SPIRE Design Description - Research Services

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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 82
Draft SPIRE Design Description Document Name Location Temp. (K) CFIL1 input filter on 4K box Table 4-4 Photometer filtering scheme. Filter Edges (cm-1) 4K 90cm-1 low- 90% 85.5 pass edge 50% 90 83 Function (B=blocking T=optimum t i i ) T 17-48cm-1 B 90cm-1+ PFIL2 10% 94 5 baffle between 4K 75cm-1 low- 90% 71.3 T 17-48cm-1 pass edge 50% 75 B 75cm-1+ PFIL3 PM7 to at 2K cold stop 2K 10% 78 8 60cm-1 low- 90% 57 T 17-48cm-1 pass edge 50% 60 B 60cm-1+ after PM9 2K 34.3cm-1* 10% 63 90% 32.6 T 17 - 34.2cm- PDIC1 low-pass edge 50% 34.3 1 10% 36.0 R 34.4 - 47.9cm-1 PDIC2 PFIL4S PFIL5S PFIL4M PFIL5M PFIL4L PFIL5L after PDIC_1 LPE definer over SW array blocker over SW array LPE definer over MW array blocker over MW array LPE definer over LW array LPE blocker over LW array 2K 24.5cm-1* low-pass edge 300mK 47.9cm-1* low-pass edge 300mK 55 cm-1 low-pass edge 300mK 34.2cm-1* low-pass edge 90% 23.3 50% 24.5 10% 25.7 90% 45.5 50% 47.9 10% 50.3 90% 52.2 50% 55 10% 57 8 90% 32.5 50% 34.2 10% 35.9 300mK 43cm-1 low- 90% 41 pass edge 50% 43 300mK 23.9cm-1* low-pass edge 10% 45 90% 22.7 50% 23.9 10% 25.1 300mK 30cm-1 low- 90% 28.5 pass edge 50% 30 10% 31 5 T 17.2 - 23.9cm-1 R 24.5 - 34.2cm-1 T 34.4-47.9cm- 1 B 50cm-1+ T 34.4-47.9cm- 1 B 55cm 1+ T 24.5-34.2cm- 1 T 24.5-34.2cm- 1 B 43cm 1 + T 17.2-23.9cm- 1 T 17.2-23.9cm- 1 B 30cm 1 + Comment Thermal blocker. Common. Thermal blocker. Common. Thermal blocker. Common. Requires prototyping. Relects SW onto array. Transmits Requires prototyping. Relects MW onto array. Transmits Band-defining edge. Requires protoyping. PDIC-1 defines Blocker for SW only. Unnecessary due to PDIC-1? Band-defining edge. Requires protoyping. PDIC 2 defines Blocker for MW only. Unecessary due to PDIC_2? Band-defining edge. Requires Blocker for LW only.
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