THE USE OF CLOSED-CYCLE COOLERS ON SPACE-BASED OBSERVATORIES

Many proposed space based observations will rely on the use of closed cycle and passive cooling systems to provide the thermal environment f or high sensitivity. The use of closed cycle mechanical coolers on spa ce telescopes poses particular integration problems; some of these dif ficulties are discussed in this paper. One of the major problems envis aged is that of exported vibration. This problem, and that of the heat sinking required, can be alleviated by siting the compressors of the Stirling cycle precooler further from the displacer unit. The effect o f the separation between the compressors and the displacer on the perf ormance of the Stirling cycle precooler has been measured. Increasing the separation from 170 mm to 565 mm decreases the cooling power at 25 K from 220 mW to 180 mW. In most applications this would be acceptabl e. The pre-cooler provides cooling at a single point. In situations wh ere refrigeration of extended objects (e.g. telescope mirrors) is requ ired, some distribution method has to be found. A scheme for achieving this is presented together with preliminary calculations on such a sy stem. Temperatures in the region of 2.5 to 4 K are required to meet th e requirements for long wavelength detectors. We have demonstrated how these temperatures can be achieved in a continuously operating closed cycle cooler that has been engineered for space applications. This co oler consists of a two-stage Stirling cycle precooling a closed cycle Joule-Thomson (JT) stage. Temperatures in the region of 4 K are achiev ed by the use of helium-4 in the JT system. The lighter isotope of hel ium is used to obtain temperatures down to 2.5 K. Under no-load condit ions the precooler reaches a base temperature of 11.3 K. The JT system achieves 4.3 K with a 10mW heat load and 2.5 K with a heat load of ov er 3 mW. The input power to the cooler is approximately 126 W. The tem perature stability of the cooler at low temperatures is important to k eep detector drift to a minimum. The temperature of the JT stage has b een measured in uncontrolled laboratory conditions and found to vary b y only 30 mK over a seventy hour period. The pre-cooler temperature va ried by approximately 0.6 K during these measurements.