SPIRE Design Description - Research Services

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.