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Hubble Space Telescope
ACS STAN, 9 Aug 2002

| STScI Analysis Newsletter (STAN)
| for the Advanced Camera for Surveys (ACS)
| ACS STAN #2, 9 August 2002


1. New WFC flat field reference files available (Aug 6)

2. Sensitivity table updates for SYNPHOT (Aug 8)

3. New version of PyDrizzle (Aug 7, in OTFR by Aug 21)

4. Reprocessing of fast-access ERO and GOODS data (Aug 28)

5. HST Calibration Workshop (abstracts due August 17)


1. New WFC flat field reference files available

The original laboratory flat fields obtained pre-launch 
resulted in photometric errors of +-5% to +-9% from 
corner-to-corner for the WFC. As of August 6, new 
flat fields for 13 WFC filters are being used in the 
pipeline, and for the calibrated data coming from the 
HST archive. Any data which was taken prior to August 6 
will need to be recalibrated. Initial results suggest 
that the photometric response for a given star is the 
same to one sigma <~ +/- 1% for any position in the WFC 
field of view. The rootnames of the new flat field 
reference files are:

m820832ej_pfl.fits  (F435W)
m820832fj_pfl.fits  (F475W)
m820832gj_pfl.fits  (F502N)
m820832hj_pfl.fits  (F550M)
m820832ij_pfl.fits  (F555W)
m820832jj_pfl.fits  (F606W)
m820832kj_pfl.fits  (F625W)
m820832lj_pfl.fits  (F658N)
m820832mj_pfl.fits  (F660N)
m820832nj_pfl.fits  (F775W)
m820832oj_pfl.fits  (F814W)
m820832pj_pfl.fits  (F850LP)
m820832qj_pfl.fits  (F892N)

The following chart provides an overview of the current 
status of each type of ACS calibration reference file:


2. Sensitivity table updates for SYNPHOT

With the flats in place, standard stars observed at the 
centers of the WFC1 and WFC2 CCDs give internally consistent 
results (which had not been the case for results mentioned 
in the previous STAN). In the interim, observations of a 
second standard star have also been obtained, with results 
generally consistent both for the two stars and the two CCDs.  
The following table shows the pivot wavelength (nm) for each 
WFC broadband filter and the mean count rate in the standard
star observations compared to predictions using SYNPHOT 
sensitivity tables based on ground-test results:

   filter   lambda     ratio (observed/predicted)

   F435W    432.0      1.210
   F475W    475.3      1.189
   F555W    536.9      1.145
   F606W    592.5      1.116
   F625W    630.7      1.115
   F775W    770.4      1.094
   F814W    809.2      1.075
   F850LP   913.4      0.996

Given that large errors relative to expected sensitivities 
exist, we are at least in the pleasant domain of having 
higher than expected response over nearly the full 
wavelength domain.  Although filter-to-filter differences 
could contribute at the level of a few percent, the 
monotonic trend with wavelength reflects uncertainties in 
the pre-launch calibrations used to populate the component 
tables, the most likely is the CCD quantum efficiency table 
(same values were and will be used for WFC1 and WFC2 
for this).  We have therefore generated a new CCD quantum 
efficiency table that factors in higher response as a 
smooth spline fit in wavelength to the above values.  
This will in turn change the pivot wavelengths by small 
amounts since this results in spectral dependent changes 
across some of the filters.
With additional absolute sensitivity changes in hand later 
this month we will: (1) assess whether there is any 
temporal component to the sensitivity, (2) possibly tweak 
(order of magnitude smaller changes if any) the QE table 
further, and (3) absorb residual discrepancies 
filter-by-filter by scaling the filter response curves.

It is important to recognize that the above approach poorly 
constrains updates to the CCD sensitivity table outside the 
wavelengths enumerated.  This has been dealt with by 
adopting a constant ratio of 1.21 below 432 nm, and 
continuing the linear trend over ~800-913 nm out to the 
assumed cutoff at 1100 nm.  Since much of the ACS science 
program involves use of F850LP (z band), we are dedicating
additional resources to refine the response over 
850-1100 nm using independent information from grism and 
ramp filter observations in this domain as well as 
assessing the magnitude error for stars over a wide color 
range observed in the z band.

If you would like to do more detailed calculations with 
the new CCD QE tables in synphot, you can create 
total-system-throughput tables (instrument plus OTA) using 
the synphot calcband task. Calcband takes any valid obsmode 
command string as input and produces an STSDAS table 
with two columns of data called "wavelength" and 
"throughput" as its output. For example, to evaluate the 
throughput for the F475W filter and the WFC detector, 
chip 1, you would use the command (the resulting throughput 
table is stored in sdssg_thpt): 

> calcband acs,wfc1,f475w sdssg_thpt


3. New version of PyDrizzle

A new version of PyDrizzle addresses several issues 
identified during SMOV calibration. It uses the 4th-order 
geometric distortion polynomials which are now in the IDCTAB 
reference file, solves a registration problem with dithered 
data, correctly processes subarray data (distortion and 
offsets), and addresses faulty input mask files. The 
standalone version also has modifications for registration, 
blot, and masking, to eventually support cosmic-ray 
rejection with PyDrizzle (not currently in the pipeline). 

This new version of PyDrizzle will be incorporated into
the OTFR calibration pipeline by August 21. All data
retrieved from OTFR prior to this date should be 
reprocessed. In addition, this version of PyDrizzle has 
been included in the latest release of STSDAS, which is 
already available via:

Note that PyDrizzle requires the installation Python and
PyRAF, available from:


4. Reprocessing of fast-access ERO and GOODS data

The ERO and GOODS data available via ftp will be 
reprocessed using the new reference files and revised
pipeline software described above. The reprocessing will be 
completed by August 28. Watch this webpage for details:


5. The 2002 HST Calibration Workshop

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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