The LWIRC has a simple cooling system consisting of a liquid
helium and a liquid nitrogen reservoir. Each reservoir has
a re-entrant fill tube and the dewar can therefore hold liquids
in any orientation. The liquid helium capacity is 20 liters.
The helium boil off rate with the window closed and the array
off is 
0.3 l/hr, while during observations the rate is 0.42 l/hr
With judicious planning, the system should
hold liquid helium for 48 hours.
It is very important that the array temperature remain stable
despite variations in radiation falling upon it, changes in
the liquid helium level, and the dewar orientation.
We have built a temperature controller for the array.
The circuit is a ``proportional-integral" controller and
it stabilizes the voltage of a temperature sensing diode
to better than 0.15 mV or 
0.005 K at 8.5 K [8].
The circuit stabilizes within 30 seconds after changing the temperature
set point.
The diode is mounted as close as possible to the focal
plane array on the ceramic chip carrier. The array
is spring loaded against an OFHC copper plug onto which a
500 ohm wirewound resistor is mounted for a heater.
The plug is connected to the helium cold plate via a heavy copper braid.
The heater can provide up to 60 mW of power, raising the
temperature of the array to 11.7 K.
The lens turrets and filter wheels can be rotated while the system is cold via a cryogenic transmission. The system permits seven motions while using only two stepper motors and two mechanical vacuum feedthroughs. The positions of the filters and optics are monitored by potentiometers mounted to each wheel. Once we established a clean electrical system, this system has provided accurate and repeatable positioning of the wheels.