Instrument Science Report WFPC2 97-01:
Impact of Focus Drift on Aperture Photometry
A. Suchkov and S. Casertano
January 31, 1997
We have examined the impact of the long term focus position variations on small aperture photometry. For both the WF cameras and the PC, the effect has been found to be small for aperture radii of about 5 pixels or larger. At smaller aperture radii, the magnitude corrections for defocusing become important for absolute photometry if high photometric accuracy is needed. The aperture corrections phased with the focus drift are given for the filters F439W, F555W, and F814W for the PC and the W3 camera. They are compared with the early results in Holtzman, et al., (1995a). With regard to short term focus variations, we have found that, depending on the current average focus position, the aperture magnitudes may vary due to the effect of focus "breathing" by a few percent, and up to ~10% for the aperture radius, r=1 pixel. If high absolute accuracy is desired in aperture photometry, it is recommended therefore that either apertures of 5 pixels or larger be used, or the aperture correction be determined from the brightest stars in each observation. We have also found that, in the PC, the photometric zero point appears to have drifted away from its original value in a way that our primary standard now looks brighter than in April 1994 by about 0.05 mag. In WF, the data were affected by a bug in CALWP2 and cannot be used for checking this effect until they are recalibrated. Preliminary indications, however, are that there has been a similar brightening in WF as well.