SPIRE Design Description - Research Services

Draft SPIRE Design Description Document
Launch-lock device: During the launch (and operation) of Herschel, failure the BSM Flexure Pivots could
result in the mirror being fixed in an undesirable off-axis position. The spectrometer functionality is
particularly susceptible to this failure mode - if the BSM gets stuck at one extreme of its angular range, then
the field of view of the FTS detectors could be projected outside the SPIRE field of view resulting in loss of
the spectrometer. To prevent this from happening, a launch lock device is incorporated into the BSM. This
device is a solenoid-driven locking device that reduces the maximum travel range of the Chop and Jiggle
axes. During launch or in the event of failure of one or more of the Flexure Pivots, the launch lock solenoids
can be energised, constraining the mirror to point ± 1”(TBC) in each axis of the telescope boresight resulting
in a soft failure mode. The state of the launch lock solenoid is monitored by a prime and redundant microswitch.
Control system: The BSM has a prime and redundant electrical interface with the FCU to provide the
necessary feedback control of the mirror pointing angle. The position of the mirror is controlled via a PID
algorithm which is part of the OBS of the MCU. The control scheme treats each rotational axis of the mirror
independently of the other. More detail on the hardware implementation of the actuator control and power
system is described in §4.1.2.2.
4.9 Spectrometer Mechanism
The Spectrometer mirror MEChanism subsystem (SMEC) controls the movement of the rooftop mirrors
inside the SPIRE spectrometer. The movement of the mirrors causes there to be an optical path difference
between the two beams that enter the spectrometer 2-K detector box. The critical performances of SMEC are
the mirror velocity and its stability, the mirror movement around its travel axis and the required accuracy of
the mirror position measurements.
94
fr-asyz1
06/05/99
Figure 4-36 - CAD drawing of the link mechanism used for the SMEC.
4.9.1 Requirements on the mirror mechanism
The design of the FTS spectrometer (see §3.4.2) means that there is an effective folding of the optical path
difference of a factor of four between the actual movement of the mirror mechanism and the change in the
optical path difference (OPD). The required resolution of the spectrometer is for a maximum ∆σ = 0.4 cm -1
at all points in the field of view with a goal of reaching 0.04 cm -1 for at least point sources viewed on axis.
There is a further requirement that the systematic noise induced by the movement of the mirrors will not
prevent low resolution spectroscopy down to at least ∆σ = 2 cm -1 . The goal resolution imposes a maximum