STScI TIPS
19 January 2006
Removing SAA-Persistent Cosmic Ray Flux from NICMOS
Anton Koekemoer (INS)
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
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STScI TIPS
19 January 2006
Removing SAA-Persistent Cosmic Ray Flux from NICMOS
Anton Koekemoer (INS)
SAA Persistence
Overview of Persistence:
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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
(Bergeron & Dickinson, NICMOS-ISR-2003-10)
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
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STScI TIPS
19 January 2006
Removing SAA-Persistent Cosmic Ray Flux from NICMOS
Anton Koekemoer (INS)
South Atlantic Anomaly (SAA):
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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
(O. Lupie, WFC3-ISR-2002-01)
(Bergeron & Dickinson, NICMOS-ISR-2003-10)
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STScI TIPS
19 January 2006
Removing SAA-Persistent Cosmic Ray Flux from NICMOS
Anton Koekemoer (INS)
Removing SAA Persistent Flux
Post-SAA darks:
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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
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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-2003-10)
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STScI TIPS
19 January 2006
Removing SAA-Persistent Cosmic Ray Flux from NICMOS
Anton Koekemoer (INS)
Subtracting the SAA model:
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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:
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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
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STScI TIPS
19 January 2006
Removing SAA-Persistent Cosmic Ray Flux from NICMOS
Anton Koekemoer (INS)
Fitting for High and Low pixels:
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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%
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STScI TIPS
19 January 2006
Removing SAA-Persistent Cosmic Ray Flux from NICMOS
(Barker & Koekemoer, 2005 HST
CalKoekemoer
Workshop (INS)
Anton
Example Results
Before SAAclean
Post-SAA Dark
After SAAclean
SAA CR Persistence model
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STScI TIPS
19 January 2006
Removing SAA-Persistent Cosmic Ray Flux from NICMOS
Anton Koekemoer (INS)
Implementation of SAAclean task
Original script:
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IDL, provided by E.Bergeron
New version:
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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)
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Output:
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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
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STScI TIPS
19 January 2006
Removing SAA-Persistent Cosmic Ray Flux from NICMOS
Anton Koekemoer (INS)
Testing Procedures
Compare IDL and Pyraf versions:
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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
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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:
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Ran on all SAA-impacted NIC1,2,3 exposures ever obtained (>6,000)
Examined images, as well as the output values of:
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chi-sq
nhigh/nlow pixels and the threshold value
scale correction factor
noise reduction values
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STScI TIPS
19 January 2006
Removing SAA-Persistent Cosmic Ray Flux from NICMOS
Anton Koekemoer (INS)
Future Plans
Off-line version:
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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
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Add some additional parameters to improve flexibility of iterations
Next release likely in Feb/Mar 2006
Pipeline version:
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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
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