The coma and astigmatism aberrations vary significantly within a camera across the field-of-view. These variations are simply part of the optical design. At the extreme corners of the WFC CCDs, away from the OTA axis, there is about 1/5 wave of astigmatism (referenced at 633 nm), which decreases to nearly zero at the CCD centers. Astigmatism at this level causes the PSF core to become elliptical and slightly less sharp; note flattening of PSF at pixels positions (54,777) and (605,148) in Figure 5.5. Coma also varies, but to a much lesser extent. Coma and astigmatism variations are considerably smaller in PC1.
The obscuration patterns due to the camera optics (relay secondary mirror and spiders) appear to shift with respect to the OTA obscurations, depending on field position. The interacting diffraction patterns of the WFPC2 and OTA spiders cause ripples in the spider diffraction spikes, which vary with field position as the two spiders shift relative to each other. In Figure 5.6the OTA spider is hidden behind the WFPC2 spider at the field center and hence the diffraction spikes there have a simple, smooth appearance (c.f. position 446,425). At the CCD corners, however, one or more vanes of the OTA spider moves out from behind the WFPC2 spider, and the double set of obscurations causes a "beating" pattern in the diffraction spikes.
The spiders also interact with light diffracted from zonal errors in the OTA mirrors, causing streaks in the scattering halo which vary in position and intensity.