Each camera has an associated 'optimum' aperture close to the geometric center of its field-of-view (FOV). These positions have been adjusted to reflect CCD performance following SMOV and to allow for pyramid vignetting. The aperture designations are WF2, WF3, WF4, and PC1 for the individual cameras and WFALL for the three-WFC combination. WFALL is located close to the apex in WF3 (see Figure 3.11). Observers are expected to place small or unresolved targets on these apertures. Note that normally all four CCDs are read out even if a specific CCD is selected with an aperture. This is discussed in section 2.8, "CCD Orientation and Readout", on page 31.
In cases where the observer does not want to use the current 'optimum' centers, a complimentary set of apertures has been implemented specifically for this purpose. These locations remain fixed and correspond roughly to the geometric center of each camera's field-of-view. They are designated WF2-FIX, WF3-FIX, WF4-FIX, PC1-FIX, and WFALL-FIX. Observers are expected to place extended targets on these apertures.
An additional set of aperture names have been defined for use with the WFPC2 filters which require partial rotations. The characteristics and uses of these filters are discussed earlier in this Chapter. In the nominal filter position, the three WFC segments of the [OII], Methane and Polarizer quad filters can be selected with an aperture for each camera corresponding to the optimum or geometric camera centers. The partially rotated quad filters, which generally fall into more than one camera, have been assigned apertures in the camera which provides the largest clear aperture. The pixel coordinates of these apertures will be reviewed on a regular basis to reflect changes in CCD and filter cosmetics. There are no analogous fixed apertures for the partially rotated filter configurations. The aperture name is generally the same as the (rotated) filter name. For the Wood's filters, the nominal filter position is used for the PC1 FOV only, while the rotated filter position is used for WFC observations. The linear ramp filters are unique because the ultimate location of the target will be determined from the central wavelength specified and so an aperture name is not required.
Occasionally the V2-V3 coordinates of the WFPC2 apertures are updated to correct slow drifts of the HST focal plane relative to the spacecraft (V1, V2, V3) system. Table 3.15 shows this history. The previous V2-V3 coordinates for any aperture can be derived by setting (V22,V32) to the values in Table 3.14, and then computing the earlier coordinates.