SIRTF Background Estimation: Methods and implementation

In order to plan observing and calibration programs with SIRTF, one of the important ingredients is the absolute brightness of the sky at the time and place of observation. In particular, to judge the feasibility of detection of astronomical sources in the presence of strong foreground signals from the zodiacal light and interstellar cirrus, observers and the science team will require knowledge of the sky brightness due to sources of emission unrelated to the astronomical source of interest. In this document we will refer to the absolute sky brightness on scales larger than a few arcmin as the “background;” a more rigorous definition is that we include the zodiacal light and emission from the diffuse interstellar medium as sources of the background. For the few arcminute-scale structure of the sky brightness, the best basis is the IRAS skymaps, for which an excellent tool already exists (IRSKY). A complete background estimation tool is already available at IPAC (in IRSKY and as batch inquiry through IBIS). In this memo, I describe the background estimation that was implemented in the SIRTF Planning Observation Tool (SPOT).