Overview of SAA persistence Algorithm for removing SAA persistent flux

1. Removing SAA-Persistent Cosmic Ray Flux from NICMOS Anton Koekemoer (INS) Removing SAA-Persistent Cosmic Ray Fluxfrom NICMOSAnton Koekemoer (INS)with Elizabeth Barker, Vicki Laidler, Eddie Bergeron • Overview of SAA persistence • Algorithm for removing SAA persistent flux • Implementation & testing of “saaclean” task • Future plans

2. Removing SAA-Persistent Cosmic Ray Flux from NICMOS Anton Koekemoer (INS) (Bergeron & Dickinson, NICMOS-ISR-2003-10) SAA Persistence • Overview of Persistence: • NICMOS HgCdTe/CdTe bulk material contains small flaws that trap electrons produced by detected photons • The trapped electrons are not read out together with all the other electrons detected in a given pixel • Thermal excitation causestraps to release the electronson longer timescales with alogarithmic decay • Resulting flux appears insubsequent exposures, witha distribution that dependson the original intensity andthe population of traps

3. Removing SAA-Persistent Cosmic Ray Flux from NICMOS Anton Koekemoer (INS) (O. Lupie, WFC3-ISR-2002-01) (Bergeron & Dickinson, NICMOS-ISR-2003-10) • South Atlantic Anomaly (SAA): • Region in Earth’s magnetosphere where Van Allen belts dip closest to the surface, resulting from misalignment in Earth’s magnetic axis • HST passes through it ~50% of the time, 8-9 orbits/day • Although NICMOS is powered off, cosmic ray electrons are still trapped • Cosmic ray rate is so high that persistence can increase total noise by 4-5x in subsequent exposures obtained long after the SAA passage

4. Removing SAA-Persistent Cosmic Ray Flux from NICMOS Anton Koekemoer (INS) (Bergeron & Dickinson, NICMOS-ISR-2003-10) Removing SAA Persistent Flux • Post-SAA darks: • After each SAA passage when NICMOS transitions from SAAOPER to OPERATE, a pair of dark exposures is obtained: • ACCUM mode, NREAD=25, EXPTIME=256s • Darks start 174s and 444s after exiting the SAA • Calibrate the darks to remove other instrumental signatures (pedestal etc) • Create average post-SAA dark image: • scale the 2nd dark to the level of the 1st dark, using the decay time constant • average the 2 darks to remove CRs accumulated during the dark exposures

5. Removing SAA-Persistent Cosmic Ray Flux from NICMOS Anton Koekemoer (INS) • Subtracting the SAA model: • The average of the 2 post-SAA darks is the SAA model, representing all the CRs accumulated during SAA passage • Scale and subtract from the calibrated (pedestal-corrected) science image • Scale factor is determined iteratively: • multiply SAA model image by small scale factors • subtract from science image • measure the resulting noise (FWHM of Gaussian fit to pixel histogram) • Plot noise as a function of scale factor, fit parabola to determine optimal scale

6. Removing SAA-Persistent Cosmic Ray Flux from NICMOS Anton Koekemoer (INS) • Fitting for High and Low pixels: • Some CRs during SAA have higher energy and/or occur later than others • Need to fit different scale factors, depending on the level of the residual signal • In practice, divide the pixel histogram into 2 regimes: low and high • Determine separate scale factors, and only apply a correction if the reduction in noise is > 1%

7. Removing SAA-Persistent Cosmic Ray Flux from NICMOS Anton Koekemoer (INS) (Barker & Koekemoer, 2005 HST Cal Workshop Before SAAclean After SAAclean SAA CR Persistence model Post-SAA Dark Example Results

8. Removing SAA-Persistent Cosmic Ray Flux from NICMOS Anton Koekemoer (INS) Implementation of SAAclean task • Original script: • IDL, provided by E.Bergeron • New version: • Python code, written by Vicki Laidler, following original IDL prototype • Installed as Pyraf/STSDAS hst_calib.nicmos.saaclean • Some slight differences in algorithms (mostly related to IDL fitting functions that are unavailable or different in Pyraf) • Inputs: • calibrated exposure (optionally corrected for pedestal effect) • Post-SAA darks (read from the image header) • Output: • calibrated exposure with the SAA model subtracted • SAA model (in off-line version) • Lots of output text containing information about the fitting process • Fitting parameters also stored in header keywords

9. Removing SAA-Persistent Cosmic Ray Flux from NICMOS Anton Koekemoer (INS) Testing Procedures • Compare IDL and Pyraf versions: • run on the full variety of datasets, ranging from extreme SAA events to datasets that are not impacted • resulting model files and output images compared, examining: • noise in the images • individual pixels • dependence on quality of bad pixel files • Results: • some differences between IDL and Pyraf; generally only in treatment of bad pixels, while SAA-impacted pixels showed the same behaviour • Full test of the Pyraf version: • Ran on all SAA-impacted NIC1,2,3 exposures ever obtained (>6,000) • Examined images, as well as the output values of: • chi-sq • nhigh/nlow pixels and the threshold value • scale correction factor • noise reduction values

10. Removing SAA-Persistent Cosmic Ray Flux from NICMOS Anton Koekemoer (INS) Future Plans • Off-line version: • Prototype version was released publicly in STSDAS 3.4 (Nov 1, 2005) • Available for use within STScI and in the outside community; feedback solicited • Finalize addressing minor issues turned up in very extreme datasets (very low or very high chi-sq, or low/high pixel regimes not well modelled): • appears that all of these can be fixed by using the correct bad pixel files • Add some additional parameters to improve flexibility of iterations • Next release likely in Feb/Mar 2006 • Pipeline version: • Aim to incorporate all improvements in off-line version by Feb/Mar 2006 • Likely release to OPUS in Apr/May 2006 • Initially only run on data that is within 1 orbit of SAA passage • Correction only applied if noise reduction is >1% • Continue to solicit feedback from community