SPIRE Design Description - Research Services

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).