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Hubble Space Telescope
ACS STAN, 2 Jan 2003

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| STScI Analysis Newsletter (STAN)
| for the Advanced Camera for Surveys (ACS)
| ACS STAN #3, 2 January 2003
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CONTENTS:

1. ACS coronagraph update for Cycle 12 proposers 

2. Cycle 12 calibration outsourcing proposals

3. ACS sensitivity for red targets in F850LP

4. Impact of the FGS realignment

5. The growth of hot pixels

6. Cosmic ray rejection

7. ACS pipeline-calibrated products

8. Should I recalibrate my ACS data?

9. Extracting spectra from WFC G800L slitless grism data

10. Recent ACS-related publications

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1. ACS coronagraph update for Cycle 12 Proposers 

Instrument Science Report ACS 2002-11 by John Krist

http://www.stsci.edu/hst/acs/documents/isrs/isr0211.pdf

Abstract: Small instabilities in the ACS coronagraph have
led to revisions of the coronagraph commanding procedures
and to the suggested methods for optimizing coronagraphic
observations. An example optimized sequence is given. Due
to the instabilities, the flat fields currently used by the
pipeline are not accurate and should not be used; new flats
will be generated.  Also, clarification is provided on the
use of the coronagraphic surface brightness profiles in the
Cycle 12 ACS Instrument Handbook.

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2.  Cycle 12 calibration outsourcing proposals

We would like to remind prospective proposers that the
Cycle 12 Call for Proposals (see Section 3.6) solicits
programs in the category of "Calibration Proposals". These
are intended to allow more complete calibration of the
instruments, than would be provided by the STScI
calibrations that are aimed at covering primary and often
used configurations.  Those contemplating such a program
should carefully read Chapters 12 and 13 of the ACS
Instrument Handbook, and consult with an ACS Instrument
Scientist well before the deadline. A successful
calibration program would be expected to provide a general
service to the community.  Internal reviews at STScI will
provide input to the TAC on feasibility, complementarity to
existing calibration plans, and the type of science to be
impacted.

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3. ACS sensitivity for red targets in F850LP

The sensitivity calibration for ACS will soon be reworked
with implied changes for most filters no larger than 1-2%
(and therefore not relevant for Phase I plans).  An
exception to this involves the z-band filter, F850LP with
the WFC for which changes for particular target
characteristics could be 10s of percent, and therefore
relevant for Phase I planning. Over the passband of the
F850LP filter the currently assumed quantum efficiency is
too high by ~2% at the short wavelengths (<8400A) sampled,
with a larger drop in the far-red, e.g. ~20% at 10,000A.  A
yet more significant effect is that the ACS point spread
function develops a large halo at these longest
wavelengths, e.g. the encircled energy in  a 3x3 pixel
aperture for an L-dwarf observed in F850LP is 74% of that
for an early type star.  The ETCs do not yet take into
account the  underlying spectral energy distribution in
setting the encircled energy, and have not yet been updated
for the new wavelength dependent QE. A more complete
discussion was provided at the 2002 HST Calibration 
Workshop, see:

http://www.stsci.edu/hst/acs/documents/calworkshop/sensitivity_red.ps

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4. Impact of the FGS realignment

On October 21, 2002 the matrices describing the positions
of the Fine Guidance Sensors in the HST focal plane were
updated. Prior to this date, pointing errors of 2 to 3
arcseconds had been observed in WFPC2 and STIS. These
errors are large enough to present a risk of missing
targets in small apertures. The ACS pointing history had
not been studied in detail. We expect that for any
observations after the update the pointing accuracy with
ACS (and the accuracy of the absolute coordinates in
pipeline reduced products) will be better than one
arcsecond. However, this may not be true for earlier
observations. We are investigating a couple of earlier
observations that had larger errors, one with a 3 arcsecond
discrepancy and one with as much as 6 arcseconds. We will
report on the cause and frequency of these aberrant results
once they are understood.

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5. The growth of hot pixels

Instrument Science Report ACS 2002-09 by Adam Riess

http://www.stsci.edu/hst/acs/documents/isrs/isr0211.pdf

Abstract: The anneal rate of hot pixels on the ACS WFC is
60%-65%, significantly lower than the characteristic anneal
rate of 80-85% seen for other CCD's flown on HST (i.e.,
WFPC2, STIS, and ACS HRC).  The ACS WFC is annealed in the
same way as the other HST CCD's and there is no firm
understanding at this time of the source of the difference.
After 7-8 successive anneals, the cumulative fraction of
annealed pixels reaches an apparent plateau at 70%. 
Additional anneals would not be expected to help as the
annealing likelihood for persistent hot pixels reaches an
apparent plateau after 7 to 8 anneals. Approximately 2
years after launch the coverage by hot pixels is expected
to exceed that by cosmic rays in a 1000 sec exposure.  At
the nominal end of the HST mission (2010) the coverage by
hot pixels would be 6%, i.e., one out of every 16 pixels. 
Because hot pixels are readily flagged and corrected or
discarded they do not pose a serious threat to science
observations, but their growing presence require careful
dithering and consideration.
 
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6. Cosmic ray rejection

Elimination of cosmic rays in CR-SPLIT images, particularly
in undersampled data, can be tricky in the presence of even
small registration differences between multiple exposures. 
The primary test for the presence of a cosmic ray in one
frame is to compare the pixel value with an expected value
(e.g. minimum of two exposures, or median of several) and
flag as a cosmic ray if more than a specified sigma-level
above that. Normal components of the noise model include
readout noise and Poisson noise from the object and sky,
but even a small fraction of a pixel offset between two
images can lead to apparent changes near a bright target
that swamp expected Poisson differences -- to account for
this an extra noise term "scalenoise" is added as a simple
multiple of the intensity. With undersampled data, the
scalenoise needs to be roughly equal to the offset in
fractions of a pixel to avoid falsely flagging cosmic rays
near the core of well exposed stars.  See discussion in
section 5.1 of ISR ACS 2002-8. This ISR also reports
finding that offsets up to 0.3 pixels are not uncommon
between ACS cr-split exposures and thus since 18 October
2002 the pipeline has used SCALENSE=30.0 (percent).  Before
that time values of 3% resulted in many valid pixels being
falsely eliminated for cases with registration differences
above a few 0.01 pixels. At the new, larger value
elimination of cosmic rays on extended objects will be
inappropriately biased against, but under the philosophy of
"first do no harm" we have adopted the conservative
approach that only rarely drops valid data.

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7. ACS pipeline-calibrated products

Many ACS observations take advantage of dithering to 
increase the effective field-of-view, reduce resolution 
loss due to undersampling, or allow more effective 
cosmic-ray and hot pixel rejection. However, the current 
standard calibration processing performed on dithered ACS 
data produces products which may not meet the expectations 
of observers in some important ways. We would like to 
point out these features and present brief details of the 
solutions which are currently being made available.
  
PyDrizzle is used in the pipeline to perform distortion 
correction and combination of all ACS dithered data. It 
invokes the 'drizzle' task to remove geometrical distortion 
but does NOT have the capability to detect cosmic-rays.  
As a result the combined dithered data sets, although
aligned and geometrically corrected, currently still contain 
cosmic-rays. CRs are only removed by the pipeline if the 
images were acquired using the CR-SPLIT mode.

A new script called MultiDrizzle has been written by Anton
Koekemoer (Koekemoer, Fruchter, Hook & Hack 2002, in the
forthcoming proceedings of the 2002 HST Calibration
Workshop) to perform cosmic-ray detection and removal using
PyDrizzle and the tasks in the STSDAS dither package.  This
can be viewed as a Python implementation of the scheme for
CR detection and flagging described in Fruchter & Hook,
2002, PASP, 114, 144.  MultiDrizzle will work best when
provided with flat-fielded calibrated products which have
not been geometrically corrected. These normally have
either '_flt.fits' ' or '_crj.fits' extensions.
Unfortunately, these products are not currently available
from the STScI archive for dithered ACS data. However, they
will become available from February 2003 when the keyword
EXPSCORR will be changed to PERFORM. The required files 
can also be created by individual users at their home 
institution. To this end, set the keyword EXPSCORR to 
PERFORM, and rerun the CALACS pipeline. The MultiDrizzle 
task is available for download from:

http://stsdas.stsci.edu/pydrizzle/multidrizzle

This new task can be used on any calibrated ACS or WFPC2 
data on any system which has PyRAF 1.0, STSDAS 3.0 and 
PyDrizzle 3.3f. If all the default values are used the 
desired output name for the final product is the only 
input. It was primarily designed for data taken during 
the same visit using the same guide stars to minimize any 
potential pointing offsets errors between observations.  
Ultimately, this task will be generalized to correct for 
pointing offsets automatically and be incorporated into 
the standard pipeline processing within the next year.
Until then, this initial release should provide 
cosmic-ray cleaned, dither-combined ACS data useful 
for analysis.

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8. Should I recalibrate my ACS data?

Our calibration and characterization of the ACS continues to
improve with time. As new reference files become available,
you may want to recalibrate your data. To decide whether
this is worthwhile, please regularly check the web page: 

http://www.stsci.edu/hst/acs/analysis/reference_files/ref_updates.html This page gives a detailed history of reference file updates and explains the type and magnitude of the changes in calibration. UPDATE:
The previous link is no longer available. See the following pages for information on reference file updates: ACS Reference File Webpage
ACS Reference File Logsheet +----------------------------------------------------------+ 9. Extracting spectra from WFC G800L slitless grism data The wavelength solution for the G800L grism with the WFC has been derived from in-orbit spectra of two Galactic Wolf-Rayet stars from SMOV and Cycle 11 calibration data. The mean dispersion is 39.2 A/pix in first order, 20.5 A/pix in second and -42.5 A/pix in negative first order. The dispersion solution is strongly field-dependent, with an amplitude of variation of about 11% from the center to the corners. A paper describing these results will appear in the proceedings of the 2002 HST Calibration Workshop, and is currently available at: http://xxx.lanl.gov/abs/astro-ph/0211651 The configuration file for the ACS spectral extraction software, aXe, incorporating these dispersion solutions, is available on the aXe calibration page: http://www.stecf.org/instruments/acs/calib/Cycle11/ The flat field for slitless spectroscopy data is both field and wavelength dependent. The effect of the wavelength dependent flat field for the WFC G800L grism mode has been investigated from observations of a flux calibrator at different positions in the field. A flat field cube has been derived by fitting the in-orbit broad-band filter flats as a function of wavelength and pixel and provides good correction (<2% residuals) across the WFC field. A description will appear in the proceedings of the 2002 Calibration Workshop, and is currently available at: http://xxx.lanl.gov/abs/astro-ph/0212021 The flat field cube is available for downloading at: http://www.stecf.org/instruments/acs/calib/Cycle11/ The flux standard also provides the (single, field independent) absolute flux calibration for the WFC G800L spectra. The first order sensitivity file, which must be used in aXe together with the flat field cube is also available on the same Web page. +----------------------------------------------------------+ 10. Recent ACS-related publications This listing was generated with STEPsheet: http://stepsheet.stsci.edu/ BOND, H.E.; PANAGIA, N.; SPARKS, W.B.; STARRFIELD, S.G.; WAGNER, R.M. "V838 Monocerotis" IAUC 7892: 1, 2002 IAUC 7892 BOND, H.E.; PANAGIA, N.; SPARKS, W.B.; STARRFIELD, S.G.; WAGNER, R.M.; HENDEN, A. "V838 Monocerotis" IAUC 7943: 1, 2002 IAUC 7943 EDMONDS, P.D.; GILLILAND, R.L.; CAMILO, F.; HEINKE, C.O.; GRINDLAY, J.E. "A Millisecond Pulsar Optical Counterpart with Large-Amplitude Variability in the Globular Cluster 47 Tucanae" ApJ 579: 741-751, 2002 ApJ 579: 741-751 GO-08267, GO-07503, GO-08219 GIAVALISCO, M.; RIESS, A.; Casertano; Dahlen; Dickinson; Ferguson; Hook; Idzi; Koekemoer; Mobasher; Moustakas; Ravindranath; Strolger; Tonry; Challis; "Supernovae 2002ez, 2002fv, 2002fw, 2002fx, 2002fy, 2002fz, 2002ga" IAUC 7981: 1, 2002 IAUC 7981 MAGEE, D.; BOUWENS, R.; ILLINGWORTH, G.; FORD, H.; BENITEZ, N.; BLAKESLEE, J.; CROSS, N.; GRONWALL, C.; TSVETANOV, Z.; CLAMPIN, M.; HARTIG, G. "Supernova 2002dc" IAUC 7908: 1, 2002 IAUC 7908 RIESS, A.; GROGIN, N.; HORNSCHEMEIER, A.; LUCAS, R.; RICHARDSON, M. "Supernovae 2002hp, 2002 hq, 2002hr, 2002hs, 2002ht, 2002hu, 2002hv" IAUC 8012: 1, 2002 IAUC 8012 TSVETANOV, Z.; BLAKESLEE, J.; FORD, H.; MAGEE, D.; ILLINGWORTH, G.; RIESS, A. "Supernova 2002dd" IAUC 7912: 1, 2002 IAUC 7912 +----------------------------------------------------------+ | Need help? +----------------------------------------------------------+ | To subscribe or unsubscribe to the STAN, send a message | to majordomo@stsci.edu with a blank subject line and | the following in the body: [un]subscribe acs_news | Previous STANs are archived at: | http://www.stsci.edu/hst/acs/documents/newsletters +----------------------------------------------------------+ | The Space Telescope Science Institute is operated by the | Association of Universities for Research in Astronomy, | Inc., under NASA contract NAS5-26555. +----------------------------------------------------------+