The OTA spherical aberration produces a Point Spread Function (PSF--the apparent surface brightness profile of a point source), as presented to the instruments, with broad wings. Briefly, the fraction of the light within the central 0.1" was reduced by a factor of about 5. The resulting PSF had "wings" which extended to large radii (several arcseconds), greatly reducing the contrast of the images and degrading the measurements of sources near bright objects or in crowded fields. Burrows et al. (1991, Ap. J. Lett. 369, L21) provide a more complete description of the aberrated HST PSF. Figure 5.1 shows the PSF in three cases.
Figure 5.1: PSF Surface Brightness.
It shows the aberrated HST PSF, the WFPC2 PSF, and for comparison the PSF that would be obtained from a long integration if HST were installed at a ground based observatory with one arcsecond seeing. All of the PSFs were computed at 4000Å. The FWHM of the image both before and after the installation of WFPC2 is approximately proportional to wavelength, at least before detector resolution and MTF effects are considered. (The WF/PC-1 core was approximately 50% broader than the core that is obtained with WFPC2). Figure 5.2 shows the encircled energy (EE), the proportion of the total energy from a point source within a given radius of the image center, for the same three cases.
Figure 5.2: Encircled Energy.
The WFPC2 curve shown is the average of measurements taken with F336W and F439W. It can be seen that the core of the image in WFPC2 contains most of the light. At this wavelength, 65% of the light is contained within a circle of radius 0.1" . However, this proportion is considerably less than the optics deliver. The reason for this is discussed in the Section 5.4. Encircled energy curves for other filters are shown in Figures 5.3 and 5.4; note that these curves are normalized to unity at 1.0" radius.
Figure 5.3: Encircled Energy for CCD PC1.
Figure 5.4: Encircled Energy for CCD WF3.