The Advanced Land Imager (ALI) of the EO-1 satellite has been calibrated under its normal operating conditions in a thermal vacuum chamber. The optical equipment for the calibrations had to be placed outside the chamber. The effects on the calibrating beam of the vacuum chamber window are described here. Since the instrument senses the reflected solar spectrum, the existing fused-silica window was found to have adequate spectral transmission properties. It also was measured to cause little wavefront error by itself. When the chamber is under vacuum, and the interior cold shroud is cooled to approximately 100 K, the window develops significant optical power. This is a result of radiational cooling of the window, coupled with change of its index of refraction with temperature. Such an effect seriously compromises the spatial calibrations of the instrument, particularly MTF measurements. This effect was overcome in two ways: A heating plate placed outside the window was used to alter the temperature distribution until its effect on the wavefront was negligible. The more practical solution was to measure the window power with an interferometer, and compensate for it by shifting the target reticle of the collimator.