STELLAR-BASED CALIBRATION IN THE FAR-INFRARED WITH APPLICATION TO IRAS BAND-4

Because stars emit very small portions of their outputs in the far inf rared, using them as calibrators requires precise measurement and corr ection for filter leaks at shorter wavelengths. Therefore, it is commo n to base far infrared calibrations on planetary objects such as aster oids. However, asteroids are complex geological bodies whose thermal p roperties depend on their evolutionary histories as well as on their g ross parameters such as mass and composition, making them difficult to model as calibrators. We propose a new method for measuring filter le aks that can be carried out using the end-to-end detector system and t herefore allows reliable use of stellar calibrators. We illustrate thi s method by showing that the IRAS 100 mum (Band 4) filters had a short wavelength leak of 14.3% +/- 3.6% on stars similar to alpha Boo, but that there is no detectable leak in the 60 mum (Band 3) filters. We de rive a calibration for Band 4 from stellar colors in a way that is clo sely analogous to the calibrations of Bands 1, 2, and 3. With correcti on for the leak, the stellar-based calibration is virtually identical to the original calibration based on asteroids; this result requires t hat the spectra of the asteroids used for the original calibration dif fer from greybody behavior between 60 and 100 mum by about 10%.