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SPIRE Design Description - Research Services

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Draft <strong>SPIRE</strong> <strong>Design</strong> <strong>Description</strong> Document<br />

Note that failure of SCAL does not result in loss of the FTS, but the performance for low resolution<br />

spectrophotometery would be degraded.<br />

Further details in of the PCAL and SCAL device may be found in the <strong>SPIRE</strong> Calibrators SSSD.<br />

4.8 Beam Steering Mechanism (BSM)<br />

The BSM is designed to allow the image of the sky to be positioned and moved across the detector arrays in<br />

a controlled manner. Its specifications are fully described in the BSM SSSD.<br />

The BSM is mounted on the photometer side of the Spire Optical Bench, with the moveable mirror<br />

coincident with an image of the system pupil stop, the Herschel secondary mirror. The BSM must be able to<br />

steer the FOV in two independently controlled orthogonal axes. Rotations of the mirror around the Y-axis of<br />

the satellite represent “chopping” motions of the FOV while rotations about the Z-axis represent jiggling<br />

motions. Various observation modes of the telescope call for simultaneous rotations in both axes.<br />

The key specifications of the BSM are summarised in Table 4-6.<br />

Table 4-6 - Key specifications of the BSM.<br />

Chop stage Jiggle Stage<br />

On-sky rotation ± 2 arcminutes ± 30 arcseconds<br />

Mirror rotation ± 2° 24’ ± 0° 36’<br />

Frequency 2 Hz (design), 5 Hz (goal) 0.5 Hz (design), 1 Hz (goal)<br />

Maximum power dissipation 4 mW<br />

Settling time to within 5% of 10 msec 50 msec<br />

demand position<br />

Angular measurement resolution 0.002°<br />

True angular position accuracy 0.04°<br />

Angular position repeatability 0.004°<br />

The BSM is composed of the following components:<br />

(i) Mechanism (BSMm)<br />

(ii) Mirror and Photometer calibration source (P-Cal)<br />

(iii) Flex-pivots<br />

(iv) Gimbal frame<br />

(v) Magnetic actuators<br />

(vi) Position sensor<br />

(vii) Stray light baffle<br />

(viii) Thermometer<br />

(ix) Structural interface to the <strong>SPIRE</strong> Optical Bench (BSMs)<br />

(x) Warm electronics (BSMe)<br />

(xi) Launch-lock device<br />

90

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