SPIRE Design Description - Research Services

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Draft SPIRE Design Description Document Figure 3-8 Ray diagram of the Herschel telescope with the SPIRE optics to the same scale. The primary mirror has a diameter of 3.5 m and the SPIRE FPU is about 500 mm wide. The SPIRE optics are therefore divided into a common portion followed by the photometer and spectrometer optical chains. The common optics form an image of the secondary at the location of the beam steering mirror and provide a well corrected intermediate focus to allow the separation of the spectrometer and photometer beams into different parts of the instrument box In addition the combination of the common optics and the rest of the photometer optical train has to provide: (i) a well-corrected, flat focal surface, perpendicular to the gut ray, at f/5 at the proposed location of the detector arrays – i.e. correction for the highly tilted focal plane of the telescope; (ii) sufficient back-focal length (BFL) to allow separation of the photometer beam into three spectral bands using dichroics; (iii) a non-aberrated image of the telescope secondary at a physical location as close to the photometer detectors as possible and physically accessible to allow a real stop to be placed there. All these functions are assured by the SPIRE common and photometer optical design as shown in Figure 3-9. CM3 is an off-axis ellipsoid projecting an image of the telescope secondary (CM2) onto CM4. This image is well-corrected and in focus at the centre of the CM4 so that the pupil image at the cold stop stays fixed during chopping and beam steering. CM4 is a flat mirror whose orientation is adjustable in flight to permit ±2 arcminutes chopping in the sagittal plane, allowing off-field chopping for a 4 x 4 arcminute subfield, and ±30 arcminutes beam steering motion in both the tangential and sagittal planes to obtain fully Nyquist sampled images. The toric CM5 mirror reimages the focal plane onto PM6 and converts the focal ratio of the beam from f/8.68 to f/5. While CM3 and CM4 are common for both photometer and spectrometer, the two systems separate at PM6. The photometer PM6 is toric and sends the beam into an Offner-type relay system consisting of three spherical mirrors: PM7 (concave), PM8 (convex), and PM9 (concave). The tilt angles, separations and curvatures of these mirrors provide enough free variables to satisfy the need for well corrected flat image planes; a long back focal length and an image of the telescope secondary mirror close to the detectors. In particular, an easily accessible pupil image is provided between PM8 and PM9 in which the cold stop is located. The physical separation between the 4-K cavity and the 2-K cavity (detector box) into which the detectors are mounted, is made here so that the entrance to the cold 34
Draft SPIRE Design Description Document environment containing the detectors is as small as possible to restrict the possibility of straylight hitting the detector arrays. Final band limiting filter is mounted at 300 mK on the front of the detector arrays M9 PM9 Third filter placed at cold stop location at entrance to cold detector box CS M8 PM8 35 PM6 M6 PM7 M7 M4 CM4 Straylight baffle placed here with second filter M5 PM5 M3 CM3 Field aperture and input filter Figure 3-9 - Ray diagram of the SPIRE photometer showing the ray paths for three points in the tangential plane, centre and extremes of the FOV (±2 arc minutes). Also shown here are the locations of the straylight baffles and optical bandpass filters. Det3 PLW Dic2 PDIC2 Fold2 PM11 PFIL3 CS PDIC1 Dic1 Det2 PMW M9 PM9 Fold1 PM10 Det1 PSW Figure 3-10 - The beam folding within the photometer detector box. The dichroics are low pass devices so PSW is the shortest wavelength array; PMW the medium wavelength array and PLW the long wavelength array.
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SPIRE Design Description - Research Services

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