Development of a Compact Cold Optics for Millimeter and Submillimeter Wave Observations

We have developed an optics for 220 GHz observations, which is a compact cold re-imaging one from a telescope focal plane, with F/# = 6 to a detector plane with F/# = 1 at 100 mK. It employs two high refractive lenses, high purity alumina (n=3.1) and silicon (n=3.4). To reduce the incident stray light into the detector, a cold nested baffle composed of four reflectors with the same spherical shape has been developed. The stray light power is simulated to be 0.2 μW which corresponds a quarter of that of a without-baffles case. The total transmittance of three kinds of IR blocking filters is 0.78 at the observation frequency, and less than 10^-10 above 6 THz. Thermal flow power into the detector, including the stray light power, is about 0.7 μW. The cold optics with an 600 pixels MKID camera has been cooled down to 100 mK.