The Improved Stratospheric and Mesospheric Sounder (ISAMS) employs two Oxford Stirling cycle refrigerators to cool eight infrared (6 MCT, 2 InSb) detectors to 80 — 100 Kelvin. Techniques to minimize heat loads on the displacer cold tip and attached cold stage conductor include unique radiation and conduction isolation designs. The designs of the cold stage conductor and detector mounts are coaxial so that all cold parts are loaded both radiatively and conductively only to an intermediate temperature. The outer stage of the detector cooling system is radiatively cooled to approximately 200 Kelvin by a recessed cold patch. This arrangement reduces total heat loads on each displacer cold tip to less than 700 milliwatts. Heaters are provided to de-gas the low emittance gold coatings if they become contaminated. A detailed nodal computer model was developed to thermally represent the hardware and sensitivity studies were performed to evaluate design parameters and orbital environmental effects. Thermal-vacuum testing at the Oxford Clarendon Laboratory showed excellent performance of the system and a correspondence with math model predictions to within 3 Kelvin. The ISAMS will be launched aboard the Upper Atmosphere Research Satellite (UARS) in 1991 .Its objectives include: measurement of the thermal structure of the atmosphere and its fluctuations in space and with season; investigation of the photochemistry of nitrogen-containing species; determination of the water vapor budget of the upper atmosphere. These objectives will be addressed by measurements of carbon dioxide, nitrous oxide, nitrogen dioxide, di-nitrogen pentoxide, nitric acid, ozone, water vapor, methane, and carbon monoxide.