In the nominal filter wheel position PC1 has a clear field-of-view but, there is significant vignetting in all three WFCs. A partial filter wheel rotation of -15 degrees produces a larger field-of-view in WF3, although some vignetting remains. The options are illustrated in Figure 3.9. The imaging performance of the Wood's filters is continually monitored for signs of aging such as visible light leaks. Additional partial rotations could be implemented in the future, to position an unaffected region of the filter into a WF or PC1, if necessary. The unvignetted filter projections associated with the two planned filter positions are shown schematically in Figure 3.9. Each filter position is associated with a unique name as summarized in Table 3.12Figure 3.9: Wood's Filters. Greyscale flat field images show the field-of-view available with the two Wood's filter options F160BW and F160BN15.
The Wood's filters can be used as a broadband filter, or in combination with the CaF_2 long-pass filter to suppress geocoronal emission, or, crossed with one of the other UV filters, such as the suprasil blocker F165LP, to define a solar-blind UV photometric system. As discussed at the beginning of this chapter, the image will be out of focus in the last case. WFPC2's Wood's filters are circular with a clear aperture of 41 mm. Two similar Wood's filters (F160AW and F160BW) were mounted in SOFA wheel 1 to provide some redundancy. In Thermal Vacuum testing F160AW showed evidence for pinholes, which cause excessive red leak in some parts of its field. Therefore the preferred filter for far UV imaging with minimal red leak in WFPC2 is F160BW.
The filter name must be selected on the basis of whether a PC or WF3 observation is required.
