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Hubble Space Telescope
WFPC2 STAN

S T A N / W F P C 2 - Number 18, August 1996

CONTENTS:

Progress Report on WFPC2 PSF Characterization:

by J. Surdej, S. Baggett, M. S. Wiggs

Making use of numerous F555W, F814W and F439W PC observations of the standard star GRW+70D5824 obtained in the context of the HST calibration proposal 6179 "Photometric zero points", we have been able to characterize the behaviour of the PSF in view of future applications such as PSF fitting photometry (especially useful in crowded fields), PSF subtraction, decomposition and/or image deconvolution.

As far as the photometric results are concerned, we have found that the use of a composite PSF constructed from optimally selected ranges in positions and breathing values (i.e. similar relative focus positions with respect to the nominal one) leads to single photometric values affected by an RMS of typically 0.01 - 0.02 mag. Breathing is defined as the change in focus caused by thermal cycling as the spacecraft goes from day to night each orbit. Very acceptable PSF residuals have also been derived after subtracting such composite PSFs from real observations. Therefore, we are in the process of populating a WFPC2 PSF library with individual and composite observed PSFs characterized by multiple keywords such as the relevant WFPC2 detector, filter, X and Y center positions of the target on the detector, central peak intensity, relative focus value if available (breathing dependent), CCD gain, etc.

A WWW tool is currently under construction to permit HST users to query the WFPC2 PSF library database on some of these keywords and then grab the desired image files. Access to the observed WFPC2 PSF library via the WWW tool should be made available to the HST community later this Fall. An Instrument Science Report presenting more details on the above results will also be soon available.

If you have WFPC2 PSF observations of your own which might be useful to include in the PSF library, or if you have comments on the construction of the library, please contact help@stsci.edu.

Change in Calibration Dark Files:

by S. Baggett, S. Casertano, M. S. Wiggs, M. Mutchler

Starting August 1, we have modified the procedure by which the dark reference files used in the default calibration pipeline are obtained. Previously, such dark files were generated by combining the dark frames - usually 10 - obtained in the week of the observation and in the previous week. The two week time frame was a compromise between noise in the dark frame (which would have been improved by a longer baseline) and timeliness of the information about hot pixels, which change on a daily scale. In the new system, the dark is obtained by combining a superdark, composed of 120 individual darks obtained over the last year, and a weekly dark, usually comprising 5 dark images. If the dark current in the weekly dark differs from the superdark value by more than a certain threshold, the value is taken from the weekly dark: the implication is that the pixel is probably hot, and thus the more recent value is preferred. If the dark current differs by less than the threshold, the more precise value from the superdark is preferred. With this scheme, the noise is minimized for the majority of the pixels where there is no indication of change, while at the same time new hot pixels are tracked properly. The threshold is set at 5 times the 3-sigma-clipped rms dispersion of the pixels in the weekly dark, measured separately in the four chips; the value actually used is reported in a HISTORY comment in the header.

The hot pixels (those whose value comes from the weekly dark) are identified in the associated Data Quality file by setting either the 10th or the 11th least significant bit (values 512 or 1024). The value 1024 is used if the pixel value is close (within 0.003 e/s) to the value from the previous week; such pixels can be considered "fixable", since their dark current has remained stable. The value 512 is used if the dark current differs from the previous week's value by more than 0.003 e/s; such pixels are probably "new" hot pixels, and dark current subtraction is inherently more uncertain, since its value changed some time within the previous week. This use of the values 512 and 1024 is consistent with that of the task for warm pixel correction, warmpix, recently made available as part of the latest STSDAS release. In addition, the second least significant bit is set (value of 2) if the pixel value is flagged in the dark frame from which it is taken; this happens, for example, if an insufficient number of the weekly dark frames contained a valid value (multiple CR hits, image residuals).

New Software to Identify Bias Jumps in WFPC2 Data Goes Online:

by C. O'Dea, M. McMaster, and J.C. Hsu

As part of the effort to automate the data quality assessment of HST data, the WFPC2 group has developed an algorithm to identify bias jumps in WFPC2 images. In the past, these regions were identified by members of the OPUS staff who manually looked at each image on a workstation and informed the observers of the anomaly. As of August 9, the calibration pipeline performs this task automatically. Besides saving manual labor, the new software provides a more objective and more sensitive method of finding bias jumps. Currently our procedure is the following (note that the exact criteria may evolve as we gain more experience with bias jump detection). In CALWP2, a routine examines a subset (from columns 5 to 14) of the bias strip (.x0h file) and determines and compares the mean values in 8 bins along the bias columns. Any change in the bias level greater than 0.09 DN is noted in the trailer and header files for the calibrated images. For jumps greater than 0.5 DN a warning message is issued in these two files and is also placed in the PDQ file for that observation. Note that large changes in the bias level can also occur if there is a highly saturated star or missing data in the image.

At this point in time the message is purely informational and no correction is done within CALWP2. The bias jumps can be corrected by the observer if so desired using separate bias levels for the appropriate ranges of rows. Please see ISR 95-06, "A Field Guide to WFPC2 Image Anomalies", for instructions on how to correct for bias jumps.

Dithering, Drizzling and Cosmic Rays:

by A. Fruchter

As is well known by WFPC2 users, the pixelation of the PC substantially undersamples the HST point spread function (PSF) in the blue, and the WF undersamples the PSF thoughout the optical spectrum. Although much high spatial frequency information in the image is permanently destroyed by smearing with the response of the "fat" pixels, the quality of the image can nevertheless be greatly improved by combining sub-pixel dithered images. In sub-pixel dithering, the pointing of the telescope is moved by small non-integral pixel amounts between exposures. Each of the pixels from the different exposures can then be thought of as sampling a final, higher-resolution image, which is the "true image" of the sky convolved with the optical PSF and the pixel-response function of the CCD. If the dithers are particularly well-placed, one can simply interlace the pixels from the images on a finer grid, but in practice imperfect offsets, and the effect of the geometric distortion on offsets as small as one arcsecond, can make interlacing quite difficult.

Another standard simple linear technique for combining shifted images, descriptively named "shift-and-add", has been used for many years to combine dithered infrared data on a finer grid. However, it is difficult to use shift-and-add in the presence of missing data (e.g. from cosmic-rays) and geometric distortion. Furthermore, shift-and-add again convolves the image with the "fat" pixel, causing an additional loss of resolution. In the presence of small shifts, where geometric distortion is not significant, one can also use Richardson-Lucy Bayesian image restoration, which is incorporated in STSDAS through the task acoadd, written by Richard Hook and Leon Lucy. However, in addition to being unable to handle large dithers, the present implementation of this technique is limited by typical computing capabilities to combining either small regions of many images, or the entire image. In addition, the present task cannot accomodate the changing shape of the PSF across the WFPC field of view, and the Richardson-Lucy method, like all non-linear techniques, produces final images whose noise properties are difficult to quantify.

For purposes of combining the dithered images of the Hubble Deep Field, Richard Hook and I developed a new algorithm for the linear combination of images known formally as variable-pixel linear reconstruction and informally as "drizzling." Drizzling can be thought of as a set of linear "functionals" that vary smoothly from the optimum linear combination technique -- interlacing -- to the old-standby, shift-and-add. The degree to which one must depart from interlacing and move towards shift-and-add is determined by the nature of the input data. Drizzling naturally handles both missing data and geometric distortion, and can largely remove the effect on photometry produced by the geometric distortion of the WFPC camera. We have now improved the code used in the HDF, making it more versatile, user-friendly, and far less CPU-intensive. This code will be incorporated into future versions of STSDAS. However, for those who wish to use the code in the near future, we plan on releasing Drizzle V1.0 on September 15, and making it available via the WWW. The code is written in fortran, and uses the IRAF F77/VOS library. It has been tested extensively on Solaris Unix, but should work on any IRAF V2.10 installation supporting F77/VOS. Installation of the software is quite simple. A "poster" paper which describes drizzling can be found at:

http://www.stsci.edu/~fruchter/dither/dither.html

This page also provides links to other documents about dithering and will provide the address for retrieval of the drizzle software. Users should also see the WFPC2 FAQ WWW page for further information about dithering.

Ivo Busko and I are presently working on a suite of IRAF tasks that will help users measure the offsets between dithered HST observations. We have based these on the cross-correlation tasks used in processing the HDF. We expect these tasks to enter wide-spread testing by the WFPC group in September, and hope to make them widely available shortly thereafter.

Finally, many observers will be interested to know that substantial progress has been made on using the drizzle code to remove cosmic rays from dithered images. Although the process involved requires multiple drizzles, and at least one inversion of the drizzle process, the computer resources required should be well within the means of most observers. The poster paper mentioned above displays an image created from twelve 2400s HST WF2 images, no two of which had the same pointing and each of which had approximately 5% of the pixels corrupted by cosmic rays.

We are now working to determine the degree to which one can remove cosmic rays while insuring accurate stellar photometry, as well as the minimum number of dithered images necessary to do an acceptable job of cosmic ray removal. The results so far are encouraging, and suggest that a significant number of users may be able to forego integral dithers or multiple images at a single dither position. We expect to report further on our progress before the next round of Phase II proposals are due.

WFPC2 Phase 1 Preparation Resources on WWW:

by J. Biretta

On-line resources useful for Phase 1 proposal preparation, including the new WFPC2 Handbook and the Exposure Time Calculator, have been collected on a single WWW page which can be accessed via the WFPC2 Homepage.

RECENT PREPRINTS:

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.

FERRARESE, L.; LIVIO, M.; FREEDMAN, W.; SAHA, A.; STETSON,
P.B.; FORD, H.C.; HILL, R.J.; MADORE, B.F.  "Discovery of a
nova in the Virgo cluster M100"  ApJ accepted

SAHA, A.; SANDAGE, A.; LABHARDT, L.; TAMMANN, G.A.;
MACCHETTO, F.D.; PANAGIA, N.  "Cepheid calibration of the
peak brightness of SNe Ia. VI. SN 1960F in NGC 4496A"  ApJS
12-96

WILLIAMS, R.E.; BLACKER, B.; DICKINSON, M.; VAN DYKE DIXON,
W.; FERGUSON, H.C.; FRUCHTER, A; GIAVALISCO, M.; GILLILAND,
R.L.; HEYER, I.; KATSANIS, R.; LEVAY, Z.; LUCAS, R.A.;
MCELROY, D.B.; PETRO, L.; POSTMAN, M.; ADORF, H.-M.; HOOK,
R.N.  "The Hubble Deep Field: observations, data reduction,
and galaxy photometry"  AJ 10-96

BELL, J.F. III; WOLFF, M.J.; JAMES, P.B.; CLANCY, R.T.;
LEE, S.W.; MARTIN, L.J.  "Mars surface mineralogy form
Hubble Space Telescope imaging during 1994-1995:
observations, calibration, and initial results"  JGR-Planets

FORBES, D.A.  "Globular cluster luminosity functions and
the Hubble constant from WFPC2 imaging: galaxies in the
Coma I cloud"  AJ accepted

GRILLMAIR, C.J.; LAUER, T.R.; WORTHEY, G.; FABER, S.M.;
FREEDMAN, W.L.; MADORE, B.F.; AJHAR, E.A.; BAUM, W.A.;
HOLTZMAN, J.A.; LYNDS, C.R.  "Hubble Space Telescope
observations of M32: the color-magnitude diagram"  AJ

LAUER, T.R.; TREMAINE, S.; AJHAR, E.A.; BENDER, R.;
DRESSLER, A.; FABER, S.M.; GEBHARDT, K.; GRILLMAIR, C.;
KORMENDY, J.; RICHSTONE, D.  "Hubble Space Telescope
observations of the double nucleus of NGC 4486B"  ApJ

SIMPSON, C.; WILSON, A.S.; BOWER, G.; HECKMAN, T.M.;
KROLIK, J.H.; MILEY, G.K.  "A one-sided ionization cone in
the Seyfert 2 galaxy NGC 5643"  ApJ 1-1-97

HEATHCOTE, S.; MORSE, J.A.; HARTIGAN, P.; REIPURTH, B.;
SCHWARTZ, R.D.; BALLY, J.; STONE, J.M.  "Hubble Space
Telescope observations of the HH47 jet: narrow band images"

MADAU, P.; FERGUSON, H.C.; DICKINSON, M.E.; GIAVALISCO, M.;
STEIDEL, C.C.; FRUCHTER, A.  "High redshift galaxies in the
Hubble Deep Field. Color selection and star formation
history to z ~ 4"  MNRAS accepted

MORRISSEY, P.F.; FELDMAN, P.D.; CLARKE, J.T.; WOLVEN, B.C.;
STROBEL, D.F.; DURRANCE, S.T.; TRAUGER, J.T.  "Simultaneous
spectroscopy and imaging of the Jovian aurora with the
Hopkins Ultraviolet Telescope and the Hubble Space
Telescope"

APPENDIX: WFPC2 Contacts:

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 on the WWW at http://www.stsci.edu/public/propinfo.html.

Analysis, STSDAS or any other questions can also be addressed to help@stsci.edu.

To subscribe or unsubscribe send a message to listserv@stsci.edu with the Subject: line blank and the following in the body:

            [un]subscribe wfpc_news YOUR NAME

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