STScI Logo

Hubble Space Telescope
ACS+WFPC2 STAN, 6 Oct 2004

+----------------------------------------------------------+
| STScI Analysis Newsletter (STAN)
| ACS+WFPC2
| 1 October 2004
+----------------------------------------------------------+

CONTENTS:

1. Addition of MultiDrizzle to the ACS Pipeline

2. ACS Data Handbook Version 3.0 

3. HST Cycle 14 Phase I Proposal Deadline: Friday January 21, 2005

4. ACS and WFPC2 Instrument Handbooks for Cycle 14

5. ACS Geometric Distortion Corrections

6. ACS Zeropoints

7. ACS Bias and Dark Reference Files

8. Discontinuation of WFPC2 Supplemental Darks

9. Recent Instrument Science Reports

10. Recent Publications

11. Recent Press Releases

+----------------------------------------------------------+

1. Addition of MultiDrizzle to the ACS Pipeline

As of September 22, 2004, the MultiDrizzle software has been
incorporated into the ACS data reduction pipeline. This means 
that when data are requested from the HST Archive, a 
"*_drz.fits" image product will be delivered which consists 
of a cosmic-ray cleaned, combined, and geometrically corrected 
image. This image product will be created for all ACS 
associations, which are defined as groups of exposures within 
a single visit, obtained using the recommended dither patterns 
or CR-SPLITs as specified in the Phase II proposal instructions. 
The pipeline previously constructed drz.fits image products 
using PyDrizzle. The advantages of MultiDrizzle over PyDrizzle 
are described in Section 4.5 of the ACS Data Handbook. Section 
4.6 describes how observers can run the MultiDrizzle software 
outside of the HST Archive to recalibrate the data if necessary. 
A future update to the ACS Data Handbook (scheduled to appear 
by the end of 2004) will include a full discussion of how the 
software was implemented into the ACS pipeline. In the meantime, 
users should refer to information at

http://www.stsci.edu/hst/acs/analysis/multidrizzle

for details about the implementation and the data products 
that are produced.

+----------------------------------------------------------+

2. ACS Data Handbook version 3.0

Version 3.0 of the ACS Data Handbook was released. 
This new version has two entirely new Chapters. Chapter 5 
covers ACS Error Sources, and discusses issues such as Readout 
Noise, A-to-D Conversion, Bias Subtraction, Dark Current, Hot 
Pixels, Cosmic Rays, Flat Fields, Image Anomalies, and Generic 
Detector and Camera Properties. Chapter 6 covers ACS Data 
Analysis and discusses issues such as Photometry, Astrometry, 
Polarimetry, Coronagraphy, Ramp Filters, and Grism/Prism 
Spectroscopy.

+----------------------------------------------------------+

3. HST Cycle 14 Phase I Proposal Deadline: Friday January 21, 2005

See the HST Call for Proposals and related information that 
will soon be made available at:

http://www.stsci.edu/hst/proposing/docs/cycle14announce.
(This link is no longer available.)

Proposers should note the new requirement for Cycle 14 to 
provide information on how the scheduling and science of a 
proposal would be affected by a potential transition of Hubble 
Space Telescope to a pointing mode with only two functional 
gyroscopes. A detailed description of this mode is contained 
in the Two-Gyro Handbook, which will be available from the 
aforementioned Cycle 14 announcement web page. Chapters 7 
and 8 contain discussions of impacts unique to observations 
with ACS and WFPC2, respectively.

+----------------------------------------------------------+

4. ACS and WFPC2 Instrument Handbooks for Cycle 14

The ACS and WFPC2 Instrument Handbooks both have undergone 
revisions to update for Cycle 14. The new versions will soon 
be made available respectively at:

http://documents.stsci.edu/hst/acs/documents/handbooks/cycle14/cover.html

http://www.stsci.edu/hst/wfpc2/documents/handbook/wfpc2_ihb_v90.pdf

These new Handbooks reflect our current understanding of the
instruments, and include calibrations and insights obtained 
during the past year.

+----------------------------------------------------------+

5. ACS Geometric Distortion Corrections

ACS images have considerable geometric distortion. Until 
recently, the geometric distortion corrections applied in the 
ACS pipeline to create drizzled "*_drz.fits" files were based 
on fourth-order polynomial solutions. However, it was found 
that more accurate results can be obtained when corrections 
are applied that: (a) are dependent on the filter; and (b) 
contain higher frequency information than can be easily 
captured with a polynomial. The version of Multidrizzle that
was installed in the ACS pipeline (see item 1 above) now 
applies such corrections for all of the most common filters 
on WFC and HRC. The corrections are based on the work of 
Anderson & King (2004; ACS ISR 04-15, hyperlink given under 
Item 9 below). For the corrections the pipeline uses new 
calibration reference files "*_dxy.fits".

+----------------------------------------------------------+

6. ACS Zeropoints 

Photometric zeropoints for ACS are available at the web page

http://www.stsci.edu/hst/acs/analysis/zeropoints

On May 10, 2004, we made minor updates to the VEGAmag 
zeropoints for ACS. The new zeropoints are based on the 
latest spectrophotometric calibration of Vega (Bohlin & 
Gilliland 2004, AJ 127, 3508). The new composite flux 
standard alpha_lyr_stis_002.fits (available at 
http://www.stsci.edu/hst/observatory/cdbs/calspec.html) 
consists of IUE data from 1256-1675A, STIS CCD fluxes from 
1675-4200A, and a specially tailored Kurucz model longward 
of 4200A (Kurucz 2003) and from 900-1256A. The STmag and 
ABmag zeropoints have not been affected by this change.

NOTE: From May 10, 2004 until August 23, 2004, the VEGAMAG 
zeropoints for filters F660N and F775W on ACS/HRC were 
erroneously listed on the aforementioned web page as 25.827 
and 25.551. The correct values are 20.827 and 24.551, 
respectively.

+----------------------------------------------------------+

7. ACS Bias and Dark Reference Files

ACS bias and dark reference files (which are made from many
contemporaneous bias and dark frames) are typically available 
in the HST Data Archive within a few weeks after any WFC or 
HRC observation. However, due to transitions in our reference 
file production processes and infrastructure over the summer, 
this turnaround time has been longer for most of the WFC and 
HRC data obtained between June 17 and August 25. We have now 
completed the process of creating and delivering the reference 
files for this period. WFC and HRC users who obtained data in 
this period are advised to re-retrieve their data from the HST 
Data Archive to obtain the optimal bias and dark calibration. 
More generally, users are advised to monitor our reference
file Web page.


These list when the best reference files for any particular 
date of observation have become available. Once dark and bias 
frames for a particular date have become available, users 
should (re-)retrieve their data from the HST Archive.

+----------------------------------------------------------+

8. Discontinuation of WFPC2 Supplemental Darks

We are attempting to reduce the number of low-priority 
exposures taken on HST in an effort to prolong the life of 
the radio transmitters which are used to down-link science 
data. Hence, we are discontinuing the WFPC2 supplemental dark 
calibration program, where typically three dark frames were 
taken per day. In principle these could be used to optimize 
removal of hot pixels from WFPC2 science images, since dark 
frames could be selected very close in time to the science 
data. However, in practice they were rarely used. We will 
of course be continuing the weekly dark calibration program, 
which provides reference files for the WFPC2 data reduction
pipeline.

+----------------------------------------------------------+

9. Recent Instrument Science Reports

ACS:

ACS coronographic flat fields (ISR04-16)
http://www.stsci.edu/hst/acs/documents/isrs/isr0416.pdf

Multi-filter PSFs and Distortion Corrections for the HRC 
(ISR 04-15)
http://www.stsci.edu/hst/acs/documents/isrs/isr0415.pdf

SBC Dark and Cumulative Images (ISR 04-14)
http://www.stsci.edu/hst/acs/documents/isrs/isr0415.pdf

Cross-Talk in the ACS WFC Detectors. II: Using GAIN=2 to 
Minimize the Effect (ISR 04-13)
http://www.stsci.edu/hst/acs/documents/isrs/isr0413.pdf

Cross-Talk in the ACS WFC Detectors. I: Description of the 
Effect (ISR 04-12)
http://www.stsci.edu/hst/acs/documents/isrs/isr0412.pdf

ACS/HRC Polarimetry Calibration III: Astrometry of Polarized 
Stars (ISR 04-11)
http://www.stsci.edu/hst/acs/documents/isrs/isr0411.pdf

ACS Polarization Calibration - II. The POLV Filter Angles 
(ISR 04-10)
http://www.stsci.edu/hst/acs/documents/isrs/isr0410.pdf

ACS Polarization Calibration I. Introduction and Status 
Report (ISR 04-09)
http://www.stsci.edu/hst/acs/documents/isrs/isr0409.pdf

Detector Quantum Efficiency and Photometric Zero Points of 
the ACS (ISR 04-08)
http://www.stsci.edu/hst/acs/documents/isrs/isr0408.pdf

Bias and dark calibration of ACS data (ISR 04-07)
http://www.stsci.edu/hst/acs/documents/isrs/isr0407.pdf

Time Dependence of ACS WFC CTE Corrections for Photometry 
and Future Predictions (ISR 04-06)
http://www.stsci.edu/hst/acs/documents/isrs/isr0406.pdf

For a list of all ACS ISRs please see:
http://www.stsci.edu/hst/acs/documents/isrs/


WFPC2:

The Accuracy of WFPC2 Photometric Zeropoints (ISR 04-01)
http://www.stsci.edu/instruments/wfpc2/Wfpc2_isr/wfpc2_isr0401.html

For a list of all WFPC2 ISRs please see:
http://www.stsci.edu/instruments/wfpc2/wfpc2_bib.html

+----------------------------------------------------------+

10. Recent Refereed Publications

ACS:   
http://www.stsci.edu/hst/acs/documents/pub_acs_2004_10.pdf
(This document is no longer available.)

WFPC2: 
http://www.stsci.edu/hst/acs/documents/pub_wfpc2_2004_10.pdf
(This document is no longer available.)

WF/PC: 
http://www.stsci.edu/hst/acs/documents/pub_wfpc1_2004_10.pdf
(This document is no longer available.)

The papers listed above are based on data from ACS, WFPC2, 
and WF/PC. The lists include all refereed papers received 
and identified by the STScI Library during the current year. 
Please include our Library in your preprint distribution 
list. The listings were generated with STEPsheet:
http://stepsheet.stsci.edu/

+----------------------------------------------------------+

11. Recent Press Releases

ACS:   
http://www.stsci.edu/hst/acs/documents/press_acs_2004_10.html
(This link is no longer available.)

WFPC2: 
http://www.stsci.edu/hst/acs/documents/press_wfpc2_2004_10.html
(This link is no longer available.)

The press releases listed above are based on data from WFPC2 
and/or ACS. The listing was compiled from the collection of 
STScI Press Releases available at
http://hubblesite.org/newscenter/newsdesk/archive/ .

+----------------------------------------------------------+
| Visit the ACS and WFPC2 websites for more news:
| ACS:   http://www.stsci.edu/hst/acs
| WFPC2: http://www.stsci.edu/hst/wfpc2
| Need help? Please contact us at the helpdesk.
+----------------------------------------------------------+
| To subscribe or unsubscribe to this STAN, send a message
| to majordomo@stsci.edu with a blank subject line and
| the following in the body: [un]subscribe acs_wfpc2_news
| Previous ACS and WFPC2 STANs are archived at:
| ACS:   http://www.stsci.edu/hst/acs/documents/newsletters
| WFPC2: http://www.stsci.edu/hst/wfpc2/documents/newsletters
+----------------------------------------------------------+
| The Space Telescope Science Institute is operated by the
| Association of Universities for Research in Astronomy,
| Inc., under NASA contract NAS5-26555.
+----------------------------------------------------------+