The WFPC2 (see [Holtzman et al. 1995a], [Voit 1997] and [Biretta et al. 1996]) is a two-dimensional imaging device covering a wavelength range from Lyman- to about 1 m. Built as a replacement for the original Wide Field and Planetary Camera (WF/PC), it includes built-in correction for the spherical aberration of the HST Optical Telescope Assembly (OTA), and was installed during the first HST servicing mission in December 1993.
A schematic of WFPC2 optical arrangement is shown in Figure 6.1. The WFPC2 field of view is located at the center of HST focal plane. The central portion of the /24 beam coming from the OTA is intercepted by a steerable pick-off mirror and is diverted into the WFPC2 proper. The beam then passes through a shutter and the filter assembly to fall onto a four-faceted pyramid mirror located at the aberrated OTA focus, which divides the image into four parts. After leaving the pyramid mirror, each quarter of the full field of view is relayed by an optical flat to a Cassegrain relay that forms a second image on a pixel CCD. Each detector is housed in a cell sealed by a MgF window, serving as a field flattener. The wavefront aberrations introduced by the OTA are thus corrected by introducing an equal but opposite error in each of the four Cassegrain relays. The resulting PSF is quite close to that originally expected for WF/PC. The four cameras have essentially identical optics, but one of these works at a smaller image scale. The Planetary Camera (PC) has a focal ratio of /12.9 and operates at an image scale of 45.5 mas/pixel, whereas the three Wide Field Cameras (WFCs) have a focal ratio of /28.3 and operate at 96.6 mas/pixel. The field of view is thus 36.4 and 77.28 arcsec in side for PC and WFCs respectively.
The HST WFPC2 data used in the simulations were retrieved from the Hubble Data Archive (HDA) (http://archive.stsci.edu) of the Space Telescope Science Institute (STScI). Only the HDA calibrated data, i.e. the data processed through the so called calibration pipeline carried out at the STScI, were used. These are affected by problems common to any astronomical instrument, e.g. limited throughput, not point-like PSF, cosmic ray hits and noise, as well as by the instrument's peculiar problems, described by [Holtzman et al. 1995a] and [Biretta et al. 1996]. It must be emphasized that no correction for these effects was applied on the HDA calibrated data before carrying out the simulations. As already noted, it was thus assumed that the calibrated data faithfully represented the ``real sky''. In other words the WFPC2 images were taken to be ideal. Accordingly, in Chapter 7 the goodness of the results expected from GAIA galaxy observations will be evaluated on the basis of a comparison between HST real data and GAIA simulated data. The reason for this simplified approach was that we were mainly interested in simulating how GAIA would reproduce a given realistic sky image containing lots of subarcsec details, rather than in deriving with the highest accuracy the real properties a field imaged by WFPC2. Note, however, that in Subsections 6.2.2 and 6.2.3 it is shown that the assumption of ideal WFPC2 images is essentially correct as far as PSF and noise are concerned.