Improved Space Object Observation Techniques using CMOS Detectors

1. Improved Space Object Observation Techniques using CMOS Detectors T. Schildknecht, A. Hinze, J. Silha Astronomical Institute, University of Bern, Switzerland J. Peltonen, T. Säntti Aboa Space Research Oy (ASRO), Turku, Finland T. Flohrer Space Debris Office, ESA/ESOC, Germany

2. Outline • Optical Space Object Observation Strategies requirement for new detector • CMOS Imaging Sensors potential benefits • Characterization of sCMOS Camera • Conclusion

3. Ground-Based Surveys

4. Space-Based Surveys • LEO sensor observing GEO/MEO/HEO • similar to ground-based GEO/MEO/HEO • Short-range observations (small-size debris surveys) • LEO to LEO, GEO to GEO, etc • similar to ground-based LEO • Specific requirements • mechanical shutters not advisable • space-proof detector (cosmic ray background!) • on-board processing desirable

5. Generic Detector Requirements • Detection, astrometry, photometry • high quantum efficiency • Low read-out noise • low dark current • stable flat field (i.e. stable gain for each pixel) • stable bias or on-chip bias reduction • limited number of dark/hot pixels (“cosmetics”) • no charge leakage from pixel to pixel • limited enlargement of PSF in detector • high full-well capacity

6. Requirements for New Detector • Electronic shutter • required for space-based sensor • required for precise epoch registration (surveys LEO) • increased reliability for ground-based sensors • Faster read-out (large sensors!) • improved duty cycle •  larger survey area are per time • more observations per tracklet (FoV crossing) •  improved orbit accuracy •  improved tracklet correlation • Extremely short exposures 1s • required for ground-based LEO, space-based short range • non-destructive readout to “subdivide” streaks • On-chip processing • spatial filtering • image segmentation

7. Silicon Detector Technologies • Charge Coupled Devices (CCDs) • CMOS sensors or Active Pixel sensors • Hybrid Visible Sensors combining silicon photodiode detection with separate CMOS electronics

8. frame transfer interline transfer CCD Detectors • Basic structure/operation principle • array of photodiodes • sequential readout (charge transfer) • one (or few) readout node(s) • no electronic shutter • Alternative architectures • “electronic shutter” function

9. CMOS or Active Pixel Sensor • Basic structure /operation principle • array of photodiodes • each pixel has own amplifier (and storage area) • multiplexed readout

10. Hybrid Visible CMOS Imagers • Combination of matrix of photodiodes with matrix of CMOS multiplexers/amplifiers

11. On-chip processing in CMOS In CMOS processing in a pixel-parallel fashion is possible Back-illuminated circuits are needed in astronomy. More complex structures can be integrated on the front surface without too much reduction in the photoactive area • ROI (region of interest) detection: Background subtraction, filtering and simple (e.g. 1-bit) segmentation may be possible if a local pixel storage for reference values can be established. • Paralleled, application specific image pipelines can be integrated on the same chip outside the active area

12. Main Advantages/Disadvantages

13. Microlenses: Cross-Talk • Problem if numerical aperture of optical system < numerical aperture of microlenses

14. sCMOS Camera Tests • Andor NEO sCMOS (CIS2051, former Fairchild Imaging) • 11 bit intrinsic • 16 "dual-gain" • front-side • QEmax 59 % • microlenses

15. sCMOS: Readout Noise • 2 single pixels (average of 1000 bias frames)

16. sCMOS: Readout Noise • Noise distribution of (512x512 pixels, best case) manufacturer spec. CCD noise distribution 10x10 pixel area

17. Non-Linearity • High gain • < 4% (spec. <1%) 10 bit 11 bit

18. Non-Linearity • Low gain • < 8% (spec. <1%) 11 bit 11 bit

19. Dual Gain

20. Non-Linearity • Dual gain error in gate array? 14 bit 16 bit

21. 1/8 * 3.5! Flat Field Pixel Variance • 1000 flat fields, distrib. of single pixel variances (512x512 pixel area)

22. Flat Field Pixel Variance • 1000 flat fields, single pixel variances • ~9% interpolated pixels (average of 9 pixels)!

23. Cross-Talk / MTF • 2-d autocorrelation of difference of 2 flat fields no explanation!.

24. Conclusions • Major challenges and design drivers for ground-based and space-based optical observation strategies are • detection of faintest objects • precise epoch registration  electronic shutter • short exposures 1s (LEO, space-based) • high readout rate 1s for full frame (LEO, space-based) • on-chip processing (space based) • CMOS Active Pixel Sensors • offer most of the required capabilities • but have still disadvantages wrt. CCDs • low quantum efficiency (no backside illuminated devices) • noise characteristics • high Pixel Response Non-Uniformity (PRNU) • low dynamic range • high percentage of dark/hot pixles

25. Conclusions • Andor NEO sCMOS (CIS2051) camera has been characterized by means of laboratory tests • noise characteristics • linearity, dynamic range • cross-Talk / MTF • Scientific CMOS devices are rapidly evolving and some disadvantages may be overcome in near future