Session 41 - Visible & UV Telescopes.
Display session, Thursday, January 08
Exhibit Hall,

## [41.09] Aperture Corrections for WFPC2 Stellar Photometry

A. Suchkov (STScI)

We present photometric corrections to compensate for the systematic change in focus position of the Hubble Space Telescope (HST) during its operations. The metering truss of HST has shrunk regularly over the years, presumably because of water desorption, at an average rate of about 0.75 \mu m/month over the active life of WFPC2. The shrinkage has been compensated by regular movements of the HST secondary mirror, ensuring that the focus has always been within 3 \mu m of the nominal value. The shrinkage of the metering truss has been much slower over the last year or so, so that future focus adjustments will be smaller and less frequent.

In between secondary mirror adjustments, the accumulating focus drift results in changing aperture corrections, which impact small aperture photometry. We have derived aperture corrections phased with the focus drift using the WFPC2 data accumulated over three years of observations of a standard star. The correction is well described by a quadratic fit to the nominal focus position, with an amplitude that depends on the filter, the chip and the aperture used. The fitted correction can be used to better assess photometric accuracy and to adjust the photometry, if needed.

Using composites of tens of standard star images, we have also derived and tabulated mean aperture corrections to large aperture radii (infinity'') for some of the most often used WFPC2 filters. The corrections may be helpful in estimating the amount of light beyond the photometry aperture in cases when large apertures cannot be used.