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Hubble Space Telescope
WFPC2 STAN

S T A N / W F P C 2 - Number 34, January 1999

CONTENTS:

Drizzling Cookbook Available:

by S. Gonzaga, J. Biretta, et al.

A "Drizzling Cookbook" has been completed to aid observers in using the new drizzling package software for combining dithered images. Most long imaging observations made with HST now use position dithers to aid in removal of detector artifacts, as well as for enhancement of the spatial resolution. After the data are obtained, the images are aligned and combined, usually with the Drizzle software package. The new Cookbook gives detailed, step-by-step examples for drizzling WFPC2 data in a variety of different situations (dense and sparse fields, extended galaxies, planetary nebulae), as well as examples for STIS and NICMOS images. The cookbook includes on-line command scripts and raw images for each example, so that users can follow along or experiment with different parameter settings on their own. For links to the cookbook, software, frequently-asked questions list, and more, see the new WFPC2 Drizzling WWW Page.

Release of Hubble Deep Field South Data:

by S. Casertano and HDF-S Team

A second Hubble Deep Field campaign (HDF-S) was carried out between late September and October of 1998. The raw, pipeline calibrated and reprocessed data were released to the community Nov 23,1998. The rationale for undertaking a second deep field campaign followed from the wealth of information that has come out of HDF-N, and from the desire to provide a point of focus for similar studies of the distant universe from southern-hemisphere facilities.

The wide public access to the HDF-N data stimulated extensive followup observations across the electromagnetic spectrum, both from major ground-based observatories and from other satellites. A similar level of effort is anticipated for HDF-S. The actual observations of the HDF-S were similar in spirit to the original HDF: approximately 150 consecutive orbits were devoted to a single telescope pointing. Additional flanking field observations were made surrounding the deep STIS, WFPC2 and NICMOS fields.

Modification of WFPC2 SAA Avoidance Contour:

by J. Biretta and S. Baggett

The South Atlantic Anomaly (SAA) contains high concentrations of energetic particles which obliterate data when they pass through pixels of the WFPC2 CCDs. In order to avoid serious image degradation, WFPC2 images are normally not taken while HST passes through the SAA. While this optimizes WFPC2 images, it also reduces the number of orbits per day where WFPC2 observations can be scheduled. In an effort to improve the efficiency of HST scheduling, an analysis was performed to investigate whether the SAA avoidance region for WFPC2 could be reduced slightly, so as to open up more orbits for scheduling.

The cosmic ray rates found in several hundred WFPC2 images taken in and near the SAA (from special observations), as well as Air Force particle flux data from the SAA, imply that the avoidance area could be reduced slightly without unduly degrading WFPC2 data. Specifically, there is a significant region inside the East edge of the current avoidance contour where the cosmic ray rates are relatively low (within a factor of 2 to 3 of the average orbital value). Based on these findings, a new, smaller SAA avoidance region has been established for WFPC2 and will be implemented in the coming weeks.

The impact on data quality should be small: for most HST orbits the spacecraft is in this region for only a short time (about one minute), so the number of extra cosmic ray events is small. Only ~0.1% of WFPC2 images should see any increase in cosmic rays as a result, but ~10% more time becomes available for scheduling long visits. The new SAA contour will increase the number of SAA-free orbits by ~1/2 to 1 orbit per day.

Dark Current Evolution:

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

The dark current level in WFPC2 has been slowly increasing over the instrument's lifetime. A recent study found that from 1994 to 1998, the dark current increased by a factor of about 2.2 in the WFC CCDs and by a factor of 1.3 in the PC (values are for the center of the CCDs). A small increase in the cold junction temperatures over this time period was detected as well; however, the amount of temperature change accounts for only a very small portion of the increase in dark current. The dark current increase is smaller in the optically vignetted regions near the CCD edges, suggesting that some of the effect may be caused by increased flourescence or scintillation in the CCD windows, rather than by the CCDs themselves. Note that the increase in dark signal we report here affects all pixels, and thus is distinct from the regular increase in the number of hot pixels reported in the WFPC2 Handbook. The latter are highly localized, and are almost certainly due to radiation-damaged sites on the CCD detectors.

Long-Range Schedule/Limited Opportunities for Long Target Visits:

by J. Biretta

A recent examination of the HST long-range schedule found that opportunities for long, contiguous observations (longer than 5 orbits) are severely over-subscribed. This is due to a number of factors, including a trend towards longer observations of fewer targets, and the recent suspension of NICMOS observing.

One solution is to split the long observations into smaller pieces. GOs with visits longer than 5 orbits have been requested to split these into smaller visits (notices sent by e-mail), and submit a revised phase 2 by 15 January. Splitting these long visits will avoid further scheduling delays and help observers obtain the same data sooner than otherwise possible. Cycle 8 observers will be required to use visits of 5 orbits or less.

A study has been performed on the science impact of splitting long observations into several shorter ones (less than 4 orbits). For most programs, the primary impact would be caused by small pointing errors introduced by the full acquisition at the start of the second half of the revised observation. From archival data, it appears that a full acquisition has a position uncertainty of about 20 mas RMS, when compared to a previous acquisition of the same target. (We note that this is different from the usual "pointing stability" between exposures, which is about 5 mas RMS once a target is acquired.) This uncertainty is directly related to the resolution of the Fixed Head Star Trackers in determining the spacecraft roll angle. GOs with long visits have also received material discussing strategies to minimizing the impacts of the pointing offsets.

To further improve the efficiency in the long-range plan, we also reduced the South Atlantic Anomaly avoidance region for WFPC2 (see Note above). Finally, for programs with short exposures, more effort is being made to utilize short visibility periods which are often available in the schedule, rather than requiring the usual ~56 minutes of visibility per orbit. We believe these changes will be transparent to most observers and will mitigate the over-subscription.

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.

BARKANA, R.; BLANDFORD, R.; HOGG, D.W.  "A Possible
Gravitational Lens in the Hubble Deep Field South"  ApJ

KUNDU, A.; WHITMORE, B.C.; SPARKS, W.B.; MACCHETTO, F.D.;
ZEPF, S.E.; ASHMAN, K.M.  "The Globular Cluster System in
the Inner Region of M87"  ApJ 3-99

PENTERICCI, L.; ROTTGERING, H.J.A.; MILEY, G.K.; MCCARTHY,
P.; SPINRAD, H.; VAN BREUGEL, W.J.M.; MACCHETTO, F.  "HST
Images and Properties of the Most Distant Radio Galaxies"
A&A accepted

STORRS, A.; WEISS, B.; ZELLNER, B.; BURLESON, W.; SICHITIU,
R.; WELLS, E.; KOWAL, C.; THOLEN, D.  "Imaging Observations
of Asteroids With Hubble Space Telescope"  Icarus accepted

CARAVEO, P.A.; MIGNANI, R.  "On the Crab Proper Motion"
Relationship Between Neutron Stars and SNRs - in Mereghetti
preprint

HULBERT, S.; NOTA, A.; CLAMPIN, M.; LEITHERER, C.;
PASQUALI, A.; LANGER, N.; SCHULTE-LADBECK, R.  "HST WFPCII
Observations of the Inner HR Car Nebula"  IAU Colloq. 169 -
in Nota preprint

SCHULTE-LADBECK, R.E.; PASQUALI, A.; CLAMPIN, M.; NOTA, A.;
HILLIER, D.J.; LUPIE, O.L.  "Imaging Polarimetry of Eta
Carinae with the Hubble Space Telescope"  IAU Colloq. 169 -
in Nota preprint

HINES, D.C.; SCHMIDT, G.D.; WILLS, B.J.; SMITH, P.S.;
SOWINSKI, L.G.  "The Misdirected Central Engine of the
Hyperluminous Infrared Galaxy and QSO-2 IRAS P09104+4109"
ApJ 2-10-99

GROSDIDIER, Y.; MOFFAT, A.F.J.; JONCAS, G.; ACKER, A.
"HST-WFPC2/H alpha Imagery of the Nebula M1-67: A Clumpy
LBV Wind Imprinting Itself on the Nebular Structure"

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 www.stsci.edu/public/propinfo.html.

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

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