Two monitoring programs are entirely new and deserve specific mention. The Supplemental Darks program (7621, 7712, 7713) aims at obtaining a large number of relatively short darks on a very frequent basis, with the main goal of helping users identify hot pixels in their observations. The program has been designed to place the least possible burden on the scheduling system; it is understood that these additional darks will have a low priority, and they will be scheduled whenever feasible. Under normal circumstances, this program will provide up to 21 additional 1000s darks per week, thus giving users a good chance of having a dark within half a day of their observations. The Astrometric Monitor program (7627) will monitor the relative placement of the four WFPC2 CCD in the focal plane, following indications that a shift of up to a pixel may have occurred since 1994. Although this is smaller than the quoted astrometric accuracy of the chip-to-chip transformations (0.\xfd 2), it appears worthwhile to follow closely the behavior of the camera in this area.
Special calibrations for Cycle 7 consist of only three programs. The first, Photometric Characterization (7628), continues our group's quest for the ultimate goal of 1% photometry. Observations taken in Cycles 5 and 6 allow us to understand and limit many of the photometric uncertainties in WFPC2 observations, but there is room for improvement, especially since the WFPC2 filters differ substantially from ground-based filter sets, and therefore stars with different properties must be observed to allow significant comparisons with ground-based photometry. In Cycle 6 we included a young LMC cluster, NGC 2100, and an old open cluster, M67, both with good ground-based photometry. This Cycle we plan to repeat some of the NGC 2100 observations and to add NGC 2419, a very distant globular cluster in the Milky Way, which will allow good coverage of the bright red giants, too bright and rare in nearby clusters. We will also, as usual, carry out a filter sweep on both our primary standard, GRW+70d5824, and our reference rich field in
Centauri.Another program, CTE Characterization (7630), aims at a thorough exploration of the various parameters that could affect the so-called "long vs. short" anomaly, that is, the observed difference in count rates between long and short exposures. Our group has been able to quantify this effect by taking advantage of archival data and of a limited number of pointed observations. However, it is clear that a complete characterization of this anomaly, comparable to what we have recently achieved for the "CTE ramp," requires an extensive set of dedicated observations in which each of the potentially critical parameters is varied in turn.
The PSF Characterization program (7629) will continue our accumulation of data for the WFPC2 PSF library, by addressing often-used filters such as F300W, F450W, F702W which were not included in previous cycles.
An additional set of Earth flats will be taken late in Cycle 7 due to the unusual length of this Cycle. Low-level temporal variations are typically discerned in the flats on time scales of about a year. These supplemental Earth flats will allow us to continue to monitor these changes, as well as make more accurate flats for use in the latter parts of Cycle 7 and early Cycle 8.
Finally, a further calibration of the Linear Ramp Filters (LRFs) will be performed. Earlier observations have demonstrated a throughput 7% lower than expectations based on laboratory filter tracings, and showed a scatter in the photometry of 10%. These observations will allow us to determine the stability of the offset and determine whether the scatter represents variability in the filter throughput or measurement errors. Additionally, exposures of the Orion Nebula at different pointings will allow a verification of the wavelength calibration of the LRFs near major nebular lines.
We now present a summary description of each calibration program, with a statement of the purpose of the program and of how it will be carried out, plus indications of the expected products. The desired and expected accuracy of each program is also explicitly stated. Table 3 summarizes the relevant data for all programs.
An electronic version of the calibration plan is available via the World Wide Web at URL:
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, [OIII], [NII] and [SII].