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Update to the WFPC2 Instrument Handbook
In This Document...
The WFPC2 PSF: Dynamic Range and Photometry / 6
Dithering with WFPC2 / 8
Other HST Imaging / 10
The WFPC2 Clearinghouse / 10
Updates to System Efficiencies and Zeropoints / 12
WFPC2 Calibration Plan / 14
References / 26
The Wide Field Planetary Camera 2 (WFPC2) is now a mature and largely well-characterized instrument. Information already available through the
WFPC2 Instrument Handbook and the
HST Data Handbook is fairly complete. In this update we provide additional information obtained from recent studies of the instrument, as well as calibration plans for the WFPC2 and a short note on present and planned imaging capabilities for HST. The major topics discussed in this document are:
- Charge Transfer Efficiency: Recent tests of the Charge Transfer Efficiency (CTE) of the WFPC2 charge-coupled diodes (CCDs) have shown that the fraction of charge "lost" to impurities in the CCD has grown with time. The CTE loss is particularly great in images with low backgrounds, and can, in some cases, significantly affect photometry. Users who plan to observe in modes that typically produce images with low backgrounds-for instance UV images, narrow-band images, and short exposures-should examine the section on CTE to see how this may affect their program.
- Point-Spread Functions: Progress has been made in recent years on the accurate subtraction of WFPC2 point-spread functions (PSFs) both to detect nearby faint companions of bright stellar objects and to obtain accurate stellar photometry. This document provides a short update describing the imaging dynamic range one can now expect to obtain near a bright stellar source, and our present understanding of the photometric accuracy one can obtain by PSF subtraction.
- Dithering: Dithering the telescope as a means of improving image resolution and removing detector defects has become increasingly popular among users of WFPC2 since the technique was successfully employed for the Hubble Deep Field. In this Update, we discuss the use of singly dithered observations (one image at each dither position). Software that we have recently developed can support such observations. The advantage of singly dithered observations is that more dither positions can be obtained in a given amount of time or number of exposures. The disadvantages are that the data is often far more difficult to reduce, and stellar photometry to better than a few percent cannot be guaranteed except in cases of excellent dithering coverage.
- Other Instruments: In addition to WFPC2, HST has two other imaging instruments, STIS and FOC, and in early 2000 the FOC will be replaced by the Advanced Camera for Surveys (ACS). A reminder of this fact, and its implications for planning observations, is provided.
- WFPC2 Clearinghouse: The WFPC2 group has developed a web-based tool, the WFPC2 Clearinghouse, to allow users to easily search journal articles, STScI documentation, and user-submitted documents for topics relating to the performance, calibration, and scientific use of WFPC2. This tool is described in the hope that the information made available by its use will help observers in the preparation of their observing plans, as well as in the reduction of their data.
- Calibration: Calibration of WFPC2 continues. We provide an updated table of system efficiencies and zeropoints, which differs slightly from that in the Instrument Handbook for certain narrow-band and UV filters. We also describe the plans for future calibration of WFPC2. Users who suspect that they will have unusual or particular calibration needs should examine the calibration plan to determine if they will need to take their own calibration observations as part of their HST observing program.
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Copyright © 1998, Association of Universities for Research in Astronomy, Inc. All rights
Last updated: 06/16/98 10:38:00