SPIRE Design Description - Research Services

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Draft SPIRE Design Description Document Figure 2-6 Overlaid photometer arrays and summary of photometer observing modes. 2.3.2 Fourier transform spectrometer 2.3.2.1 Optical design and FPU layout The layout of the FTS and its optical scheme are shown in Figure 2-7 and Figure 2-8 respectively. The main design features of the FTS are described in Swinyard et al. (2000) and in §3.4.2 below. It uses two broadband, high-efficiency, intensity beam splitters in a Mach-Zehnder configuration rather than the traditional polarising beam splitters. This configuration has the advantage that all four ports are separately accessible, as in the classical Martin Puplett (M-P) polarising FTS (Martin, 1982). But the throughput is a factor of two higher than for the M-P as none of the incoming radiation is rejected. This design is also insensitive to the polarisation of the incident radiation. The performance of the beam splitters and of a bench-top implementation of this design has been demonstrated (Ade et al. 1999). A thermal calibrator is located at a pupil image in the second input port of the FTS, and provides a thermal input that mimics the dilute 80-K black body emission of the telescope. This allows the large telescope background to be nulled, thereby reducing the dynamic range requirements for the detector sampling. Two band-limited detector arrays are placed in the two output ports, covering 200-300 µm and 300-670 µm. A single back-to-back moving roof-top mechanism serves both arms of the interferometer, with a frictionless carriage mechanism using double parallelogram linkage and flex pivots. The pick-off mirror (on the photometer side of the optical bench panel and located at the intermediate field image) directs the spectrometer field of view through a hole in the optical bench panel into the FTS side of the instrument. A 4-K pupil stop is located between the pick-off mirror and the input fold mirror. The input relay mirror focuses the beam to an intermediate image plane located just after the first beam splitter, after which the beam is collimated and sent to the moving corner cube assembly. The corner cube shifts the beam and sends it towards the camera mirror, which produces an image plane just before the output beam splitter. The output relay mirror focuses the beam onto the detector arrays. A pupil image is located near the final fold mirror, making this a convenient location for the entrance aperture to the 2-K enclosure. As this pupil moves when the optical path difference changes, it is not a good place for a limiting cold stop. Instead, the limiting aperture is located at the 4-K pupil plane between the pick-off mirror and the input fold mirror. 16
Draft SPIRE Design Description Document Det SSW Fold SM12A Rout SM11A BS2 SBS2 Figure 2-7 - Physical layout of the spectrometer. Cam SM10A CC SCCA Coll SM9A M6s SM6 BS1 SBS1 17 Rin SM8A M5 CM5 M7s SM7 M4 CM4 Figure 2-8 - Optical design of the spectrometer. The FTS design is optimised for the 200-400 µm band. The wavelength coverage is extended to 15 cm -1 (670 µm) to give access to the astrophysically important 609-µm line of CI in our own and nearby galaxies, and to increase the range over which the spectral energy distribution of sources can be measured in the FTS low-resolution mode. A filtering scheme similar to the one employed for the photometer channel is used to restrict the passband of the instrument. Filters on the bolometer arrays themselves define the passband for each array. 2.3.2.2 Spectrometer detector arrays The field of view of the FTS is approximately 2.6 arcminutes in diameter, and is covered by 37 hexagonally close-packed detectors in a short-wavelength array and 19 in a long-wavelength array. The detector modules will be similar to those used for the photometer, with a mechanical interface to the wall of the 2-K enclosure. The two FTS arrays cover the 200-300 and 300-670 µm bands. The detectors and feedhorns for the short wavelength band are similar to those for the photometer 250-µm channel. The long wavelength band is M3 CM3
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SPIRE Design Description - Research Services

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