17 September 2012 Using a laser tracker for active alignment on the Large Binocular Telescope
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Abstract
The Large Binocular Telescope (LBT) currently achieves collimation using a combination of collimation models and closed-loop active correction schemes. Shack Hartmann wavefront sensors with off-axis guide stars are used for Gregorian modes, and a closed-loop correction scheme is used for the prime-focus cameras. While in general this combination serves to produce alignment residuals well below a good seeing limit within a few minutes of obtaining a given target field, the uniquely asymmetrical structure of the LBT is prone to producing large deflections of the telescope optics when the ambient temperature is changing unusually rapidly. These deflections are difficult to model satisfactorily, and are an ongoing source of inefficiency in telescope operations. Furthermore, none of the current approaches to telescope collimation are particularly "piston aware"; a situation that needs to be improved on now that the LBT is commencing operations with the first of its beam combining instruments, LBTI. The laser tracker is a metrology instrument capable of automatically measuring optical element positions with better than 100 micron precision within a spherical volume of 30 m radius centered on the tracker head. With the ability to directly measure optics into position to this accuracy built into the Telescope Control System (TCS), the LBT would always be starting observations from a point of near-collimation, the component telescopes would be co-pointed, and the OPD would be well within the capture range of the beam combining instrument's internal phasing systems. This paper describes first results from engineering investigations into using the laser tracker to automatically align the optics on the LBT.
© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
A. Rakich, A. Rakich, } "Using a laser tracker for active alignment on the Large Binocular Telescope", Proc. SPIE 8444, Ground-based and Airborne Telescopes IV, 844454 (17 September 2012); doi: 10.1117/12.928706; https://doi.org/10.1117/12.928706
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