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 telescope consists, in part, of an 84-hexagonal paneled 10.4-meter primary dish designed by Robert Leighton.  The DSOS corrects the dish surface figure for imperfections and gravitational deformations, as the dish moves in elevation during observations.  This improvement in the telescope's aperture efficiency aids observations at the shorter wavelengths, specifically in the 350-µm-wavelength range.

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 stored holography maps of the dish surface.

April 2005 Holography Maps - DSOS On (10.5µm / 10.5µm / 7.1 µm)

From holography, the latest measured optimum surface accuracy of the telescope is 10.0 µm RMS, with the DSOS on.  The decrease from 25 µm RMS (without the DSOS) to 10.0 µm RMS, translates linearly to an improvement in aperture efficiency from 33% to 79% at the 350-µm-wavelength range.  However, the CSO's holography instrument is frequency limited to a range of roughly 230 to 460 GHz.  Additionally improvement in efficiency does not translate linearly to higher frequencies.  As a result course efficiency measurements at the 350-µm-wavelength range were done. About a 71% best and 60% average improvement in peak signal power was measured.  Which is about 56% efficiency.

* These excellent results were measured with instruments mounted on the Cassegrain focus of the telescope. Instruments mounted out on N2 (Nasmyth 2) see marked improvements but do not benefit as much from the newer correction table. Steps are being taken to determine the adjustments of the DSOS to best optimize performance of the telescope out to N2.

The DSOS has been in operation on the CSO since February 2003.  Observers using the SHARCII (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 including with the help of this unique active optics system.

Please read and follow the Users Procedure!  This will help the system function properly (you get good data) and avoid unnecessary down time (no surface correction during your observation run).