On the basis of the measured NICMOS performance in HST-Cycle 7 and Cycle 7N programs, we analyze the behavior of the HST optical assembly at IR wavelengths. An accurate analysis of the telescope thermal status allows us to estimate the background flux observed by NICMOS, and compare it with the flux actually measured in different filters. The very close match between expected and measured fluxes confirms the validity of our model. A good understanding of the HST emissivity, which turns out to be lower than previous estimates, allows to predict with higher accuracy the performance of the future IR instruments on HST like NICMOS+cooling system and to specify critical design parameters for WFC3. Also, issues related to the long term stability of the system can be addressed more properly, providing useful quantitative insight on future missions such as the Next Generation Space Telescope.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.