Talk on OOF holography at Cambridge

I gave a talk here in Cambridge this week on the OOF holography technique, link and screen shot below

Title: The OOF Holography Technique: Correcting the Effects of Gravity and Thermal Gradients on Large Filled-Aperture Telescopes

http://www.mrao.cam.ac.uk/~bn204/publications/2009/MRAOApril09.pdf

Abstract:

The maximum practical size of large single-dish telescopes is mainly defined by the effects three forces: gravity, thermal gradients, and wind. All of these can in principle be corrected by the new generation of active surfaces on telescopes if we knew the actual deformations that are caused by them. At current cm, mm and sub-mm telescopes, measuring these deformations corresponds to measurement of one part in a million which is challenging for conventional survey techniques. In this talk I will introduce the phase-retrieval holography technique based on Out-Of-Focus (OOF) beam maps that is particularly suited to measuring these deformations. I will also present the application of this technique to the 100m-diameter Green Bank Telescope where we have been able to eliminate residual gain variation due to gravity and open up the daytime for efficient observations at mm-wavelengths.