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

Speaker: Dr. Bojan Nikolic(Cavendish Laboratory/Cambridge University)

Date: January 18，2018 (Thursday)

Time: 14:00-15:00

Location：Middle conference room, 3rd floor

Abstract:

The maximum practical size of large single-dish telescopes is largery defined by the effects three forces: gravity, thermal gradients, and wind. Measuring these effects is critical for testing and comissioning of telescopes, and furthermore, if known, all of these can in principle be corrected by the new generation of active surfaces on telescopes.

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 discuss the phase-retrieval holography technique based on Out-Of-Focus (OOF) beam maps that is particularly suited to measuring these deformations. One of the key advantages of this technique is that it makes a measurement of the full optical train through to the astronimical detectors.

I will also present some results of 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 largery correct for thermal effects even in daytime. I will also review some of the other applications of OOF at telescopes around the world