SPIRE Design Description - Research Services

Draft SPIRE Design Description Document
Note that failure of SCAL does not result in loss of the FTS, but the performance for low resolution
spectrophotometery would be degraded.
Further details in of the PCAL and SCAL device may be found in the SPIRE Calibrators SSSD.
4.8 Beam Steering Mechanism (BSM)
The BSM is designed to allow the image of the sky to be positioned and moved across the detector arrays in
a controlled manner. Its specifications are fully described in the BSM SSSD.
The BSM is mounted on the photometer side of the Spire Optical Bench, with the moveable mirror
coincident with an image of the system pupil stop, the Herschel secondary mirror. The BSM must be able to
steer the FOV in two independently controlled orthogonal axes. Rotations of the mirror around the Y-axis of
the satellite represent “chopping” motions of the FOV while rotations about the Z-axis represent jiggling
motions. Various observation modes of the telescope call for simultaneous rotations in both axes.
The key specifications of the BSM are summarised in Table 4-6.
Table 4-6 - Key specifications of the BSM.
Chop stage Jiggle Stage
On-sky rotation ± 2 arcminutes ± 30 arcseconds
Mirror rotation ± 2° 24’ ± 0° 36’
Frequency 2 Hz (design), 5 Hz (goal) 0.5 Hz (design), 1 Hz (goal)
Maximum power dissipation 4 mW
Settling time to within 5% of 10 msec 50 msec
demand position
Angular measurement resolution 0.002°
True angular position accuracy 0.04°
Angular position repeatability 0.004°
The BSM is composed of the following components:
(i) Mechanism (BSMm)
(ii) Mirror and Photometer calibration source (P-Cal)
(iii) Flex-pivots
(iv) Gimbal frame
(v) Magnetic actuators
(vi) Position sensor
(vii) Stray light baffle
(viii) Thermometer
(ix) Structural interface to the SPIRE Optical Bench (BSMs)
(x) Warm electronics (BSMe)
(xi) Launch-lock device
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