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

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

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 2003 Special LASP Seminar at GSFC

JWST Needs Very Good Near Infrared Detectors! 0.1 1 10 Wavelength [mm] Signal [e-/sec/pix] Zodiacal Light Sunshield JWST requirement JWST goal R=5 R=1000 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 2003 Special LASP Seminar at GSFC

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

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

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

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

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

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

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

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 2003 Special LASP Seminar at GSFC

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

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

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

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 2003 Special LASP Seminar at GSFC

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

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 14 January 2003 Special LASP Seminar at GSFC

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 2003 Special LASP Seminar at GSFC

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

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

Special LASP Seminar at GSFC 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. MULTIACCUM 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. 14 January 2003 Special LASP Seminar at GSFC

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 Amplifier glow JWST detectors should be much better than NICMOS 14 January 2003 Special LASP Seminar at GSFC

Special LASP Seminar at GSFC 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 2003 Special LASP Seminar at GSFC

Special LASP Seminar at GSFC 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 2003 Special LASP Seminar at GSFC

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 2003 Special LASP Seminar at GSFC

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

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

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 2003 Special LASP Seminar at GSFC

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 2003 Special LASP Seminar at GSFC

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 2003 Special LASP Seminar at GSFC

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

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

Special LASP Seminar at GSFC 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 2003 Special LASP Seminar at GSFC

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

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 2003 Special LASP Seminar at GSFC

Special LASP Seminar at GSFC 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 14 January 2003 Special LASP Seminar at GSFC