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CHAPTER 2. THE SPIRE INSTRUMENT
evaporator and uses porous material which absorbs or releases gas depending on its temperature. The refrigerator contains 6 litres of liquid 3 He. At the beginning of the cold phase,
all of this is contained in liquid form in the evaporator. The pump is cooled to ⇠ 2 K, and
cryo-pumps the 3 He gas, lowering its vapour pressure and so reducing the liquid temperature.
The slow evaporation of the 3 He provides a very stable thermal environment at 300 mK for
around 48 hours under constant heat load in normal observing and operational circumstances.
Once most of the helium is evaporated and contained in the
pump then the refrigerator must be recycled. This is carried out
by heating of the sorption pump to ⇠ 40 K in order to expel the
absorbed gas. The gas re-condenses as liquid at ⇠ 2 K in the
evaporator. Once all of the 3 He has been recondensed, the pump
is cooled down again and starts to cryo-pump the liquid, bringing the temperature down to 0.3 K once again. This recycling
takes about 2.5 hours and is usually performed during the daily
telecommunications period (DTCP). Gas gap heat switches control the cooler and there are no moving parts. The confinement
of the 3 He in the evaporator at zero-g is achieved by a porous
material that holds the liquid by capillary attraction. A Kevlar
wire suspension supports the cooler during launch whilst minimising the parasitic heat load. Copper straps connect the cooler
Figure 2.11: SPIRE cooler 0.3 K stage to the five detector arrays, and are held rigidly at
various points by Kevlar support modules. The supports at the
entries to the spectrometer and photometer 1.7 K boxes are also designed to be light-tight.
2.4.3
Warm electronics
There are three SPIRE warm electronics units. The Detector Control Unit (DCU) provides
the bias and signal conditioning for the arrays and cold electronics, and demodulates and
digitises the detector signals. The FPU Control Unit (FCU) controls the 3 He cooler, the Beam
Steering Mechanism and the FTS scan mirror, and also reads out all the FPU thermometers.
The Digital Processing Unit (DPU) runs the on-board software interfaces with the spacecraft
for commanding and telemetry. The 130 kbs available data rate allows all photometer or
spectrometer detectors to be sampled and the data transmitted to the ground with no onboard processing.