JWST Radiation Environment 1March 13, 2003 Reference pixels and readout modes: What we have learned thus far Don Figer, Bernie Rauscher, Mike Regan March 13, 2003

JWST Radiation Environment 2March 13, 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 Detectors Are Important for JWST

JWST Radiation Environment 3March 13, 2003 NIR Detector Characteristics lDark current lRead noise lLinearity lLatent charge (persistence) lQuantum efficiency (QE) lIntra-pixel sensitivity lThermal stability lRadiation immunity

JWST Radiation Environment 4March 13, 2003 IDTL Test System Controller Electronics Vacuum Hose He Lines Entrance Window Dewar

JWST Radiation Environment 5March 13, 2003 JWST MIR Detector Requirements

JWST Radiation Environment 6March 13, 2003 Dark Current Lowest measured dark current is ~0.005 e  /s/pixel.

JWST Radiation Environment 7March 13, 2003 Read noise is ~10 e  for Fowler-8. (system read noise is ~2.5 e  ) IDTL Measurements: Read Noise

JWST Radiation Environment 8March 13, 2003 Reference Pixels Raytheon 2Kx2K NIR Module Rockwell 2Kx2K NIR Module lAll candidate JWST detectors have reference pixels lReference pixels are insensitive to light lIn all other ways, designed to mimic a regular light-sensitive pixel lNIR detector testing at University of Rochester, University of Hawaii, and in the IDTL at STScI -> reference pixels work! lReference pixel subtraction is a standard part of IDTL data reduction pipeline Raytheon 1024x1024 MIR MUX

JWST Radiation Environment 9March 13, 2003 Use of Reference Pixels lJWST’s NIR reference pixels are grouped in columns and rows lMost 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 lWe have begun to explore how reference pixels should be used. Approaches considered include the following. –row-by-row subtraction –maximal averaging (average all reference pixels together and subtract the mean) –spatial averaging –temporal averaging lSpatial averaging is now a standard part of IDTL calibration pipeline

JWST Radiation Environment 10March 13, 2003 A Picture of IDTL System Noise lShorting resistor mounted at SCA location l1/f “tail” causes horizontal banding. lTotal noise is =7 e- rms per correlated double sample.

JWST Radiation Environment 11March 13, 2003 Averaging small numbers of reference pixels adds noise lAveraged the last 4 columns in each row and performed row-by-row subtraction Before After

JWST Radiation Environment 12March 13, 2003 Spatial Averaging lIn spatial averaging, data from many (~64 rows) of reference pixels are used to calibrate each row in the image lA Savitzky-Golay smoothing filter is used to fit a smooth and continuous reference column lThis reference column is subtracted from each column in the image lUsing this technique, we can remove some 1/f noise power within individual frames lIn practice, this technique works very well This is a standard part of the IDTL data calibration pipeline

JWST Radiation Environment 13March 13, 2003 Spatial Averaging: Before & After Before After

JWST Radiation Environment 14March 13, 2003 Temporal Averaging lDwell on the reference pixel and sample many times before clocking next pixel lPotentially removes most 1/f lNot tried this in IDTL yet. U. Hawaii has reported some problems with reference pixels heating up

JWST Radiation Environment 15March 13, 2003 Temporal Averaging: Before & After Before After

JWST Radiation Environment 16March 13, 2003 Summary of Reference Pixel Calibration Methods lSpatial averaging works well using a Rockwell HAWAII-1RG detector lBased on conversations with U. Rochester, we foresee no problems with SB-304 lTemporal Averaging is promising. More work needed using real detectors.

JWST Radiation Environment 17March 13, 2003 Summary lReference pixels work and are an invaluable part of the data calibration pipeline lWe have explored three techniques for using reference pixels –row-by-row subtractions, –maximal averaging, –spatial averaging, & –temporal averaging lAveraging at the end of row will not work lSpatial averaging works well and is robust lWe have found: –dark current is low (~0.01 e-/s/pixel) –glow is very small –noise goes down as roughly 1/root(N) up to 8 reads (at least) –persistence is observed –JWST requirements seem realizable –saving all the data are necessary to mitigate unforeseen detector effects, such as the non-linear bias drift after reset ("shading" in NICMOS). Note that ref pixels do not get rid of all of the effect. lCosmic ray rejection requires careful handling of reference pixels, output voltage drifts, and knowledge about previous history (persistence)

JWST Radiation Environment 18March 13, 2003 Appendix

JWST Radiation Environment 19March 13, 2003 NIR Detector Effects - NICMOS lDark current lBias drifts lQE variations lAmplifier glow

JWST Radiation Environment 20March 13, 2003 NIR Detector Effects - NICMOS lPersistence

JWST Radiation Environment 21March 13, 2003 NIR Detector Effects - NICMOS lDC bias level drift lGhosts

JWST Radiation Environment 22March 13, 2003 NIR Detector Effects - NICMOS lLinearity lWell depth

JWST Radiation Environment 23March 13, 2003 NIR Detector Effects - NICMOS lQE lDark current “bump”