In addition to determining the calibration coefficients of the various thermistors monitoring the thermal condition of the instrument, and measuring the spectral response function of the individual channels, the pre-flight calibration includes subjecting the instrument in its normal operating mode to conditions it will encounter after launch. A "thermal vacuum" chamber simulates the thermal regime that the instrument is expected to experience in space. During these tests, the characteristics of the instrument are determined, in particular, the radiometric influence of the telescope foreoptics which must be correctly modeled in the in-flight calibration procedure (see section below on effect of foreoptics).