Optical Filters

3.7 Wood's Filters

WFPC2 features two solar-blind Wood's filters, for FUV (<2000Å) imaging. It was shown by Wood in the 1930s (Physical Optics, 1934, R. W. Wood) that thin layers of alkali metals transmit FUV wavelengths while providing very efficient long wavelength blocking due to the plasma frequency of the free electrons. Wood's filters have been built for WFPC2 at JPL using thin (5000Å) layers of sodium sandwiched between two MgF_2 substrates. These Wood's filters have a broad bandpass from 1200Å to 2100Å with visible-light transmission lower than 10^-8. The best conventional UV filters exhibit visible-light transmission of 10-3 to 10^-4. Many astronomical objects emit 10^4 to 10^7 visible photons for every FUV photon. In such cases, a Wood's filter (or "solar blind" detector as on STIS) is essential for FUV imaging so that the visible light leak does not dominate the observation. The main problem experienced with Wood's filters is long term instability. Sodium is a very reactive metal, and attempts to passivate the sodium layer have met with limited success. It is possible that as the Wood's filters age pinholes will form which transmit visible light. This transmitted light will appear as an increase in the background level at the focal plane. No indications of any degradation on-orbit have been observed.

Figure 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.

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 3.12.

The filter name must be selected on the basis of whether a PC or WF3 observation is required.

Table 3.12: Wood's Filters. The filter and aperture names should be specified on the Phase II proposal as shown below.

Figure 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.