Radiometer Design Jessup reviewed his proposed radiometer design, which calls for enclosing the sensors and blackbodies within a sealed, temperature-controlled housing. The housing would be filled with dry nitrogen, and the sensors would look out through an IR-transparent window, equipped with a wiper and fluid reservoir to periodically clean salt, spray, etc. Jessup acknowledged this, but said that if he can find an IR-transparent window that can be very accurately characterized, the protection it would afford the sensors could more than account for the added uncertainty introduced. The attempt to isolate the radiometer passively with the insulation appears to have caused temperatures to drift up significantly. Nightingale described the SISTeR, developed at Rutherford Appleton Lab.
SISTeR is based on an ellipsoid mirror that allows a small foreoptics window.
This permits smaller internal blackbody calibration targets and a narrow exit slit, which protects against the elements. The chopper is close to the detector, and the filter is between chopper and detector. Barton pointed out that the chopper could change temperature when the hot and cold blackbody are viewed. For an SST measurement including a sky scan, the measurement cycle is about 2 minutes, which includes a look at the two blackbodies. A conical mirror above the radiometer is to be used to feed the radiometer and create a large footprint.