SPIRE Design Description - Research Services

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Draft SPIRE Design Description Document The main disadvantages are that the observing modes are complicated (jiggling or scanning) and that the efficiency for mapping is a less than the ideal (Griffin, 2000). This means that the full collecting area of the telescope is not used with maximum efficiency. In order to collect the radiation from a point source with good efficiency, the detector through put must be of the order of λ 2 . The diffraction-limited beamsize of a telescope of diameter D is approximately θ ≈ 1.2λ/D. The corresponding solid angle on the sky is Ωsky = π(θ/2) 2 . The throughput (the product of area and solid angle, which is conserved in an ideal optical system) is therefore AtelΩsky ≈ [πD 2 /4][π(1.2λ/2D) 2 ] ≈ λ 2 . The SPIRE photometer uses smooth wall, single-mode feedhorns - the circular waveguide section allows propagation of only one mode (TE11). The corresponding throughput (defined as the area-solid angle product) is exactly λ 2 . The spectrometer uses multi-mode feedhorns that pass both TE and TM modes. 4.4.6 Bolometer array thermal-mechanical design The design of the SPIRE array units (called Bolometer Detector Arrays, BDA's) is illustrated in Optical entrance Kevlar supports 76 300 mK strap attachment 2 K mounting flange Readout cable attachment Figure 4-21. Each BDA unit is split into a 2-K and a 300-mK section. The 300-mK section contains the feedhorns, waveguides, bolometer array wafer and the supporting structure. The 2-K detector structure forms the mechanical interface between the 300-mK structure and the photometer or spectrometer 2-K enclosures. The enclosures are connected via thermal straps to the 1.7-K stage of the Herschel cryostat. The heat load from the 2-K structure to the 300-mK structure of the BDA is minimised by suspending the 300mK section from the 2-K structure with two pre-tensioned 3000-denier Kevlar cords. Kevlar has high mechanical strength in tension and a very low thermal conductivity at this temperature, and forms gives good thermal isolation between the assemblies. The estimated heat load is less than 1.6 µW per array. The high mechanical rigidity of the Kevlar cord yields a high resonant frequency (> 200Hz).
Draft SPIRE Design Description Document Optical entrance Kevlar supports Figure 4-21 Bolometer Detector Array module 77 300 mK strap attachment 2 K mounting flange Readout cable attachment Table 4-2 lists the main design and performance parameters for the SPIRE detectors (update from SSSD): 4.5 Mirrors Table 4-2 main design and performance parameters for the SPIRE detectors Yield > 90% Operating Resistance ~ 5 MΩ Time constant < 30 ms (photometer) < 16 ms (spectrometer) Responsivity 0.4 GV W -1 Detector noise 14 nV Hz -1/2 Efficiency of bolometer-cavityfeedhorn combination > 0.6 BDA mass budget 0.5 kg BDA heat load budget on 3 He cooler 1.6 µW The basic design of the mirrors is the same as the one used for ISO-LWS. The mirrors are made from Al- 6061. They all have a standard interface with the structure, i.e. an M8 screw and a pin. Each mirror is machined in a single block of aluminium (diamond cutting). Before the final finishing of the machining process, the mirrors are annealed to remove any residual stresses that may cause distortions at cryogenic temperatures. The screw part of the attachment exerts pressure only on the shoulder part of the mirror, avoiding deformation of the optical surface. The mount of each mirror is located on the optical bench by
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