SPIRE Design Description - Research Services

Draft SPIRE Design Description Document
4.6 Filters, beam splitters
Filters are placed at a number of positions within the SPIRE FPU, serving the following functions:
(i) definition of the spectral passbands;
(ii) minimisation of the thermal loading on the 3 He fridge, 2-K, and 4-K stages by rejecting short
wavelength thermal energy;
(iii) minimisation of stray light getting to the detectors;
(iv) maximisation of the in-band spectral transmission.
The filters are implemented as combinations of capacitative and inductive grids formed by evaporation of
copper on thin (~ 2 µm) mylar or polypropylene substrates (Lee et al., 1996). Several grids are used in each
filter. The grids must be maintained at accurate spacings. This is achieved either by the use of annular rings
for vacuum-gap filters or solid dielectric spacers for “hot-pressed” filters. The latter are formed by pressing
together a stack of grids and spacers while heating the combination. The result is a solid and robustly selfsupporting
filter.
The SPIRE filter chains are designed on the pessimistic assumption that the telescope surface is perfect even
out to UV wavelengths. For complete blocking out to UV, 3 or 4 blocking filters are needed in the chain
The filters can withstand multiple cycling to low temperature, and are robust with respect to low temperature
vibration. They are unaffected by UV and high energy particle irradiation and have a very low outgassing
rate. They are based on technology developed and implemented in the ISO Long Wavelength Spectrometer
and also used in numerous ground-based submillimeter instruments. The filters are interfaced to the structure
by simple 3-point mounts to ensure that flatness of the grids is not affected by mechanical stress on the
support rings.
4.6.1 Photometer filtering scheme
The photometer filtering and dichroic scheme is summarised in Table 4-4. The relative positioning of these
items is illustrated in Figure 4-25 and Figure 4-26. The two dichoics allow the spectral bands to be directed
to the three arrays with high efficiency
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