Temperature-regulated 22 GHz water vapor radiometers for CARMA

摘要

The Combined Array for Research in Millimeter-wave Astronomy (CARMA) have carried out a water vaporradiometer (WVR) project to test the WVR phase correction technique for better observational efficiency. Wehave built two uncooled, but temperature-regulated, 22 GHz WVR prototypes to explore the feasibility of thetechnique. To better isolate the effects of instrumental and atmospheric instabilities, we have optimized the WVRdesign for simplicity with less high frequency components. The calibration system is Dicke switch with a singleambient load. The thermal regulation system consists of heaters and multi-stage insulation. We have completedtesting of the WVR prototypes in a laboratory and at the CARMA site. The gain stability is about 20-100 mKand the front-end temperature rms is about a few mK to hundreds, depending on weather conditions. Based onthe site tests, the sky temperature at 22 GHz usually varies a few K in 15 minutes, which is not necessary dueto the atmospheric water vapor. Such short time-scale background temperature variation overwhelms the limitof the WVR dynamic range. Moreover, we have compared the WVR data rms with the phase monitor at thesite and obtain a scale factor of the 22 GHz water vapor line, 6-12, which is consistent with the results of otherWVR projects. We suggest that expanding the WVR dynamic range with diode detector models and a betterthermal regulation system are keys to the success of the CARMA WVR phase correction.