Wide Field Camera 3 Instrument Handbook for Cycle 19 - Space ...

10 Chapter 2: WFC3 Instrument Description
2.2 Field of View and Geometric Distortions
WFC3 replaces WFPC2, Hubble’s first large-area camera that included corrections
for the spherical aberration of the HST primary mirror. The appearance of the HST
focal plane following SM4 is shown in Figure 2.2.
Figure 2.2: The HST focal-plane layout, showing the instrument complement following SM4.
Although the WFC3 IR and UVIS detectors appear co-aligned in this figure, IR alignment activities
have confirmed that the two detector centers are actually offset by 4.8 arcsec. The diameter of the
outer black circle, projected onto the sky, is about 28 arcminutes.
WFC3 images are subject to significant geometric distortions. These result
primarily from the tilt of the focal plane relative to the optical axis (required for
constant focus across the detectors; see Figure 2.1), which leads to a modest