SPIRE Design Description - Research Services

Draft SPIRE Design Description Document
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Figure 4-32 Beam Steering Mechanism: A. P-Cal, B. Chop stage motors, C. Jiggle Stage Motors, D. Chop Stage
Flexure Pivots, E. Gimbal frame, F. Mirror surface, G. Chop Stage Magneto-resistive position sensor, H. BSM
Structure, I. Electrical connectors. Not shown; stray light baffle.
Mirror/Chop Stage: The mirror has a diameter of 32 mm and is machined from a monolithic block of Al-
6160. The mirror has an integrally machined shaft about which it rotates. And thus forms the chop stage of
the BSM. The front reflecting surface is machined flat to within 1 µm rms and polished to within 10 nm
RMS. The mirror has a specular reflectivity in excess of 99% and an emissivity of less than 1% in the
wavelength range 200-670 µm. Perpendicular stiffening ribs machined into the rear surface of the mirror
minimise the mass while retaining sufficient in plane stiffness. There is a 2.0-mm diameter (TBC) hole at the
centre of the mirror, through which PCAL, the photometer calibration source (see §4.7.1), radiates when
operated. This hole is conjugate with the central obscuration of the Herschel telescope. The PCAL source is
mounted behind the BSM mirror with a short length of light-pipe coupling the radiation to the aperture at the
centre of the mirror. The underside of the mirror is light-weighted and has pockets for the iron plates for the
magneto-resistive position sensors. The moment of inertia of the chop stage is 2.1 kg.mm 2 and has been
minimised to reduce power consumption during chop transitions.
Flexure Pivots: The Flexural Pivots allow rotation of the mirror while constraining translational degrees of
freedom. Lucas 5010-800 Flexure Pivots are used for the chop axis while Lucas 5010-600 Flexure Pivots are
used for the jiggle axis. These pivots have high radial stiffness, which virtually eliminates unwanted rotation
of the mirror due to bearing flexibility. They have minimal static friction to overcome which would greatly
complicate the design of the control system and possibly increase the power demand on the actuator. The
torsional spring rate of these pivots is 5.02 Nm/° for the 5010-600 and 0.631 Nm/° for the 5010-800. This
restoring force ensures that the mirror returns to the neutral position in the event of power failure to the
BSM. The mounts for the pivots are designed in such as way so that if the pivots fracture, they are held in
place and effectively become journal bearings. This aspect of the design renders this contingency a soft
failure mechanism.
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