SPIRE Design Description - Research Services

Draft SPIRE Design Description Document
will produce a power at the detector of (40/80)(1/0.04)(1/30) 2 = 1.4% of the telescope background power.
The latter is typically a few pW, so the signal level provided by PCAL is around 4 x 10 -14 W. With a detector
NEP of a few x 10 -17 W Hz -1/2 , this provides a very large instantaneous S/N.
The 150 ms time constant requirement (30 ms goal) for PCAL is that it be comparable to or faster than the
photometer detectors. A power dissipation requirement of < 2 mW has been adopted to ensure that PCAL
operation makes a negligible contribution to the average load of the SPIRE FPU on the Herschel helium
tank, and that local heating of the environment will not result when it is operated.
The baseline PCAL device is essentially an inverse bolometer, developed by J. Beeman of UC Berkeley for
use in the SIRTF MIPS instrument. Figure 4-30 shows a photograph of a SPIRE PCAL prototype.
The device is similar in principle to the one used in the ISO Long Wavelength Spectrometer (but with lower
power dissipation). A thin 1 x 1 mm dielectric substrate is coated with a metallic film and suspended from
its 4-K copper housing by thin nylon threads. Electrical contacts allow a current to be passed through the
metal film, heating it up to a temperature of 40 K or more. A photograph of a prototype device is shown in
Figure 4-30. A typical resistance is 300 Ω, requiring ~ 2.5 mA drive current for 2 mW power. There is a 1%
stability requirement on the constant-current drive, which translates to a comparable stability for the radiant
power output. Prototype devices tested to date show that the radiant output requirement can be met
comfortably, with some design iteration still needed to achieve the required time constant.
4.7.2 Spectrometer calibrator (SCAL)
Figure 4-30 - Photograph of prototype PCAL device
The signal output from one of the FTS detectors represents the difference between the spectra presented at
the two input ports. When the FTS is at its zero path difference position, all frequencies are in phase and the
signal is at its maximum. Astronomical signals will invariably be much smaller than the thermal background
from the telescope. To minimise the required dynamic range for detector signal measurement, it is desirable
to replicate the telescope spectrum in the second input port. This is the purpose of SCAL, which is designed
to null the telescope emission to within 20% over the 200-400 µm range by minimising its spectrum and
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