Development of an optical device 'Field Stacker' for achieving accurate photometry in ground-based mid-infrared observations

摘要

Strong time variation of atmospheric transmittance is a crucial problem for monitoring observations at mid-infrared wavelengths from the ground. To overcome this problem, a new device called " Field Stacker " has been developed. It is an optical device to combine two discrete fields in the telescope FoV into a single field and feed it in the camera. It enables us to observe a science target and a reference star simultaneously, and improve the photometric accuracy dramatically based on real-time calibration. To practically achieve highly accurate photometry, the tilt of the mirrors in the Field Stacker should be accurately adjusted. Acceptable error of the misalignment of each pick-up mirror is estimated to be ﹤ 0.0085 deg from a simple geometric calculation. The actual tilt error measured in the laboratory almost met this requirement. Spatial variation of the water vapor in the atmosphere is another concern for the accurate photometry. Assuming a simple model of the atmospheric structure, the spatial variation was estimated from time variation of infrared background radiation. The estimated variation of the water vapor was 0.00036 mm within the telescope FoV (Φ25 arc-minutes), suggesting that it does not significantly affect the photometric accuracy even at 31 and 37 μm. Number density of reference stars was examined based on all-sky infrared catalogues to estimate the availability of the Field Stacker. The estimated availabilities at 9 and 18 μm were 99.9% and 59.6%, respectively.