SPIRE Design Description - Research Services

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Draft SPIRE Design Description Document ~12 mm ~100 mm 40 ~20 mm Figure 3-15 - Sketch of the final optical beams onto the spectrometer focal planes. There is a real pupil image of ~20 mm at about 119 mm from the focal plane which has a physical diameter of 23.8 mm. Off axis images therefore come onto the array at an angle of up to 3.5º – shown by the dashed line. The detectors themselves, because of the feedhorns, can only accept a certain range on input angles and, as they are configured to stare straight ahead, they only partially illuminate the pupil – shown in pink. Whilst this only causes a small loss of signal at the centre of the FTS mirror movement, at large mirror displacements the pupil images from the two interferometer arms shear past each other and the loss in fringe contrast will be greater. 3.5 Straylight control Straylight control is defined as the reduction of any unwanted optical power falling onto the detectors in an instrument to as low as practically possible. This includes both the removal of out of spectral band optical power and the removal of in band optical power from sources outside of the field of view. These might be radiating surfaces within the structure of the instrument or satellite directly viewed by the detectors or seen via reflection or diffraction from other parts of the optical chain and structure. 3.5.1 Bandpass filtering Any surface with a temperature greater than ~10-15 K will emit radiation in the detection band of the SPIRE instrument. The Herschel telescope will be at a temperature of about 70-80 K with an effective emissivity of 3-4%. There will, therefore, be a relatively large photon flux falling on the SPIRE instrument entrance aperture from the telescope alone. The bolometer detectors are sensitive to optical power at all wavelengths, therefore the first task in managing the amount of unwanted optical power falling on the detectors themselves is to limit the spectral band of the radiation falling on the detectors. This is achieved by a series of optical filters as described in the SPIRE Filter Specification Document. In practice a single passband filter does not have enough out of band rejection on its own and four filters are used strategically placed along the optical path to reduce the out of band radiation entering each part of the instrument structure. The overall passband provided by these filters is shown in Figure 3-16; the physical locations of the filters are shown in Figure 3-9 and Figure 3-13 above.
Draft SPIRE Design Description Document Transmission 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 10 30 50 70 Wavenumber [cm-1] 90 110 130 41 25.5cm-1 HPE 37cm-1 LPE 43cm-1 75cm-1 90cm-1 160cm-1 Total transmission Figure 3-16 - Example showing the definition of a 350 micron photometer channel using real filters. The passband (λ/∆λ) in this case is ~3. 3.5.2 Baffling Controlling unwanted optical power that is in the spectral band of the detectors is effected in the SPIRE instrument by having a series of compartments within the instrument separated by physical barriers or baffles. The overall concept for the photometer is shown in Figure 3-18 and for the spectrometer in Figure 3-19. The optical design, as described above, is arranged so that the entrance to the instrument itself is at the field plane of the Herschel telescope and a physical aperture plate is placed here together with an optical filter that rejects most of the out of band radiation. The aperture plate is made just large enough to allow the beams generated by the detectors to pass out of the instrument with the minimum vignetting. In fact the beams will be chopped and jiggled around the sky so the aperture plate is slightly oversized compared to the detector footprint on-axis – Figure 3-17 (also see Definition of a combined focal plane plate for the SPIRE instruments).
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