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

Abstract

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 $\mu$ W 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 ${\hbox{10}}^{-10}$ above 6 THz. Thermal flow power into the detector, including the stray light power, is about 0.7 $\mu$ W. The cold optics with an 600 pixels MKID camera has been cooled down to 100 mK.