9918(  1) - 04/04/03 10:15  - [  1]

    PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS   ST ScI Use Only
                                                       ID:      9918
                                                       Version: 1
                                                       Check-in Date: 04-Apr-2003
                                                                      10:15:07

1.Proposal Title:
CTE Corrections for WFPC2 and ACS
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2. Proposal For  3. Cycle
AR               11
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4. Investigators
                                                                                     Contact?
    PI: Andrew Dolphin                    National Optical Astronomy Observatories,
                                          AURA

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5. Abstract

The error budget for optical broadband photometry is dominated by three factors: CTE
corrections, long-short anomaly corrections, and photometric zero points. Questions
about the dependencies of the CTE have largely been resolved, and my CTE corrections
have been included in the WFPC2 handbook and tutorial. What remains to be done is
the determination of the "final" CTE correction at the end of the WFPC2 mission,
which will increase the accuracy of photometry obtained in the final few cycles. The
long-short anomaly is still the subject of much debate, as it remains unclear
whethere or not this effect is real and, if so, what its size and nature is.
Photometric zero points have likewise varied by over 0.05 magnitudes in the
literature, and will likely remain unresolved until the long-short anomaly is
addressed (given that most calibration exposures are short while most science
exposures are long). It is also becoming apparent that similar issues will affect
the accuracy of ACS photometry, and consequently that an ACS CTE study analogous to
my WFPC2 work would significantly improve the calibration of ACS. I therefore
propose to use archival WFPC2 images of omega Cen and ACS images of 47 Tuc to
continue my HST calibration work. I also propose to begin work on "next-generation"
CTE corrections, in which corrections are applied to the images based on accurate
charge-trapping models rather than to the reduced photometry. This technique will
allow for more accurate CTE corrections in certain cases (such as a star above a
bright star or on a variable background), improved PSF-fitting photometry of faint
stars, and image restoration for accurate analysis of extended objects.
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                                                9918(  1) - 04/04/03 10:15  - [  2]

            Summary Form for Proposal  9918

  Item                         Used in this proposal
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  Proposal Category            AR
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