JWST's Near-Infrared Detectors: Ultra-Low Background Operation and Testing

1. JWST's Near-Infrared Detectors:Ultra-Low Background Operation and Testing Bernard J. RauscherSpace Telescope Science Institute Extracted fromLASP Seminar at GSFC

2. Outline • What is a Near-Infrared Array Detector? • JWST Science Drivers • Detector Requirements • Detector testing at STScI/JHU • Optimal Use • Summary Extracted fromLASP Seminar at GSFC

3. JWST’s IR Arrays are “Hybrid” Sensors • PN junctions are “bump bonded” to a silicon readout multiplexer (MUX). • Silicon technology is more advanced than other semiconductor electronics technology. • The “bump bonds” are made of indium. Extracted fromLASP Seminar at GSFC

4. 1.E+02 Sunshield 1.E+01 JWST requirement 1.E+00 Signal [e-/sec/pix] JWST goal 1.E-01 R=5 1.E-02 Zodiacal Light 1.E-03 R=1000 1.E-04 0.1 1 10 Wavelength [mm] JWST Needs Very Good Near Infrared Detectors! • Completing the JWST Design Reference Mission “on time” requires background limited near-infrared (NIR) broadband imaging • Zodiacal light is the dominant background component in the NIR • The total NIR detector noise requirement is therefore =10 e- rms in a t=1000 seconds exposure. • NIRSpec will probably be detector noise limited. The total noise goal is =3 e- rms per 1000 seconds exposure Extracted fromLASP Seminar at GSFC

5. JWST Near Infrared (NIR) Detector Requirements Extracted fromLASP Seminar at GSFC

6. Detector Testing at STScI/JHU:Independent Detector Testing Laboratory Extracted fromLASP Seminar at GSFC

7. Tom Reeves Lab Technician Robert Barkhouser Optical Engineer Eddie Bergeron Data Analyst Past and present personnel Bernie Rauscher Project Scientist Utkarsh Sharma Graduate Student Mike Telewicz Intern Steve McCandliss JHU Lead Ernie Morse Data Analyst Gretchen Greene Mechanical Engineer Scott Fels Intern Don Figer Director Monica Rivera Intern Sito Balleza Systems Engineer Mike Regan System Scientist Russ Pelton Technician Extracted fromLASP Seminar at GSFC

8. Dark Current • Lowest measured dark current is ~0.006 e-/s/pixel. Extracted fromLASP Seminar at GSFC

9. IDTL Measurements: Read Noise • Read noise is <10 e- for Fowler-8. (system read noise is ~2.5 e-) Extracted fromLASP Seminar at GSFC

10. IDTL Measurements: Conversion Gain Per correlateddouble sample Extracted fromLASP Seminar at GSFC

11. Hawaii Shirt IDTL Test System Hawaii Detector Extracted fromLASP Seminar at GSFC

12. Then & Now November 2000 November 2002 Extracted fromLASP Seminar at GSFC

13. Raytheon ALADDIN Rockwell HAWAII-1R Jan. ‘01 (MUX) Feb. ‘02 (MUX) Apr. ‘02 (SCA) Raytheon SB-304 Rockwell HAWAII-2RG Nov. ‘02 (MUX) Jan. ‘03 (MUX) IDTL First Light Images Rockwell HAWAII-1RG Jun. ‘02 (MUX) Jul. ‘02 (SCA) Extracted fromLASP Seminar at GSFC

14. IDTL Test System Leach II Controller Electronics Dewar Entrance Window Vacuum Hose He Lines Extracted fromLASP Seminar at GSFC

15. Detector Readout System T=30-50 K Unix Instrument Control Computer Warm Harness COTS Leach II IR Array Controller T~293 K Cryogenic Harness JWST SCA Detector Customization Circuit Extracted fromLASP Seminar at GSFC

16. Rockwell HAWAII-2RG Detector Customization Circuit (DCC) An Adaptable Readout System • The only hardware change required to run a different detector is swap-in a DCC. • We have DCCs for the following detectors. • Raytheon • SB-290 • SB-304 • Rockwell • HAWAII-1R • HAWAII-1RG • HAWAII-2RG • Each DCC is a multi-layer PCB. Extensive use of surface mount technology. Includes flexible “neck” to simplify interfacing. Extracted fromLASP Seminar at GSFC

17. Raytheon SB-290/SB-304 Rockwell HAWAII-2RG Close-up ofDetector Customization Circuits (DCCs) Extracted fromLASP Seminar at GSFC

18. Optimal Use • JWST Detector Readout Strategies • Anomalies seen in other instruments • Other effects… • Use of Reference Pixels Extracted fromLASP Seminar at GSFC

19. MULTIACCUM Detector Readout MULTIACCUM parameters: texpose = exposure time, t1 = frame time, and t2 = group time. The small overhead associated with finishing the last group of samples is not included in the exposure time. Detector Readout • JWST science requires MULTIACCUM and SUBARRAY readout. • Other readout “modes” can be implemented using parameters. • For example, Fowler-8 can be implemented as MULTIACCUM-2x8. • Cosmic rays may be rejected either on the ground or on-orbit. Extracted fromLASP Seminar at GSFC

20. NICMOS Anomalies (& how JWST will avoid them) • Dark current • JWST detectors already designed to minimize glow • Careful detector characterization & selection • Do not exceed max temp. requirement! • Bias drifts • Good electronic design • Avoid power supply coupling • Avoid ground coupling • Reference pixels will help • Synchronous readout can help • QE variations • Careful detector characterization & selection • Amplifier glow • JWST detectors should be much better than NICMOS Extracted fromLASP Seminar at GSFC

21. NICMOS Anomalies: 2 • Persistence • There will be persistence on JWST • Strongly dependent on detector fabrication process • Careful detector characterization & selection needed to choose best detectors • In IDTL, we are exploring mitigation measures Extracted fromLASP Seminar at GSFC

22. NICMOS Anomalies: 3 • DC bias level drift • Good electronic design is first line of defense • Reference pixels should eliminated “Pedestal” drifts. • Depending on reference pixel layout, reference pixels may help reject “bands”. • Ghosts • In NICMOS, may result from ground plane coupling within the MUX. • Also seen in SIRTF InSb radiation testing. • Good cable harness and electronic design help Extracted fromLASP Seminar at GSFC

23. NICMOS Detector Effects • Linearity • In NICMOS, ~10% intrinsic non-linearity can be calibrated out to within ~0.2%. • Well depth • Well-depth is a function of reverse bias in photo-voltaic detectors. • Well-depth can also depend on temperature. • In the IDTL, we will study well depth as a function of reverse bias and temperature. Extracted fromLASP Seminar at GSFC

24. NICMOS Detector Effects: 2 • QE • Can depend on wavelength and temperature. • Dark current “bump” • This is a curious effect seen in NICMOS. Extracted fromLASP Seminar at GSFC

25. Raytheon 1024x1024 MIR MUX Raytheon 2Kx2K NIR Module Rockwell 2Kx2K NIR Module Reference Pixels • All candidate JWST detectors have reference pixels • Reference pixels are insensitive to light • In all other ways, designed to mimic a regular light-sensitive pixel • NIR detector testing at University of Rochester, University of Hawaii, and in the IDTL at STScI -> reference pixels work! • Reference pixel subtraction is a standard part of IDTL data reduction pipeline Extracted fromLASP Seminar at GSFC

26. Use of Reference Pixels • JWST’s NIR reference pixels will be grouped in columns and possibly rows • Most fundamentally • reference pixels should be read out in exactly the same manner as any “normal” pixel • Data from many reference pixels should be averaged to avoid adding noise to data • We have begun to explore how reference pixels should be used. Approaches considered include the following. • Maximal averaging (average all reference pixels together and subtract the mean) • Spatial averaging • Temporal averaging • Spatial averaging is now a standard part of IDTL calibration pipeline Extracted fromLASP Seminar at GSFC

27. A Picture of IDTL System Noise • Shorting resistor mounted at SCA location • 1/f “tail” causes horizontal banding. • Total noise is =7 e- rms per correlated double sample. Extracted fromLASP Seminar at GSFC

28. After Before Averaging small numbersof reference pixels adds noise • Averaged the last 4 columns in each row and performed row-by-row subtraction Extracted fromLASP Seminar at GSFC

29. This is a standardpart of the IDTL datacalibration pipeline Spatial Averaging • In spatial averaging, data from many (~64 rows) of reference pixels are used to calibrate each row in the image • A Savitzky-Golay smoothing filter is used to fit a smooth and continuous reference column • This reference column is subtracted from each column in the image • Using this technique, we can remove some 1/f noise power within individual frames • In practice, this technique works very well Extracted fromLASP Seminar at GSFC

30. Spatial Averaging: Before & After Before After Extracted fromLASP Seminar at GSFC

31. Rockwell HAWAII-1RG Double Correlated Sampling image. Read noise is ~15 e- rms (=5.3 e- using Fowler-8 sampling). Fit to reference columns using Savitzky-Golay filtering to smooth averaged reference pixel data in each row.. Spatial Averaging:Example using Rockwell HAWAII-1RG Detector Extracted fromLASP Seminar at GSFC

32. Temporal Averaging • Dwell on the reference pixel and sample many times before clocking next pixel • Potentially removes most 1/f • Not tried this in IDTL yet. U. Hawaii has reported some problems with reference pixels heating up Extracted fromLASP Seminar at GSFC

33. Temporal Averaging: Before & After Before After Extracted fromLASP Seminar at GSFC

34. Summary of Reference Pixel Calibration Methods • Spatial averaging works well using a Rockwell HAWAII-1RG detector • Based on conversations with U. Rochester, we foresee no problems with SB-304 • Temporal Averaging is promising. More work needed using real detectors. Extracted fromLASP Seminar at GSFC

35. Summary • The Independent Detector Testing Laboratory (IDTL) at STScI/JHU is up and running • Test results including dark current, read noise, conversion gain, relative quantum efficiency, and persistence are in good agreement with other JWST test labs • Reference pixels work and are an invaluable part of the data calibration pipeline • We have explored three techniques for using reference pixels • Maximal averaging, • Spatial averaging, & • Temporal averaging • Spatial averaging works well and is robust • Early reports from U. Hawaii using temporal averaging are not encouraging due to reference pixel self-heating. More work is planned in the IDTL Extracted fromLASP Seminar at GSFC