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Hubble Space Telescope
ACS STAN, 28 Jun 2002

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| STScI Analysis Newsletter (STAN)
| for the Advanced Camera for Surveys (ACS)
| ACS STAN #1, 28 June 2002
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CONTENTS:

1. The first ACS STAN -- subscribe now for future issues!

2. Preliminary filter sensitivities

3. Flat field calibrations

4. Image quality

5. Image tracking stability 

6. Reference file status chart

7. Calibration status of ACS data from the archive

8. Geometric distortion correction

9. Updated aperture locations

10. Notes on observing modes: coronograph, grisms

11. HST Calibration Workshop, 17-18 Oct 2002

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1. The first ACS STAN

This is the first "STAN" for the Advanced Camera for Surveys 
(ACS). ACS was installed onboard HST in March 2002, and the
orbital verification (SMOV) program is nearing completion.

Keep up with the latest ACS news by subscribing to receive
future issues of this STAN -- see the appended instructions
(Cycle 11 ACS GOs have been automatically subscribed).
Every issue of the ACS STAN will be archived at:

www.stsci.edu/hst/acs/documents/newsletters


Explore the ACS website for more information, including
"New in the last 40 days" items -- currently featuring
preliminary reports on hot pixels and cosmic rays on ACS:

www.stsci.edu/cgi-bin/acs

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2. Preliminary filter sensitivities

The following chart shows a comparison of the preliminary 
filter sensitivity measurements to the SYNPHOT models for 
the ACS CCD imaging modes:

www.stsci.edu/hst/acs/performance/delta_synphot.pdf


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3. Flat field calibrations

The current "ground" flats are LP-flats for which our SMOV
photometry is expected to be good to +-5% for the WFC. 
Using on-orbit SMOV data, images of 47 Tuc are currently 
being analyzed to create new L-flat images which will 
improve the flatfielding to ~1%. We are working to deliver 
these new reference files by August 2002. Any data which 
is taken before these flats are delivered will need to be 
recalibrated.

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4. Image quality

The current synphot PSF curves are based on ground 
calibration data and will be updated once on-orbit data has 
been further analyzed. A first look shows some evidence of
internal scattering of infrared light. This is just a few
percent of the total stellar flux in the z band, but becomes
more serious at longer wavelengths. The scattering manifests 
itself in HRC images as prominent PSF wings in the long
wavelength filters and as spikes in WFC images. Further
work will focus on characterizing the wavelength dependence
of the PSF.

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5. Image tracking stability  

A recent SMOV test involved tracking the position of stars 
on the CCD over several orbits during which time HST was 
repositioned to attitudes leading to orbital heat input 
differences near the upper range of those normally 
encountered. The results from this provide confirmation 
that guiding with HST is excellent, but not perfect.
The tracking accuracy while on the same guide stars showed 
offsets of up to about 40 milliarcsecond accumulating over 
a period of seven hours. This behavior has been seen before 
with other instruments and has been correctly described in 
past Call for Proposal documentation: "A drift of up to 
0.05" may occur over a timescale of 12 hours..."

With the very sharp point spread function supplied by HST, 
offsets between successive exposures much less than this 
(e.g. 0.005" is still 10% of a WFC pixel scale) can 
complicate the rejection of cosmic rays, especially in 
images dominated by unresolved stellar sources. The 
scheduling system allows for two components of a CR-SPLIT 
image to be separated by times approaching an hour, this 
can lead to offsets at the 10th pixel level and may require 
special care in analysis to avoid rejection of valid data
from cosmic ray detection algorithms.  Consideration of 
such issues should be an integral part of observation 
planning and post-observation data analysis procedures.

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6. Status of ACS calibration reference files

For a brief overview of the current status (availability, 
pedigree, plans, etc) of ACS reference files, see:

www.stsci.edu/hst/acs/analysis/reference_files/ref_status.pdf

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7. Calibration status of ACS data from the archive

All ACS data retrieved from the archive through OTFR get 
processed using the standard calibration pipeline software, 
CALACS. As of mid-August, CALACS Version 4.0a should be in
place in the archive for use on data to be retrieved. This 
version of CALACS incorporates many of the lessons learned 
from the initial calibration period following installation 
of ACS in HST. Prior to this, errors in the error (ERR) 
and data quality (DQ) arrays were present and would affect 
the final distortion-corrected product.

The latest version of CALACS will also be made available 
through the public release of the next version of STSDAS, 
due out by the end of July. The STSDAS WWW pages at 
stsdas.stsci.edu
can be checked for news on when the new release will be
available, and for downloading the software. This would
provide the capability for re-calibrating your ACS data
with the latest software and calibration files. 

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8. Geometric distortion correction

Standard calibration of ACS data applies a flat-field 
correction using a sky flat. This results in an output 
image whose extended source photometry remains consistent, 
but whose pixels cover different areas of the sky, seriously
impacting point-source photometry. Also, the off-axis 
position of the ACS field-of-view results in significant 
optical distortion.  Both of these affects are corrected 
during standard pipeline calibration through the use of 
PyDrizzle, a Python task running under PyRAF which controls 
the operation of the IRAF image combination task 'drizzle'.  
This step not only corrects the distortion in single images, 
but also combines the separate WFC chips images into a single 
distortion-corrected image. In addition, ACS data taken as a 
dither pattern as specified in the proposal with the use of 
a pattern parameter will also be automatically combined in 
the pipeline using PyDrizzle. All these distortion corrected, 
and possibly dither-combined, products will have the 
suffix '_drz.fits' when retrieved from the archive.

However, many lessons were learned during the initial 
calibration program undertaken after ACS was installed that 
revealed inaccuracies in how PyDrizzle was applying the 
corrections to the data.  These lessons were used to 
update PyDrizzle to provide a much more reliable and 
accurate calibrated product. Unfortunately, this new version, 
Version 3.0, will not be available for use in the calibration 
pipeline and the archive until mid-August. The trailer file 
for the calibrated product will contain messages detailing 
which version of PyDrizzle was used.  This can be used to 
verify that PyDrizzle Version 3.0 was used to calibrate 
your data.

As with all other calibration software, PyDrizzle can also 
be run offline on your own system to allow for customized 
parameters for the output product to better match the 
science in the observations. PyDrizzle can be obtained as 
part of the next release of STSDAS, which should be 
available by the end of July. More information about 
PyDrizzle can be obtained online from the ACS Data Handbook, 
and from the ACS Drizzling WWW page at:

www.stsci.edu/instruments/acs/

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9. Updated aperture locations 

Aperture locations have been updated as a result of ACS-FGS 
alignment measurements. They differ by a few arcseconds 
from those in the ACS Handbook. The results are referenced 
on the ACS home page and have been incorporated in flight 
software and the VTT. The WFCENTER aperture is now fully 
supported.

A further refinement using the results of the distortion 
analysis is in progress and will cause changes of, at most, 
a few tenths of an arcsecond. An exception to this may be 
the SBC which has not been directly measured and had been 
assumed to be at the same position as the HRC. The full 
update will be installed at the end of July.

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10. Notes on observing modes: coronograph, grisms

The coronographic mode is not yet ready for operation.
The optimal aperture location is still being tuned and 
calibrated against pre-launch models.

The ACS group at the ST-ECF reports their initial 
results from the "first light" grism data:
http://www.stecf.org/instruments/acs/SMOV/WFC/index.html
(This link is no longer available.)

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11. The 2002 HST Calibration Workshop


www.stsci.edu/stsci/meetings/cal02

The 2002 HST Calibration Workshop will be held on October 
17 and 18 at the Space Telescope Science Institute. All 
observers who have new ideas to contribute regarding the 
calibration of HST instruments are encouraged to present 
their work. Abstracts for all presentations are due by 
Aug 17, 2002. The registration deadline is Sep 17, 2002. 
Also note that the HST Calibration Workshop immediately
follows the Astronomical Data Analysis Software & Systems
Conference (ADASS XII) that will be held in downtown
Baltimore between October 13 and 16.


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