14 January 2003Special LASP Seminar at GSFC1 JWST's Near-Infrared Detectors: Ultra-Low Background Operation and Testing Bernard J. Rauscher Space Telescope Science Institute

14 January 2003Special LASP Seminar at GSFC2 Outline What is a Near-Infrared Array Detector? JWST Science Drivers Detector Requirements Detector testing at STScI/JHU Optimal Use Summary

14 January 2003Special LASP Seminar at GSFC3 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.

14 January 2003Special LASP Seminar at GSFC4 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E Wavelength [  m] Signal [e-/sec/pix] Zodiacal Light Sunshield JWST requirement JWST goal R=5 R=1000 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

14 January 2003Special LASP Seminar at GSFC5 JWST Near Infrared (NIR) Detector Requirements

14 January 2003Special LASP Seminar at GSFC6 Detector Testing at STScI/JHU: Independent Detector Testing Laboratory

14 January 2003Special LASP Seminar at GSFC7 Past and present personnel Eddie Bergeron Data Analyst Mike Telewicz Intern Gretchen Greene Mechanical Engineer Monica Rivera Intern Russ Pelton Technician Tom Reeves Lab Technician Bernie Rauscher Project Scientist Steve McCandliss JHU Lead Scott Fels Intern Sito Balleza Systems Engineer Robert Barkhouser Optical Engineer Utkarsh Sharma Graduate Student Ernie Morse Data Analyst Don Figer Director Mike Regan System Scientist

14 January 2003Special LASP Seminar at GSFC8 NIR Detector Characteristics Dark current Read noise Linearity Latent charge (persistence) Relative and Absolute Quantum efficiency (QE) Intra-pixel sensitivity Thermal stability Radiation immunity

14 January 2003Special LASP Seminar at GSFC9 Dark Current Lowest measured dark current is ~0.006 e  /s/pixel.

14 January 2003Special LASP Seminar at GSFC10 Read noise is ~10 e  for Fowler-8. (system read noise is ~2.5 e  ) IDTL Measurements: Read Noise

14 January 2003Special LASP Seminar at GSFC11 IDTL Measurements: Conversion Gain Per correlated double sample

14 January 2003Special LASP Seminar at GSFC12 Hawaii 1R with 5 um Cutoff

14 January 2003Special LASP Seminar at GSFC13 IDTL Measurements: Relative and Absolute Quantum Efficiency Relative Quantum Efficiency for H1RG, 10/18/2002 We are currently working on better calibration to enable measurements of absolute QE vs. wavelength.

14 January 2003Special LASP Seminar at GSFC14 Relative QE Maps Relative QE maps show significant structure

14 January 2003Special LASP Seminar at GSFC15 IDTL Test System Hawaii Detector Hawaii Shirt

14 January 2003Special LASP Seminar at GSFC16 Then & Now November 2000 November 2002

14 January 2003Special LASP Seminar at GSFC17 IDTL First Light Images Jan. ‘01 (MUX) Raytheon ALADDIN Feb. ‘02 (MUX)Apr. ‘02 (SCA) Rockwell HAWAII-1R Rockwell HAWAII-1RG Jun. ‘02 (MUX)Jul. ‘02 (SCA) Raytheon SB-304 Nov. ‘02 (MUX) Rockwell HAWAII-2RG Jan. ‘03 (MUX)

14 January 2003Special LASP Seminar at GSFC18 IDTL Test System Leach II Controller Electronics Vacuum Hose He Lines Entrance Window Dewar

14 January 2003Special LASP Seminar at GSFC19 Detector Readout System Unix Instrument Control Computer COTS Leach II IR Array Controller Warm Harness Cryogenic Harness Detector Customization Circuit JWST SCA T~293 K T=30-50 K

14 January 2003Special LASP Seminar at GSFC20 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. Rockwell HAWAII-2RG Detector Customization Circuit (DCC)

14 January 2003Special LASP Seminar at GSFC21 Close-up of Detector Customization Circuits (DCCs) Rockwell HAWAII-2RG Raytheon SB-290/SB-304

14 January 2003Special LASP Seminar at GSFC22 Optimal Use JWST Detector Readout Strategies Anomalies seen in other instruments Other effects… Use of Reference Pixels

14 January 2003Special LASP Seminar at GSFC23 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. MULTIACCUM parameters: t expose = exposure time, t 1 = frame time, and t 2 = group time. The small overhead associated with finishing the last group of samples is not included in the exposure time. MULTIACCUM Detector Readout

14 January 2003Special LASP Seminar at GSFC24 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

14 January 2003Special LASP Seminar at GSFC25 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

14 January 2003Special LASP Seminar at GSFC26 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

14 January 2003Special LASP Seminar at GSFC27 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.

14 January 2003Special LASP Seminar at GSFC28 NICMOS Detector Effects: 2 QE –Can depend on wavelength and temperature. Dark current “bump” –This is a curious effect seen in NICMOS.

14 January 2003Special LASP Seminar at GSFC29 Reference Pixels Raytheon 2Kx2K NIR Module Rockwell 2Kx2K NIR Module 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 Raytheon 1024x1024 MIR MUX

14 January 2003Special LASP Seminar at GSFC30 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

14 January 2003Special LASP Seminar at GSFC31 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.

14 January 2003Special LASP Seminar at GSFC32 Averaging small numbers of reference pixels adds noise Averaged the last 4 columns in each row and performed row- by-row subtraction Before After

14 January 2003Special LASP Seminar at GSFC33 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 This is a standard part of the IDTL data calibration pipeline

14 January 2003Special LASP Seminar at GSFC34 Spatial Averaging: Before & After Before After

14 January 2003Special LASP Seminar at GSFC35 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

14 January 2003Special LASP Seminar at GSFC36 Temporal Averaging: Before & After Before After

14 January 2003Special LASP Seminar at GSFC37 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.

14 January 2003Special LASP Seminar at GSFC38 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