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Hubble Space Telescope
WFPC2 STAN

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| STScI Analysis Newsletter (STAN)
| ACS+WFPC2
| 19 April 2004
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CONTENTS:

 1. UDF Data Released

 2. Cycle 13 Phase II Update 

 3. HST Pure Parallel Observations

 4. ACS/WFC Cross-talk and Gain Settings

 5. MultiDrizzle Released in STSDAS 3.2

 6. Pixel Area Maps

 7. Dithering Necessary for Hot Pixel Removal

 8. Pointing Pattern Library 

 9. Recent Instrument Science Reports 

10. Recent Publications

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1. UDF Data Released

On March 9, in the presence of MD Senator Barbara Mikulski,
STScI Director Steven Beckwith unveiled the Hubble Ultra
Deep Field (UDF), the deepest view into the Universe to date.
The total exposure time of the ACS/WFC image is 400 orbits
in the filters F435W, F606W, F775W and F850LP, reaching to
about 30th magnitude, and detecting more than 10,000 objects.
The UDF was also observed with NICMOS in F110W and F160W.
The Space Telescope Science Institute, in collaboration with 
the Space Telescope - European Coordinating Facility (ST-ECF), 
and the Canadian Astronomy Data Centre (CADC) are pleased to 
release the final reduced images, object catalogs, and 
ancillary files from the Hubble Space Telescope UDF program.

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2. Cycle 13 Phase II Update

This edition of the STAN provides information that may be 
of particular relevance to proposers about to prepare 
Cycle 13 Phase II observing programs. This information 
covers developments since the Cycle 13 ACS Instrument 
Handbook release in October 2003.
The Cycle 13 Phase II deadline is May 14, 2004.

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3. HST Pure Parallel Observations

STScI is reducing the amount of pure parallel
data taken with HST in order to reduce the number of on/off
cycles on the S-band Single Access (SSA) transmitters. While
not an immediate threat to the scientific performance of
HST, the SSA transmitters are a key subsystem and their
failure could ultimately limit the observatory lifetime.
The change has gone into effect on April 4, 2004.
There is no change with regard to coordinated parallels.
They will continue to be executed as in the past. 

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4. ACS/WFC Cross-talk and Gain Settings

Images obtained with the ACS/WFC are affected by a small 
amount of electronic cross-talk between the four CCD quadrants 
that correspond to the four amplifiers of the two detectors. 
The effect produces mostly negative electronic "ghost" images 
in a given quadrant that mirror real images recorded on other 
quadrants, although very faint positive ghosts have also been
observed.

The stretch in these images was chosen to highlight the ghost 
images. The counts in the ghost images are typically low by a 
few (1 to 4 seems typical) DN/pixel, and quantitative analysis 
(e.g., photometry) is rarely affected at a significant level. 
A detailed analysis of the cross-talk effect is underway and 
will soon be reported in an Instrument Science Report.

Preliminary analysis suggests that the strength of the cross-talk 
is significantly weaker in images acquired with gain setting 
GAIN=2 than in images taken with GAIN=1 (see the aforementioned 
webpage for examples). Therefore, Cycle 13 ACS/WFC observers 
who are concerned about the quantitative influence or visual 
appearance of cross-talk are advised to use GAIN=2 instead of 
the default GAIN=1.
 
Specifically, GAIN=2 yields better dynamic range than GAIN=1, 
but has the disadvantage of slightly increased read noise 
relative to GAIN=1.

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5. MultiDrizzle Released in STSDAS 3.2

The MultiDrizzle script provides automatic image registration,
cosmic ray rejection and final image combination using Drizzle.
It is available within PyRAF (the Python-based interface to 
IRAF). The first official release of MultiDrizzle has now 
taken place, as part of the latest release of STSDAS V3.2 
(February 2004).
The earlier beta version of MultiDrizzle is still available 
for download to support those who have older installations 
of STSDAS, although it is strongly recommended to upgrade to 
the latest STSDAS release if possible.

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6. Pixel Area Maps

When HST/ACS images are flatfielded by the CALACS pipeline 
the resultant FLT files are flat if the original sky intensity 
was also flat. However, because there is very significant 
geometric distortion in such images, the relative photometry 
of point sources in FLT images cannot also be correct because 
the pixel areas on the sky vary around the field. To help 
users who may wish to do photometry on the FLT (undrizzled) 
images we are providing a relative pixel-area map (PAM) for 
both the HRC and WFC channels of the ACS as well as an example 
script which will allow the construction of such a PAM when 
required. The flux of an object on an FLT file should be 
multiplied by the PAM value at the appropriate pixel position 
before the published zeropoints are applied.The PAM for the 
WFC is close to unity at the center of the WFC2 chip, close 
to 0.95 near the center of the WFC1 chip and close to 1.12 
near the center of the HRC.

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7. Dithering Necessary for Hot Pixel Removal

Hot pixels continue to accrue, especially on the ACS/WFC. 
We therefore remind Cycle 13 observers that while the 
standard CR-SPLIT approach allows for cosmic-ray 
subtraction, without additional dithering it will not 
eliminate hot pixels in post-observation processing. Hence, 
we recommend that observers who would have otherwise used 
a simple CR-SPLIT use some form of dithering instead. For 
example, a simple ACS-WFC-DITHER-LINE pattern is available, 
based on integer pixel offsets, which shifts the image by 
2 pixels in X and 2 in Y along the direction that minimizes 
the effects of scale variation across the detector. 
The specific parameter values for this pattern are given 
in Section 8.4.3 of the Phase II Proposal Instructions.

However, any form of dithering providing a displacement of 
at least a few pixels can be used to simultaneously remove 
the effects of cosmic ray hits and hot pixels in 
post-observation processing. The developments of PyDrizzle 
and MultiDrizzle in the IRAF/STSDAS environment now makes 
this task easy. These tasks are described in detail in 
Chapter 4 of the ACS Data Handbook.

Further details on the issue of hot pixels for ACS can be
found in the ACS Instrument Science Report 02-09.

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8. Pointing Pattern Library

The use of dithering and mosaicing patterns for ACS is
described in Chapter 8 of the Phase II Proposal Instructions.
We have now also posted an annotated "library" of pointing 
patterns on the web. The library is intended to give ACS 
users a quick way to identify and use, with confidence, 
a carefully designed pointing pattern which suits their 
observational goals. Users can always design their own 
freelance patterns, but for most, the patterns presented 
in the library should eliminate the need to calculate 
pointing parameters oneself. 

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9. Instrument Science Reports since the last STAN:

Results of UV Contamination Monitoring of the Advanced 
Camera for Surveys (ISR 04-05)

Elevated temperature measurements of ACS charge transfer
efficiency(CTE) (ISR 04-04) 

Best Gyroscope Usage to Maximize the HST Mission Lifetime 
(ISR04-03) 

Lossy Compression of ACS images (ISR 04-02) 

ACS CCD Gains, Full Well Depths, and Linearity up to and 
Beyond Saturation (ISR 04-01)

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