The Far-Infrared Photometer on the Infrared Telescope in Space

We describe the design and calibration of the Far-Infrared Photometer (FIRP), one of four focal plane instruments on the Infrared Telescope in Space (IRTS). The FIRP will provide absolute photometry in four bands centered at 150, 250, 400, and 700 microns with spectral resolution wavelength/wavelength spread is approximately 3 and spatial resolution delta theta = 0.5 degrees. High sensitivity is achieved by using bolometric detectors operated at 300 mK in an AC bridge circuit. The closed-cycle He-3 refrigerator can be recycled in orbit. A 2 K shutter provides a zero reference for each field of view. More than 10% of the sky will be surveyed during the 3 week mission lifetime with a sensitivity of less than 10(exp -13) W per sq cm per sr per 0.5 degree pixel.