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Hubble Space Telescope

S T A N / W F P C 2 - Number 8, July 1995



WFPC2 Exposure Time Estimator Tool

by John Biretta

A new WFPC2 exposure time estimation tool is now available on WWW/Mosaic. This program uses the tables in Chapter 6 of the WFPC2 Instrument Handbook (version 3.0, June 1995) to estimate exposure times and signal-to-noise ratios. It handles both point and extended sources, with either stellar or power-law spectra. It does not currently handle emission line targets, or complex situations (e.g. detection of a point source against a bright galaxy; high-z galaxies). Noise sources include read noise, photon noise, and noises contributed by dark current and sky background.

Visit the WFPC2 Website, and click on "Exposure Time Calculator". At the next page one then requests either the point source calculator, or the extended source calculator. Both calculators first request information about the target. One can either specify a magnitude (type value in box) and spectral class (click on window and choice) for a stellar target/galaxy, or one can specify a flux density and spectral index for a power-law target. If the source is strongly reddened, a color excess can also be given. The approximate target sky coordinates are entered next to allow estimation of the sky background.

The observation mode is specified next - the camera (PC or WFC), A-to-D gain (7 or 14 electrons per DN), and filter are entered by clicking on appropriate values. For point sources, the location of the target PSF on the pixel is also entered. Finally one enters either the desired signal-to-noise ratio, or the desired exposure time, and clicks on "calculate." If the signal-to-noise ratio was given, the program returns the exposure time to reach that signal-to-noise. Alternatively, one can enter the exposure time, and it will compute the resulting signal-to-noise ratio.

The accuracy is that of the Chapter 6's tables in the WFPC2 Handbook (version 3.0): for broad band filters it is about 5%; for narrow band filters the accuracy can be reduced (typically ~20%) if there are strong spectral features near the filter band passes. For situations requiring greater accuracy, SYNPHOT should be used (see next article).

The Exposure Time Calculator was written by H.-M. Adorf (ECF), A. Suchkov (STScI), J. Biretta (STScI), and M. S. Wiggs (STScI). Comments and suggestions should be forwarded to


by S. Baggett

The latest SYNPHOT tables will be of interest primarily to:

1) observers preparing their Cycle 6 proposals, who may want to use SYNPHOT for estimating exposure times for HST observations,

2) observers with data in-hand who wish to use the science data header photometric keywords (PHOTFLAM, PHOTZPT, etc) to reduce their data and,

3) observers who wish to use synthetic photometry to calibrate their observations.

SYNPHOT is an STSDAS (Space Telescope Science Data Analysis Software) software package that uses throughput tables of each optical element in order to dynamically compute throughput functions and calibration information based on an input observation mode. The WFPC2 throughput tables, including wfpc2_optics, dqe, filter, and gain tables, were recently updated (July 1995) and all files are available for ftp retrieval. The new tables bring SYNPHOT into general agreement with the results presented in the WFPC2 Instrument Handbook (Version 3.0, June 1995) and in "Photometric Performance and Calibration of WFPC2" by Holtzman et al. (to be published in PASP this November). See also "Photometry with the WFPC2", the PostScript version of a talk presented at the April 1995 HST Calibration Meeting: the plot in Figure 1 compares the recent SYNPHOT results to standard star calibration observations.

For ease in retrieving and setting up the new SYNPHOT files, there is now an online README available, containing a brief summary of what SYNPHOT does, a list of the required files, how and where to retrieve them if you don't already have a copy, and a couple of examples of how they may be used to estimate exposure times as well as recalibrate observations you may already have. The README file can be found in the WFPC2 SYNPHOT table directory and can be accessed via ftp:

 login: anonymous  (use your email address as password)
 cd cdbs/comp/wfpc2

Paper copies of any of these documents may be obtained by sending email to

New Mask File

by Michael S. Wiggs

A new static pixel mask reference file was recently installed into the calibration pipeline (file f8213081u.r0h). This file contains locations of defective CCD pixels, and is used in generating the data quality files which appear with calibrated data (.c1h files). The new version has the locations of CCD traps marked in the 256's bit, in addition to previous features. Later this Fall, a new version of the STSDAS WFIXUP task will be released, which can use this new information to automatically interpolate over CCD traps and bad columns.

Focus Move Planned for 28 August

by Stefano Casertano and John Biretta

The next semi-annual focus adjustment of the Optical Telescope Assembly (OTA) is planned for 28 August (day 240) around 15 hours UT. The OTA secondary mirror will be moved outwards by 6.5 microns, to compensate for continued desorption and shrinkage of the OTA truss assembly. From WFPC2 monitoring observations it appears that we are currently about 3.5 microns inside optimal focus.

Recent Reports and Documentation

Calibration Workshop papers:

 "Photometry with the WFPC2" by B. Whitmore

 "WFPC2 Flat Field Calibration" by J. Biretta

 "WFPC2 Ghosts, Scatter, and PSF Field Dependence" by J. Krist

Instrument Science Reports:

 "WFPC2 Photometry Analysis Script" by I. Heyer and B. Whitmore.

 "A Field Guide to WFPC2 Image Anomalies" by J. Biretta, C. Ritchie, 
 and K. Rudloff


by Harry Ferguson (

There has been considerable interest in the 150 orbit "Hubble Deep Field" (HDF) observations that will be carried out in December 1995. The observations will be of a single field at J2000 coordinates 12 36 49.42, +62 13 46.3, using a portion of the Director's Discretionary time, and will be available to the full community.

Two issues are worth highlighting here.

First, archival proposals for analysis of the HDF observations will not be reviewed by the cycle 6 TAC. There will be a separate call for proposals and a separate review panel at a later date.

Second, we invite anyone contemplating follow-up observations or detailed analysis to share information through the HDF web page. The aim is to establish a forum for publicizing the status of follow-up work in order to avoid needless duplication of effort, and to provide points of contact for cooperation/collaboration between various investigators. If you are interested in participating, please contact


by John MacKenty

We have initiated a new parallel program known as the "WFPC2 Ultraviolet Parallel Survey" (DD 6253) in July. This program is designed to utilize those WFPC2 parallel observing opportunities which would otherwise not be used by the Cycle 5 parallel programs and would therefore be lost. Many of these pointings are relatively short (<= 2 orbits) and may not be CR-SPLIT. At the request of the STScI Director, we have formed an internal team to define a observing program and to collect these data which are placed immediately into the public Archive.

After consideration of the existing programs and the contents of the HST archive, we have chosen to explore a less visited region of phase space with the intention of re-visiting this decision after a few months of data are obtained. Accordingly we have begun a program of observations in the ultraviolet to search for interesting objects. This program is designed to find extragalactic sources with near UV excesses (3000A) outside of the galactic plane and far UV sources (1600A) in the galactic plane. The primary goal of this survey is to discover sources with excess ultraviolet flux. The observing strategy is to use the WFPC2 filter which provides wavelengths slightly shorter than those available from the ground but still has the maximum available sensitivity. Ground based observations (initially the digitized sky survey plates) at longer wavelengths will be used to estimate the colors of objects and (for the single exposure instances) to discriminate between sources and cosmic rays.

Our team consists of John MacKenty (PI), Sylvia Baggett, John Biretta, Daniela Calzetti, Stefano Casertano, Harry Ferguson, Andrew Fruchter, Marc Postman, Alex Storrs, and David Taylor. We hope investigators will find these observations of interest and please contact you have questions or comments.


We draw your attention to these papers, based on WF/PC and WFPC2 data, that will appear in the next few months. This list includes all preprints received by the STScI Library not yet published in the journals. Please remember to include our Library in your preprint distribution list.

"Radio and emission-line jets in the type 2 Seyfert galaxy 
Mkn 1066 (UGC 2456)" Bower, G.; Wilson, A.; Morse, J.A.; 
Gelderman, .R; Whittle, M.; Mulchaey, J. ApJ 11-20-95

Elson, R.A.W.; Gilmore, G.F.; Santiago, B.X.; Casertano, S.
"HST observations of the stellar population of the globular 
cluster omega Cen"  AJ 8-95

Knapen, J.H.; Beckman, J.E.; Heller, C.H.; Shlosman, I.; 
De Jong, R.S.  "The central region in M100: observations 
and modeling"  ApJ 12-1-95


Any questions about the scheduling of your observations should be addressed to your Program coordinator. Post-Observation questions can be addressed to your Contact Scientist. If you do not know who these persons are, you can find the information in:

Analysis, STSDAS or any other questions can also be addressed to

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