WFPC2 Cycle 9 Calibration Plan
S. Baggett, S. Gonzaga, J. Biretta, S. Casertano, I. Heyer, A. Koekemoer,
M. McMaster, C. O'Dea, A. Riess, A. Schultz, B. Whitmore, M. S. Wiggs
Abstract
This report presents the details of the WFPC2 Cycle 9 calibration plans. The proposal suite is aimed at maintaining the calibration accuracy of WFPC2 via monitoring programs, as well as continuing some previous proposals into Cycle 9 and performing several new tests. The standard monitoring programs will be continued (e.g., decontaminations, photometric and astrometric monitors, darks, biases, internal flats, and earthflats). In addition, the CTE monitor, PSF and photometric checks will be performed. Three new programs are planned as well: a red leak check, a verification of the wavelength stability of the narrowband and linear ramp filters, and a CTE calibration. The CTE calibration, in conjunction with WIYN groundbased observations (if approved), will permit a direct verification of the absolute photometric calibration of WFPC2 in observations that may be affected by CTE, providing a more robust determination of the zero point for many WFPC2 observations.
Calibration plans for Cycle 9 are being finalized and as always, observers are welcome to suggest new calibrations or comment on the existing plans (email help@stsci.edu). The Cycle 9 calibration program is aimed at maintaining the calibration of WFPC2 via monitoring programs, as well as continuing two proposals from previous Cycles into Cycle 9 (photometric and PSF characterizations) and performing several new tests (on-orbit redleak check, CTE, wavelength stability check of narrowbands and linear ramp filters). A short overview is provided in Table 1; the entire program is estimated to require 67 external orbits (including several extra orbits to allow for smooth transitions between Cycles and a ~10% reserve for unanticipated items) and 2452 internal or occultation periods.
As in the past, a large part of the program consists of routine monitors and decontamination (decon) procedures. The decons will continue to be performed on a monthly basis, to remove the UV contaminants and anneal hot pixels. The monitoring programs will be used to track the health and performance of the cameras via internal exposures (biases, darks, flats, kspots), Earth flats, and external exposures (imaging of standard stars and fields, e.g., UV throughput, CTE, and astrometric monitors).
Continuations of Special Programs
The photometric and PSF checks will be done once during Cycle 9. The photometric check (observations of the standard star GRW+70D5824 in PC1 and WF3 using filters that are not routinely monitored) will verify the zeropoints and contamination rates in the non-standard filters. The PSF check will consist of dithered observations of the crowded ω Cen field in the standard wide-band photometric filters, providing a high S/N, critically sampled PSF across the full WFPC2 field of view and over most of the visible wavelength regime. The program will support PSF fitting photometry, provide data to test PSF subtraction as well as dithering techniques (e.g., effects of OTA breathing and gain) and be a source of PSFs for the online WFPC2 PSF Library.
Three new programs are planned for Cycle 9: a redleak check, a CTE calibration, and a wavelength stability check of the narrowband and ramp filters.
The redleak check will provide on-orbit verification of the red leaks in the WFPC2 UV filters. Two solar analog targets, for which FOS spectrophotometry is available, will be observed; measured countrates will be compared to SYNPHOT predictions of the expected count rates from the UV proper and from the red leak. The results should benefit programs that rely on precision multicolor photometry and comparison with model spectra; some discrepancies reported thus far could be explained by a significant (> 10%) error in the estimated red leak.
There are two parts to the planned CTE program: a monitor and a suite of observations to complement a proposed groundbased CTE program. The monitor, an extension of proposal 8447, will consist of observations of ω Cen (NGC 5139) taken every 6 months, in F814W and F555W, with a variety of preflash levels (20 to 1000 electrons).
The absolute calibration portion of the CTE program will consist of observations of previously observed globular clusters, matching targets selected for a companion ground-based proposal - subject to approval of the latter. Three globulars will be chosen from a list of five (Eridanus, NGC 2419, Pal 3, Pal 4, and Pal 14); the ω Cen field, while ideal for HST observations, is too crowded for groundbased observations at the relevant magnitudes (V~22). The HST observations will maximize the benefit from the groundbased observations and provide: 1) An independent, absolute determination of the current CTE effect, 2) A measure of time variations of CTE (by comparison with archival data), and 3) A set of observations bracketing the range of signal and background in science images (200 to 1000 electrons per star, on backgrounds ranging from 10 to 300 electrons). Comparisons of the HST and groundbased datasets will provide a direct verification of the zeropoints for many WFPC2 observations: while there is no evidence that the current WFPC2 photometric calibration is inapplicable to faint sources, enough corrections need to be applied that a direct verification is extremely desirable.
Finally, a check of the wavelength stability of the narrowband and ramp filters will be performed: on-orbit ramp filter VISFLATs, crossed and uncrossed with narrowband filters, will constrain the wavelength and transverse calibration of the ramps filters relative to the narrow band filters. Comparison with similar Cycle 4 data will allow us to evaluate whether the filter properties have evolved with time due to annealing / shrinkage of the thin film materials. In addition, to provide an absolute test for changes in the ramp filters, four external orbits will be used for observations of an extended line emission source (planetary nebula) through the ramp filters.
Planning has also started for the health and safety monitoring, as well as calibration verification, during and following the up-coming Service Mission 3b which is nominally scheduled for June 2001. The Servicing Mission plans, however, will be developed separately from Cycle 9, and details posted at a later date.
Format
Table 1 summarizes the Cycle 9 calibration programs for WFPC2; the remainder of this report contains details for each proposal, including purpose, observing description, and estimates of the accuracy goals and resources required. The proposal pages are intentionally in landscape format, to facilitate reporting of the results at the end of Cycle 9. Phase II versions of the proposals can be viewed at http://www.stsci.edu/public/propinfo.html .
WFPC2 Cycle 9 Calibration Plan
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Decons, phot. & focus monitor, internals, UV throughput, VISFLATs, and UV FLATs |
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8826 - |
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Includes INTFLAT monitor, for possible future preflashed observations. |
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Includes monitor as well as followup to groundbased observations. Outsourcing candidate. |
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Check of wavelength/aperture mapping and test for changes in LRFs. |
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