Sub-picowatt precision radiometry using superconducting transition edge sensor bolometers

We describe a cryogenic radiometric calibration system suitable for high se nsitivity, low background power bolometers, The system uses a variable temp erature blackbody source capable of temperatures of 4 to 20 K, whose output covers the sub-millimeter wavelength regime. The detector is an eight-chan nel, filled-focal-plane array of superconducting transition edge sensor (TE S) bolometers, The system is designed to test components of the optical sys tem such as polarizers, filters and stray light suppressors as well as the performance of the detector array elements, The bolometer response to incid ent submillimeter radiation is measured and compared to the calculated outp ut of the source seen by the bolometer.