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The detector should not be the coldest part of the instrument
during the warm-up procedure. This is to avoid it collecting
ice if there is a leak. It can be heated using the the Lakeshore
LS330 Temperature Controller, but be sure to avoid shock heating
the array (temperature change > 1K per minute), by using
the 'Low' heater setting when the detector is below ~120K, and
only the 'Medium' heater setting at higher temperatures. Additional
heaters are attached to the 4K (nominally 5 Watt dissipation)
and first (20W nominal dissipation) stages of the Joule-Thomson
cooler. |
The bulk of the cryostat is warmed back up to room temperature
using heater panels attached to the optics box and the radiation
shield, with fixed heating powers of 450 Watts and 150 Watts
respectively. Typical warm-up curves are shown in
Figure 1. |
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Figure 1. Warming up Michelle. The cryostat can
be backfilled with dry nitrogen gas once the temperatures
have risen above 120K.
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| Make the Array safe by loading the "Protect_Array"
waveform via the waveform Generator control screen (LINK TO
DJI STUFF) |
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| Open the JT and Bypass valves (clockwise) to
5 turns and close again immediately to 4 turns ( to remove backlash).
Switch off the JT Compressor followed by the associated GM motor
and finally the compressor (see below re Gemini). |
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If the Instrument is remaining on UKIRT to warm-up completely
no further action is needed with the JT Helium system. If,
however, the cryostat is to be removed from UKIRT during the
warm-up, the disconnection of the JT Helium supply lines (with
system pressures equalized) will result in a pressure build-up
within the cooler which will vent via the pressure relief
valves.
On the Gemini Telescope the compressors will run permanently
and all the Helium supply lines will have to be disconnected
at the Cassegrain wrap interface once the GM coldheads are
depowered.
In both cases some system recharging with N6 Helium will
be required once the Instrument is warm. At UKIRT only the
JT pipework will need recharging to 150psi whilst on Gemini
all three manifolds will probably need to be done.
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| The heater at the Detector is controlled via
the Lakeshore LS330 controller. Check/adjust the setpoint to
300K and switch on the heater at the 'Low' setting. |
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Using a dual DC Power supply apply the following voltages
to the first and third stages of the Joule-Thomson cooler:-
First (70K) Stage:
~23V giving ~20W power input (25.7 ohm resistor)
Third (4K) Stage:
11.3V giving 5W power input (25.7 ohm resistor)
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Switch off the Main GM Coldhead motor and subsequently
the Compressor (see note above re Gemini). The GM Coldhead
can be safely disconnected from the Helium pipework without
subsequent recharging.
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| 1.2 Apply heat
to the Main Optical Bench and Radiation Shield using the Heater
Control Unit. Position the unit near the cryostat and terminate
to the Heater connector. By setting "setpoint 1" on
each channel to 25 deg C and "enabling" each channel
300W input is provided to the MOB and 150W to the Radiation
Shield |
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1.3 When the coldest temperature
anywhere inside the cryostat is above 120K the cryostat can
be backfilled. First, however, the bolts securing the two
main Vacuum covers should be loosened to slightly more than
finger tight. If not these bolts will be virtually impossible
to undo when the vacuum is removed. Backfill the cryostat
with clean, dry nitrogen. For the volume to be filled this
can take over 30 mins and is only terminated when the pop-off
plate on the motor connector plate has operated. (i.e. DO
NOT RELY ON VACUUM GAUGE MONITOR!)
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Ensure that you have an adequate supply of gas before starting
- remember that the volume of the cryostat is ~2 cubic metres.
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| 1.4
With backfilling complete and the detector temperature above
120K, set the LS330 controlled heater to the 'Medium' setting. |
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| When all temperatures have reached
approximately 290 K (15 deg C) the warm-up heaters can be switched
off and the cryostat prepared for opening. Log the timing to
update the Instrument cooldown log |
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Recharge or purge teh Helium system as required to 150
psi.
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