CRYOGENICS

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 $\sim$0.3 l/hr, while during observations the rate is $\sim$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 $\pm$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.