Since the beginning of 1988, the Focal 5 facility of CSL ( Centre Spatial de Liège ) has been upgraded in order to perform the cryogenic optical tests of the single optical components as well as the overall telescope assembly of the optical subsystem of the ESA ISO telescope. The main objectives of these vacuum thermal tests were: to measure the optical surface quality of the various components and of the fully assembled telescope ( WFE measurements ) at cryogenic temperatures (5k). to measure the misalignment of the telescope and its associated experiments when in cryogenic conditions (5k). Consequently, a special experimental set-up has been design and manufactured in order to reach these objectives taking into account that: Alignment between the different optical elements had to be kept during the cryo-test inside given ranges ( +/- 15 arcmin ) from 300k to 5k. The cool down and the warm-up phases of the specimens had not to induce excessive constrains in the materials. The measurements were achieved through interferometric methods which required high stability. Special care had to be taken for the thermal control, thermal monitoring and thermal measurements.