In conducting thermal/vacuum testing on a test article that is to operate at cryogenic temperatures (below 77 K, liquid nitrogen temperature), the designer is faced with unique problems peculiar to these temperatures. Care must be taken to account for these problems in the design of the test and its associated test equipment if a successful test program is to be achieved. These problems are often aggravated when the test article has special test requirements of its own that must be met. Frequently, trade-offs must be made. When the test article is an optical system, as it is in this case, the designer has especially difficult trade-offs to make. The design that evolves may involve compromise, but it must still insure that the desired cryogenic test conditions will be satisfactorily provided.