S T A N / W F P C 2 - Number 15, May 1996
- WFPC2 NEWS:
- WFPC2 Instrument Handbook Expanded and Updated
- WFPC-2 CTE and Long vs. Short Exposure Calibration Problems
- REMINDER ABOUT CYCLE 6 SCHEDULING
- RECENT PREPRINTS
- APPENDIX: WFPC2 CONTACTS
WFPC2 Instrument Handbook Expanded and Updated:
by John Biretta
The WFPC2 Instrument Handbook has been updated and expanded, and copies are now available. New and significantly revised material includes:
- Observation Strategies -- a new chapter (Ch. 7) including:
- Faint targets
- Bright targets
- Faint targets near bright ones
- Choosing exposure times
- Choosing ORIENTs and POS TARGs w/ examples
- Pointing accuracy / dither strategies
- Linear Ramp Fiters
- Performance comparisons to FOC, NICMOS, and STIS.
- Signal-to-noise and exposure time estimation. Generalized equations are now given, as well as simplified forms for faint and bright target limits. Many examples are given for both manual calculation and for the WFPC2 on-line Exposure Time Calculator (Ch. 6)
- Plots of point source SNR vs. exposure time vs. magnitude are given for the 14 most popular filters (Appendix B). Saturation limits are included.
- CCD dark current and MgF2 window glow (Ch 4.)
- Charge Transfer Efficiency problems (Ch 4).
- PSF variations with time, OTA focus, and field position (Ch. 5).
- Examples of common image artifacts (Ch. 4, 5, 7).
- Time dependence of the UV throughput (Ch. 6).
- Overhead times and camera timing (Ch. 2, 7).
- Corrected FQCH4N aperture names (Ch. 3).
- Cycle 6 calibration plan (Ch. 8).
- An index.
Paper copies of this new version (V.4) can be obtained by sending e-mail to email@example.com.
WFPC-2 CTE and Long vs. Short Exposure Calibration Problems:
by Harry Ferguson, Stefano Casertano, and John Biretta
There are two small but significant systematic calibration uncertainties that can affect stellar photometry with the WFPC2 under certain conditions. These have been described in the WFPC2 Handbook, in previous STANs, and in Holtzman, et al. (1995). The first is the charge-transfer efficiency (CTE) problem which causes some signal to be lost when charge is transferred down the chip during readout.This has the effect of making objects at higher row numbers (more charge transfers) appear fainter than they would be if they were at low row numbers. The second is an apparent difference in the measured flux from stellar sources in short exposures (~200s) vs. that measured in long exposures (~1000s). We briefly describe these effects here, and give WWW links to detailed reports of on-going studies.
For sources exposed to more than 1000 DN with backgrounds typical of wide-bandpass optical filters CTE effects introduce systematic errors up to 0.03 mag in photometry, which can be largely removed by the photometric ramp (or row number) correction previously described elsewhere. For sources exposed to only a few hundred DN the systematic effects are potentially larger. For exposures with significant background (>50 electrons, i.e. >600s in F555W) the error should be 0.05 mag or less. For exposures with little background (exposures <100s, or narrow band filters) the errors are harder to quantify in the existing calibration observations, but may be in the range ~0.05-0.1 mag at 100 DN. For sources exposed to a few 10's of DN, CTE may introduce photometric errors of up to 0.2 mag, in cases where the background is very low (i.e. very short exposures and narrow band filters). It is also apparent that this effect should be largely removed by a preflash of ~160 electrons (although the overhead time and photon noise associated with preflashing is significant).
While the CTE effect shows a strong dependence on row number, the magnitude offset in short vs. long exposures persists even at low row numbers, and results in faint targets being too faint by up to ~0.05 mag in short exposures. This offset appears to be significantly smaller in small apertures (2 or 3 pixel radius). An empirical correction for this effect is discussed, which consists of adding ~2 electrons to each pixel in the target aperture.
The STScI WFPC2 group is working to understand these problems and provide a calibration. In the meantime, it was thought useful to make available some of the analysis done in the last few months. The CTE web pages show a partial analysis of frames taken with different exposure times and different preflash levels. The descriptions are culled from viewgraphs and are a bit terse. These pages will evolve over the next few months as our understanding progresses.
Please send comments or questions to the undersigned.
Harry Ferguson (firstname.lastname@example.org) Stefano Casertano (email@example.com)
Reminder about Cycle 6 Scheduling:
The Institute is currently constructing the Long Range Plan for Cycle 6. Since the TAC awarded roughly half of the time in cycle 6 to FOS and GHRS observations and since all of these observations must be completed before the FOS and GHRS are removed during the servicing mission (currently scheduled for February 1997), most FOC, WFPC2, and FGS observations, except for those which are time-critical, will execute during the second half of Cycle 6. As has been true in previous cycles, there may be a few observations which take place beyond the formal end of the cycle (June 1997) to ensure their completion.
Users should also be aware that the Long Range Plan is intended to increase the predictability of when your observation is taken, especially for the purpose of obtaining coordinated observations and for allowing everyone to make final checks on their HST observing setups. There are many reasons which the LRP can change, including, in Cycle 6, the need to reschedule any failed FOS and GHRS observations quickly.
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.
BALLY, J.; MORSE, J.; REIPURTH, B. "The birth of stars: Herbig-Haro jets, accretion and proto-planetary disks" Sci. with the HST 2 DE KOFF, S.; BAUM, S.A.; SPARKS, W.B.; BIRETTA, J.; GOLOMBEK, D.; MACCHETTO, F.; MCCARTHY, P.; MILEY, G.K. "HST snapshot survey of 3CR radio sources counterparts. I. Intermediate redshifts" ApJS accepted EDMONDS, P.D.; GILLILAND, R.L. "K giants in 47 Tucanae: detection of a new class of variable stars" ApJ 6-20-96 KOO, D.C.; VOGT, N.P.; PHILLIPS, A.; GUZMAN, R.; WU, K.L.; FABER, S.M.; GRONWALL, C.; FORBES, D.A.; ILLINGWORTH, G.D.; GROTH, E.J.; DAVIS, M.; KRON, R.G.; SZALAY, A.S. "Redshift z ~ 1 field galaxies observed with the Keck telescope and the HST" ApJ accepted LUNDGREN, S.C.; FOSTER, R.S.; CAMILO, F. "Hubble Space Telescope observations of millisecond pulsar companions: constraints on evolution" IAU Colloq. 160 - in Camilo preprint VOGT, N.P.; FORBES, D.A.; PHILLIPS, A.C.; GRONWALL, C.; FABER, S.M.; ILLINGWORTH, G.D.; KOO, D.C. "Optical rotation curves of distant field galaxies. I. Keck results at redshift z ~ 2" ApJ accepted BAUM, S.A.; O'DEA, C.P.; DE KOFF, S.; SPARKS, W.; HAYES, J.J.; LIVIO, M.; GOLOMBEK, D. "HST observations of obscuration rings in Hercules A: implications for energy transport in powerful radio galaxies" ApJ 6-20-96 FERRARESE, L.; FORD, H.C.; JAFFE, W. "Evidence for a massive black hole in the active galaxy NGC 4261 from Hubble Space Telescope images and spectra" ApJ accepted RENZINI, A.; BRAGAGLIA, A.; FERRARO, F.R.; GILMOZZI, R.; ORTOLANI, S.; HOLBERG, J.B.; LIEBERT, J.; WESEMAEL, F.; BOHLIN, R.C. "The white dwarf distance to the globular cluster NGC 6752 (and its age) with the Hubble Space Telescope" ApJ accepted VON HIPPEL, T.; GILMORE, G.; TANVIR, N.; ROBINSON, D.; JONES, D.H.P. "The metallicity dependence of the stellar luminosity and initial mass functions: HST observations of open and globular clusters"
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.
Analysis, STSDAS or any other questions can also be addressed to: firstname.lastname@example.org.
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