SPIRE Design Description - Research Services

Draft SPIRE Design Description Document
System Description/Issues Sub-systems Design
analysis
Calibration To ensure that the data produced by
the instrument can be converted into
meaningful physical units to allow the
correct operation of the instrument in
all modes and the processing of the
instrument data into the required data
products
Observing mode calibration definition
Ground commissioning and
calibration plan
Flight commissioning and calibration
plan
Instrument to ground facility
interfaces
Ground facility definition
Ground based observing programme
definition
3.3 Structural design and FPU integration
31
Photometer
Calibrator
Spectrometer
Calibrator
DPU (FSDPU)
ICC
Tools
Systems
analysis
Instrument
performanc
e models
Design
verification
methods
Prototype subsystem
tests
CQM instrument
level performance
verification
Ground based
observing
programme
We have already discussed the need to have various temperature zones with the SPIRE FPU. This, combined
with the need for two essentially separate instruments in the SPIRE instrument, has dictated the design
approach to be taken for the SPIRE structural design. Figure 3-7 shows the conceptual design of the FPU
structure. A single stiff optical bench is used to mount all the subsystems and optical components, including
two detector boxes that are thermally isolated from the optical bench on stiff space frames. On one side of
the bench the components for the common entrance optics and the photometer channel are mounted, and on
the other the components for the spectrometer channel. Each side of the optical bench has a cover that forms
a structural “monocoque” element in the design. The integrated instrument box is mounted from the
Herschel optical bench via three thermally isolating supports. One of these is directly mounted from the
SPIRE optical bench and forms a fixed reference point, the other two are mounted from the two covers and
are bipods with flexibility in one direction to allow for any differential thermal contraction during system
cool down.
The FPU covers also form both a straylight shield to protect the instrument from the ambient thermal
radiation environment in the Herschel cryostat and an RF shield to protect the detectors from any radiated
EMI. All sub-system wiring entering the instrument box must pass through passive RF filters mounted in
boxes from the SPIRE Optical Bench on the spectrometer side. When the cover is integrated with the optical
bench the RF filter boxes will be sealed to the cover. The exception to this are the harnesses for the detectors
themselves that connect the bolometer arrays to the externally mounted JFET units. These are filtered within
the JFET units and then pass to the instrument box via a drilled plate hard mounted to the SPIRE Optical
Bench. The wiring harnesses therefore form part of the RF shield therefore and careful attention must be
paid their electrical shielding.
In addition to sealing the instrument box against RF, it must also be sealed against the possibility of stray
optical radiation entering via routs other than the legitimate path defined by the telescope and SPIRE optical
elements. To this end the thermal straps that must broach the covers to connect the sorption cooler and the
detector boxes directly to the Herschel helium tank at 1.7 K must pass through light baffles.