperature constants for Engelke function calculation.......................................................................................12 4. Predicted flux values calculated by correlating Equation (5) with the published sensor responsivity curves.......................................................... 14 5. Predicted sensor count values calculated from Table 4 flux values and Equation (7)..................................................................................................16 ix LIST OF SYMBOLS/ABBREVIATIONS CCD Charge Coupled Device epro End Product (Data File) HgCdTe Mercury-Cadmium-Telluride IR Infrared LD Level of Detection SWIR Short Wave Infrared X Wavelength (in pm) Q Solid angle subtended by star E Radiant flux (of star) Engelke Engelke function-derived radiant flux Eo Radiant flux of star at X=0.55pm I Irradiance (of star) K Kelvin LxI b Blackbody spectral radiance R Absolute responsivity of sensor Sx Visible response function S(X) Sensor filter response function Tb Brightness temperature Te Effective (published) temperature x CHAPTER I INTRODUCTION The sensor addressed to in this thesis requires on-orbit radiometric calibration to correctly identify the one-of-a-kind targets it detects. A more detailed description of the sensor is included in Appendix A. Most often these targets are sensed against a “warm” earth background and extensive post­ processing is required to extract low intensity targets from the existing background clutter level. If the absolute response for each pixel of the sensor’s focal plane array is successfully determined for each spectral band, then a larger portion of the clutter level could be subtracted from the remaining signal during post-processing to obtain a more accurate photon count for a given target. Because the sensor has already been deployed to orbit, normal laboratory techniques for calibration cannot be followed. For example, calibrated laboratory blackbody sources that are set to radiate at known temperatures could not be used to create a response on the sensor’s focal plane array. On- orbit calibrations require the use of either a high powered laser or some other existing “hot” source. Stars were chosen as the radiation source because they are readily accessible and can be used repeatedly for re-calibration of the sensor with minimal effort. Stars are desirable because they are point sources, radiate similarly to a blackbody, and can be measured against a very cold, low clutter background. Appendix B addresses reasons for treating stars as 1 blackbody sources for this particular sensor. The blackbody approximation for stars is especially true for those that do not show emission lines associated with Carbon and other elements. Additionally, the point source-like