S T A N / W F P C 2 - Number 11, December 1995

CONTENTS:

• Issues and Highlights for Phase II Proposal Preparation
• Upgrade to Exposure Time Calculator
• Photometric Zeropoint Difference Between Long & Short Exposures
• Preliminary Results of Contamination Study
• Planned Changes to Dark Calibration Reference Files
• Improved Flat Fields
• WFPC2 Cycle 4 Calibration Summary now Available
• Distribution List for Instrument Science Reports
• Recent Preprints
• WFPC2 Contacts

Issues and Highlights for Phase II Proposal Preparation:

by Chris Odea, Anatoly Suchkov, and John Biretta

The WFPC2 is performing normally and as described in the Instrument Handbook.

The updates include information on PSF subtraction, polarization observations, dithering strategy, contamination, new software tools, linear ramp filters, scattered light, etc. Some of the issues are summarized briefly below.

1. PSF Subtraction:

PSF subtraction (to search for faint objects near bright ones) is complicated by the large dynamic range required, the spatially dependent PSF, ghosts, and the undersampled PSF. Observers may want to consider various observation strategies including (1) taking multiple exposures with different exposure times (e.g., 1, 10, 100 sec), (2) observing a star in the same position as the object (also using multiple exposures), (3) taking 2 or more images with sub pixel dithering, and (4) obtaining observations at different roll angles.

2. Polarization:

Observers planning WFPC2 polarization observations should be aware that the WFPC2 polarizers are not designed for high precision work. Fractional polarization values measured with WFPC2 may have systematic errors of 0.03. These are due largely to the instrumental polarization induced by the pick-off mirror, and by uncertainties in the polarizer flat fields. Also, the polarizers become ineffective outside the wavelength range spanning filters F255W to F675W. Tradeoffs between ease of scheduling, obtaining optimum polarizer position angles, inter-CCD flatfielding, and use of the PC1 CCD must be considered. See ISR "WFPC2 Polarization Observations: Strategies, Apertures, and Calibration Plans" available on the WFPC2 WWW pages.

3. Dithering:

Integer pixel dithering is useful for removing detector artifacts. Sub pixel dithering is recommended for programs which would benefit from sampling structure on scales comparable to the pixel size. Two-position dithering produces a substantial gain in resolution, although four positions are recommended in cases where the additional overhead of dithering will not substantially affect the final signal-to-noise of the image. Observers should generally take at least two exposures at each position to allow cosmic ray removal. For Cycle 6 there is a new Optional Parameter "Dither-type" which can be used to generate simple box or line dither patterns (see section 9.1 of the Phase 2 proposal instructions). Position dithers should be kept to less than ~0.5 arcseconds if precise sub-pixel positioning is needed.

4. Photometry:

Currently 2-5% photometric accuracy is routinely attainable at visual and red wavelengths. UV performance is poorer, and is limited by the continuously changing contamination. One of the problems for high-precision photometry is the imperfect CCD Charge Transfer Efficiency (CTE). Experiments are underway to find observation strategies which can reduce this effect.

5. Contamination:

Far UV photometry is affected by contamination building up on the detector windows, and by monthly decontamination procedures to remove contaminants. For example, the throughput in F160BW can degrade by roughly 40% in a month, while the throughput changes at F336W are 5%. Special scheduling of UV observations (e.g. UV campaigns) are currently not supported. Results of routine photometric monitoring can be used to correct UV photometry at the few % level.

6. Software tools:

In preparing Phase 2 proposals, the observer may find the WFPC2 exposure time calculator (ETC) available on the WWW useful. ETC enables one to quickly estimate exposure times needed to reach a specified signal-to-noise ratio, for both point sources and extended sources.

7. Linear Ramp Filters:

The WFPC2 linear ramp filters provide a narrowband imaging capability at a central wavelength which is continuously adjustable from 3710 to 9762 Angstroms (with no wavelength gaps). The usable field of view is limited to a ~10 arcsecond diameter, and the filter passband FWHM is ~1.3% of the central wavelength. To use these filters, proposers should specify filter and aperture "LRF" on the phase 2 proposal, along with the desired central wavelength.

can help observers determine which CCD (WF or PC) will be automatically assigned for their observation, if this is an important consideration.

8. Red Leaks:

Observers using UV filters should recall that several of them have significant red leaks (e.g. F336W, F300W, F170W, ...). See section 6.6 of the WFPC2 Instrument Handbook.

9. Serial Clocks:

The serial transfer registers of the CCDs can be kept running during an observation (CLOCKS=YES). However, this mode is rarely used and is not well characterized. Thus we recommend its use only if star images will be so saturated (greater than 10^8 electrons) that significant bleeding off the chip is expected (see section 2.6 of the Instrument Handbook).

10. Stray Light Patterns:

Bright stars near the WFPC2 field of view (typically <14th mag.) can cause various reflection patterns on the CCDs. The largest effects occur for stars just outside the PC1 field. See memo on WFPC2 Advisories WWW page for further details, and also the "Field Guide to WFPC2 Image Anomalies," available on WWW.

11. Scattered Earth Light in Broad Band Visual/Red Filters:

Occasionally observations in broad visual and red filters are impacted by scattered Earth light in the OTA. This occurs most frequently for long (>600 sec.) exposures in the Continuous Viewing Zone (CVZ), but, in principle, can occur for any observation. In the most serious (and rarest) cases the background increases by ~10 DN/pixel and is modulated by strong diagonal bars across the CCD centers. Observers with deep imaging programs in the CVZ may wish to consider strategies (such as offsetting targets from the CCD centers) to reduce these effects; such strategies should be discussed with your Contact Scientist. See also "A Field Guide to WFPC2 Image Anomalies," available on WWW.

12. Tracking modes:

Two guiding modes are routinely available, (1) Gyro Hold, and (2) Fine Lock. Fine Lock is used by default since use of Coarse Track may be harmful to the Fine Guidance Sensors. Use of Gyros only is not generally recommended (even for SNAPs) since (1) the pointing accuracy is only 14" and the target may be missed on the PC, and (2) the drift rate is 0.0014" per sec and moderate exposures (> 100 s) will result in smearing of the image (see section 7.1.1 of the Phase 2 Instructions).

Upgrade to Exposure Time Calculator:

by John Biretta and Michael S. Wiggs

Version 2.0 of the WFPC2 Exposure Time Calculator will be installed around 15 Jan. 1996. Upgrades include full support of Linear Ramp Filters, optional stellar background light in SNR calculations, and improved help texts.

Photometric zeropoint difference between long & short exposures:

by Stefano Casertano and Massimo Stiavelli

Evidence for a possible photometric zeropoint difference for long versus short exposures has surfaced. This effect was first reported by GOs performing careful calibrations in support of the extragalactic distance scale key project. Further investigation shows that "long" vs. "short" is probably a misnomer. The zeropoint difference appears to be related to total counts, rather than to exposure length. The magnitude difference measured between short and long exposures is more pronounced for faint stars in large apertures, where it can reach 0.05 mag, and is essentially absent for stars with more than 1000 total counts. The dependence on aperture and magnitude appears consistent with a charge transfer efficiency problem. The offset of faint star magnitudes can be explained by a loss of 0.3 DN (2 e-) in each pixel used in the aperture. We are actively pursuing this problem with further testing and analysis and encourage those who require accurate absolute photometry for their observations to pay special attention to information concerning this topic on the WWW or to discuss it with their Contact Scientist.

Preliminary Results of Contamination Study:

by Brad Whitmore and Inge Heyer

The UV throughput of the WFPC2 is affected by the buildup of contaminants on the detector window. This is removed each month by heating the camera during a decontamination procedure. As part of our project to determine whether 1 % photometry can be achieved with the WFPC2 we are analyzing the set of Omega Cen calibration observations taken before and after each decontamination.

Our preliminary results indicate that: 1) the monthly contamination rate in F336W is about 4.5 % on the WF and about 3 % on the PC (Note: these values are slightly different than values quoted in the WFPC2 Instrument Handbook based on observations of a white dwarf), 2) F555W shows only 1% contamination in a month, 3) there is a weak gradient across the detectors with the largest contamination in the center. These results are based on aperture photometry with radii of 3 pixels. No obvious aperture dependence is seen for radii from 1 to 5 pixels.

The contamination can be well modeled by a linear decline as a function of time since decontamination, resulting in errors of 1.3% RMS in F336W observations once the contamination is removed. Much of the remaining scatter is probably due to focus variations, spectral variations, and other effects that we hope to model. Hence, better than 1 % photometry is quite possible in principle. However, a better understanding of other effects, some of which are discussed elsewhere in this newsletter (e.g., CTE), will be required to achieve 1 % accuracies in less idealized conditions (i.e., short exposures of bright stars).

The contamination analysis should be completed later this winter. If you have comments or suggestions on how this project should proceed please send them to Brad Whitmore at whitmore@stsci.edu, or Inge Heyer at heyer@stsci.edu.

Planned changes to dark calibration reference files:

by John Biretta, Stefano Casertano, and Harry Ferguson

We are planning to change the way dark calibration reference files are generated. The change should both reduce the noise contributed to science data by the dark reference file, and provide a better identification and correction of time-variable hotpixels. The change will take effect in late February 1996.

The current method for generating dark reference files is to "average" ten on-orbit 1800 sec. dark exposures using CRREJ in STSDAS to eliminate cosmic rays. Ten exposures was deemed necessary to reduce the noise to levels which would not impact most observers. However, we must usually wait a week or more to collect that many frames (the darks are scheduled with low priority to avoid displacing science observations). Hence, the dark reference files used for routine calibration (i.e. calibration pipeline) are typically 1 to 2 weeks out of date. Data calibrated though the pipeline therefore have only a partial correction for hotpixels.

The new method would use a "superdark" (average of ~100 dark frames) for most pixels. In addition, on a weekly (or semi-weekly) basis, the time-variable hotpixels would be identified in stacks of 4 or 5 on-orbit dark frames and placed in the "superdark." This would then be delivered as a reference file and used in the calibration pipeline. These new reference files would give improved signal to noise in most pixels, together with a more up-to-date correction for time-variable hotpixels.

This new scheme would also provide a very great improvement in the dark calibration for data taken immediately after decontaminations. Most of the time-variable hotpixels are removed during decontaminations. In the old scheme, data taken for ~1 week after the decon was still calibrated with the pre-decon dark reference file, which contains many hotpixels. In the new scheme, the "bare" superdark would be installed and used immediately after the decon, thus providing fairly accurate calibration of post-decon data.

Obtaining the best possible identification and correction of hotpixels will still require observers to either re-calibrate or use the WARMPIX task in STSDAS. This is because there will still be some delay (although reduced) between science observations and installation of reference files. Re-calibration requires obtaining a full set of calibration reference files, including the dark reference file for the epoch of the observation, from STScI and then running CALWP2 on the raw data. The later option, the WARMPIX program, requires obtaining only a hotpixel table for epoch of the observation, and the flat field reference file from STScI; WARMPIX can be run on the calibrated data. Note that these methods will identify or correct most hotpixels. However, some hotpixels show rapid variability, and hence the only method guaranteed to remove all hotpixels is to dither the telescope pointing during observation, and then discarding anomalous pixels when stacking the images during analysis.

While we are fairly committed to making this change, we will still welcome any suggestions or comments from observers. Please address comments to biretta@stsci.edu.

Improved Flat Fields:

by John Biretta and Michael S. Wiggs

A set of improved flats in filters F300W, F375N, F450W, F555W, F606W, and F814W have been generated and installed in the calibration pipeline. These primarily correct errors (up to 7% on WF2) in the illumination pattern near the CCD edges. Residual errors over the remaining field of view are thought to be less than 1% peak-to-peak at visual and red wavelengths. We anticipate delivering a second generation of improved flats in a few weeks which will include all (~43) filters. This second generation will correct slight gradients (~0.5%) across the field of view.

WFPC2 Cycle 4 Calibration Summary now Available:

by Sylvia Baggett

A new WFPC2 Instrument Science Report "WFPC2 Cycle 4 Calibration Summary" is now available on-line. Paper copies can be requested from help@stsci.edu.

"WFPC2 Cycle 4 Calibration Summary" WFPC2 ISR 95-07 by 
S. Baggett, S. Casertano, J. Biretta, December 22, 1995.

The Cycle 4 Calibration program was executed from late Spring 1994 through early Summer 1995. The main purpose of the Cycle 4 programs was to provide calibration for all Cycle 4 GO/GTO science programs as well as to monitor the health and photometric stability of the instrument. This report summarizes the proposals executed, products generated including accuracies where possible, a full list of documentation, and work in progress to complete the Cycle 4 calibration goals.

Distribution List for Instrument Science Reports:

If you would like to be included on the distribution list to receive paper copies of all Instrument Science Reports as they become available, please send your request to help@stsci.edu.

Of course, we will continue to post all ISRs on the WWW, and paper copies of individual reports can be requested by sending email to help@stsci.edu.

Recent Preprints:

AJHAR, E.A.; GRILLMAIR, C.J.; LAUER, T.R.; BAUM, W.A.;
FABER, S.M.; HOLTZMAN, J.A.; LIGHT, R.M.; LYNDS, R.;
O'NEIL, E.J. JR.  "Hubble Space Telescope observations of
globular clusters in M31. I. Color-magnitude diagrams,
horizontal branch metallicity dependence, and the distance
to M31"  AJ

BIGNAMI, G.F.; CARAVEO, P.A.; MIGNANI, R.; EDELSTEIN, J.;
BOWYER, S.  "Multiwavelength data suggest cyclotron feature
on the hot thermal continuum of Geminga"  ApJ accepted

DRIVER, S.P.; WINDHORST, R.A.; GRIFFITHS, R.E.  "Caught in
the act: the identification of the galaxies responsible for
the faint blue excess"  IAU Symp. 171

DRIVER, S.P.; WINDHORST, R.A.; PHILLIPPS, S.; BRISTOW, P.D.
"HST counts of elliptical galaxies: constraints on
cosmological models?"  ApJ 4-20-96

GUHATHAKURTA, P.; YANNY, B.; SCHNEIDER, D.P.; BAHCALL, J.N.
"Globular cluster photometry with the Hubble Space
Telescope. V. WFPC2 study of M15's central density cusp"
AJ accepted

KNEIB, J.-P.; ELLIS, R.S.; SMAIL, I.; COUCH, W.J.;
SHARPLES, R.M.  "Hubble Space Telescope observations of the
lensing cluster Abell 2218"

PASCARELLE, S.M.; WINDHORST, R.A.; DRIVER, S.P.; OSTRANDER,
E.J.; KEEL, W.C.  "The serendipitous discovery of a group
or cluster of young galaxies at Z ~ 2.40 in the deep HST
WFPC2 images"  ApJ 1-1-96

WINDHORST, R.A.; PASCARELLE, S.M.; KEEL, W.C.  "Deep HST
imaging of a galaxy cluster at z=2.40"  IAU Symp. 171

WFPC2 Contacts:

Any questions about the scheduling of your observations should be addressed to your PRESTO contact. Post-Observation questions should be addressed to help@stsci.edu (410-338-1082).

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