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 ban ds centered at 150, 250, 400, and 700 mum with spectral resolution lam bda/DELTAlambda almost-equal-to 3 and spatial resolution DELTATHETA = 0.5 degrees. High sensitivity is achieved by using bolometric detector s operated at 300 mK in an AC bridge circuit. The closed-cycle He-3 re frigerator can be recycled in orbit. A 2 K shutter provides a zero ref erence for each field of view. More than 10% of the sky will be survey ed during the 3 week mission lifetime with a sensitivity of less than 10(-13) W cm-2 sr-1 per 0.5 degree pixel.