A Caltech Submillimeter Observatory Active Optics System

Abstract

Active surface correction of the Caltech Submillimeter Observatory (CSO) primary mirror has been accomplished. The Dish Surface Optimization System (DSOS) has been designed and built to operate at the CSO, on Mauna Kea, Hawaii. The DSOS is the only active optics system of its kind in the world. There are 99 steel rod standoffs that interface the dish panels to its backing structure. Each standoff is now fitted with a heating/cooling assembly. Applying a controlled potential to each of the 99 assemblies adjusts the surface of the dish. Heating elongates and cooling shortens the standoffs, providing the push or pull on the primary’s panel surface. The needed correction for each standoff, for a given elevation, is determined from prior holography measurements of the dish surface. Without the DSOS the optimum surface accuracy was 25 μm RMS, yielding a beam efficiency of 33% at the 350 μm wavelength range. With the DSOS on, this has been improved to 10 μm RMS. The best beam efficiency obtained is 56%, with an average beam efficiency of 53%. The DSOS has been in operation on the CSO since February 2003. Observers using SHARC-II (Dowell et al. 2003; a 384 pixel submillimeter high angular resolution camera) and the 850 GHz heterodyne receiver, have been able to detect new weak and/or distant objects with the help of this unique active optics system.