Optical Filters

3.6 Methane Quad Filter

The methane band quad filter, known as the jewel-quad, was designed for a four-faceted WF/PC configuration to permit imaging with both four WFC CCDs and four PC CCDs. WFC imaging is recovered for the first quadrant element of the filter (6193Å) by a partial SOFA wheel rotation of -33 degrees which moves quadrant 1 into WF2 and WF3 with some vignetting of both camera fields. PC imaging with all four elements of the methane band jewel-quad cannot be recovered, but partial SOFA wheel rotations of -15 degrees and +15 degrees are implemented to recover two of the four methane band filters (8929Å and 6193Å). The +15 degrees rotation of the filter wheel, however, results in some vignetting of PC1's field-of-view. The filter projections associated with the methane band jewel-quad are shown in Figure 3.8. Each of the four filter wheel positions are associated with unique filter names, as summarized in Table 3.11 on page 54.

The required central wavelength is selected by filter name and aperture location. Filter element FQCH4N (Filter Quad Methane Narrow) has three possible apertures, each of which is located in one of the three WF channels and associated with a unique central wavelength, while FQCH4N33 is associated with one possible central wavelength. FQCH4N15 and FQCH4P15 are both associated with one central wavelength for PC1 observations. In addition to the filter name and aperture, a central wavelength is also requested in the proposal instructions in order to provide a consistency check.

Table 3.11: Methane Band Quad Filter. The filter and aperture names should be specified on the Phase II Proposal as shown here.

Figure 3.8: Methane Quad Filter. The mapping to the focal plane for nominal and rotated (-33, -15, and +15 degrees) SOFA positions is shown. Dashed lines indicate the limits of the unvignetted field-of-view on each quad.