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The CSO is one of a very few radio telescopes with an active surface correction system. This Dish Surface Optimization System (DSOS) improves the telescope efficiency by altering the primary mirror surface to correct for static imperfections and for changes in gravitational deformation as the telescope moves in zenith angle (elevation). The consequent improvement in the telescope's aperture efficiency is significant at shorter wavelengths, in particular at 350 µm wavelength.
The CSO's 10.4 m primary mirror is made up of 84 hexagonal panels with aluminum skins and honeycomb cores. The panels are connected to the backing structure by 99 steel rod standoffs. Each standoff is fitted with a (Peltier) heating/cooling assembly so it can be elongated (heated) or contracted (cooled). A controlled potential is applied to each of the 99 assemblies to adjust the shape of the primary mirror. The necessary standoff lengths were determined from holography measurements of the telescope over a range of zenith angles.
Holography measurements indicate the DSOS improves the surface accuracy of the telescope to about 11 µm rms compared to about 25 µm rms without the system. This would correspond to an improvement in aperture efficiency from 33% to 79% at the 350 µm wavelength. The CSO's holography instrument, however, operates in the range of roughly 230 to 460 GHz. Additionally improvement in efficiency does not translate linearly to higher frequencies. As a result, of 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. Instruments mounted on the Nasmyth focus (N2) also 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 SHARCII 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.
CSO Technical Briefing, Hilo, HI, October 2007
Update to the CSO Submillimeter Active Optics System,
Leong, M. M., 2007, CNC/USNC/URSI North American Radio Science Conference, Ottawa
A CSO Submillimeter Active Optics System,
Leong, M., 2005, USNC/URSI National Radio Science Meeting, Boulder, p. 426
Dish Surface Optimization System: Surface Correction on a 10.4-meter Leighton Primary Mirror,
Leong, M., Houde, M., Peng, R., Yoshida, H., Chamberlin, R., Phillips, T. G., 2003, in AMOS Conference Proceedings, ed. P. Kervin, and J. Africano, AFRL/MEDB, Kihei, HI
A method for implementing the CSO surface correction hardware: Memo No. 1,
CSO Staff (M. Leong, R. A. Chamberlin, M. Houde, R. Peng, T. G. Phillips, H. Yoshida), 2002 October 18