STScI TIPS 19 January 2006 Removing SAA-Persistent Cosmic Ray Flux from NICMOS Anton Koekemoer (INS) 1 Removing SAA-Persistent Cosmic Ray Flux from NICMOS Anton 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

STScI TIPS 19 January 2006 Removing SAA-Persistent Cosmic Ray Flux from NICMOS Anton Koekemoer (INS) 2 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 causes traps to release the electrons on longer timescales with a logarithmic decay Resulting flux appears in subsequent exposures, with a distribution that depends on the original intensity and the population of traps (Bergeron & Dickinson, NICMOS-ISR )

STScI TIPS 19 January 2006 Removing SAA-Persistent Cosmic Ray Flux from NICMOS Anton Koekemoer (INS) 3 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 (Bergeron & Dickinson, NICMOS-ISR )(O. Lupie, WFC3-ISR )

STScI TIPS 19 January 2006 Removing SAA-Persistent Cosmic Ray Flux from NICMOS Anton Koekemoer (INS) 4 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 (Bergeron & Dickinson, NICMOS-ISR )

STScI TIPS 19 January 2006 Removing SAA-Persistent Cosmic Ray Flux from NICMOS Anton Koekemoer (INS) 5 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

STScI TIPS 19 January 2006 Removing SAA-Persistent Cosmic Ray Flux from NICMOS Anton Koekemoer (INS) 6 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%

STScI TIPS 19 January 2006 Removing SAA-Persistent Cosmic Ray Flux from NICMOS Anton Koekemoer (INS) 7 Example Results (Barker & Koekemoer, 2005 HST Cal Workshop Before SAAcleanAfter SAAclean SAA CR Persistence model Post-SAA Dark

STScI TIPS 19 January 2006 Removing SAA-Persistent Cosmic Ray Flux from NICMOS Anton Koekemoer (INS) 8 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

STScI TIPS 19 January 2006 Removing SAA-Persistent Cosmic Ray Flux from NICMOS Anton Koekemoer (INS) 9 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

STScI TIPS 19 January 2006 Removing SAA-Persistent Cosmic Ray Flux from NICMOS Anton Koekemoer (INS) 10 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