SPIRE Design Description - Research Services

Draft SPIRE Design Description Document
brightness in the second input port of the FTS. It is located at the second input port to the FTS, at an image
of the telescope pupil (diameter = 30 mm). The telescope is assumed to be at 80 K and to have an overall
emissivity of 4%. It is assumed that the overall emissivity of the system is uncertain by a factor of two, and
that the actual value will not be known before launch, and SCAL must therefore have sufficient adjustability
to accommodate this uncertainty. The baseline SCAL design is shown in Figure 4-31. SCAL incorporates
two active elements:
(i) a device similar to PCAL, designed to operate at a high temperature, filling a small fraction of the pupil
area, which allows the responsivity of the spectrometer detectors to be monitored in the same way as for
the photometer;
(ii) a large area heated plate to provide uniform illumination of the pupil and to replicate the dilute 80-K
telescope spectrum.
The heated plate is supported on a thermally isolating tripod using Torlon struts, with the temperature
measured by a Cernox thermometer. Full redundancy is implemented for both the heater and thermometer.
The unit has a mass of < 200 gm and an allowed volume envelope of 50x50x70 mm. It interfaces directly to
the 4-K structure.
Torlon supports
Heated metal
plate filling pupil
PCAL type
source
89
Kapton film heater
Cernox thermometer
Figure 4-31 - Schematic diagram of SCAL. The diameter of the heated metal plate is approximately 30 mm.
In order to replicate the spectral shape of the telescope emission, SCAL must operate in the Rayleigh-Jeans
part of the black body spectrum even at the longest wavelength covered by the FTS. This requires a
temperature in excess of ~ 40 K. For a source of this temperature, filling the pupil, the emissivity must be
considerably less than unity (approximately 8% if the telescope is at 80 K and has 4% emissivity). Two
options are under consideration for achieving the desired SCAL heated plate emissivity:
(i) a neutral density filter may be placed in front of the device;
(ii) the plate may be coated with a suitable material to achieve the desired emissivity.
SCAL is required to have stable emission over the timescale of an FTS observation. The radiant output is
required to be stable to within 1% over a period of > 1 hr. A long time constant is therefore desirable. A PID
controller, implemented in software is baselined to control the SCAL temperature, and the device shall be
designed to have a long intrinsic time constant. However, the cool-down time should not be excessive to
prevent emission persisting after the FTS has been switched off (this could, for instance, create stray light
problems for photometer observations ). SCAL will be designed to have a cool-down time of ~ 15 minutes.
The required SCAL power dissipation when switched on is < 5 mW with a goal of 2 mW.
Thermal testing of SCAL prototypes using a 2-mm thick aluminium plate indicate that an operating
teperature of 80 K for the heated plate is achievable with power input of ~ 2 mW, comfortably within the
requirement. A modelling and prototyping programme is continuing to optimise the device time constant and
to control accurately the emissivity of the plate.