Long-term performance of 4 K Gifford-McMahon refrigerators on the BIMA array

Eight radio telescopes in the Berkeley-Illinois-Maryland Array (BIMA) are outfitted with 4 K Gifford-McMahon refrigerators. The refrigerators cool superconducting detectors (SIS mixers) on the radio receivers; the heat load at 4 K is 10 to 20 mW. The refrigerators were constructed by welding third stages onto standard CTI model 1020 cryocoolers; Er3Ni or ErNi spheres are used as the low temperature regenerators. The refrigerators are operated almost continuously; one has been in service for 4 years. Maintenance consists of disassembling the cold heads every 1 to 2 years to clean out water and dust.