2.2 Field of View and Geometric Distortions

WFC3 replaced 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 in 2009 is shown in Figure 2.2.

WFC3 images are subject to 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 elongation of the field of view in both channels. In the UVIS detector, most of the distortion runs approximately parallel to the diagonal direction of the CCD, while in the IR channel the distortion is parallel to the sides of the detector. As a result, the UVIS field projected onto the sky is shaped like a rhombus, with an acute angle between the x- and y-axes of the detector of approximately 86.1°. The IR channel projected onto the sky is rectangular, with an aspect ratio of about 0.90. Individual pixels projected onto the sky have the same geometry; thus the UVIS pixels are rhomboidal, measuring 0.0395 arcsec on each side, while the IR pixels are rectangular, measuring 0.135 × 0.121 arcsec.

Although the WFC3 IR and UVIS detectors appear co-aligned in Figure 2.1, 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.

For further discussion of geometric distortions in WFC3, see Appendix B.

Figure 2.2: The HST focal-plane layout, showing the instrument complement following SM4.