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Design of an holographic off-axis calibration light source for ARGOS.

Abstract 131

Submitted by Christian SCHWAB


C. Schwab (1), D. Peter (2), W. Gaessler (2), A. Quirrenbach (1)


Landessternwarte Heidelberg (1), Max-Planck Institute for Astronomy (2)


Future adaptive optics (AO) systems for extremely large telescopes (ELTs) face additional challenges compared to conventional single conjugated AO-systems. First of all, the AO system is directly incorporated into the telescope design; this means the telescope has a large deformable mirror (DM) in the strongly converging optical train to correct the atmospheric turbulence. Secondly, the AO systems will potentially use more than one laser beacon in order to correct a large field of view. It is highly desirable to have artificial light sources illuminating the DM for calibration tasks during day time. This saves expensive night time and allows for extensive tweaking of the system. The design of an artificial light source is challenging since the described setup forces one to place the lightsource in a beam with a small F-number and additionally the light source has to feign the aberrations due to the off-axis positions of the laser beacons. We report on the design of an artificial light source for the ARGOS rayleigh laser guide star facility that will arrive at the LBT in the near future. ARGOS utilizes three laser beacons per eye of the LBT. They are projected on a circle with 2’ radius around the optical axis at a height of 12 km above the telecope. The calibration light source is positioned in the Gregorian focus illuminating the deformable secondary mirror with an F/1 beam. It consists of three off-axis laser sources for calibration and testing of the system, and additionally two on-axis light sources for alignment, and the simulation of a source at infinity to control the PSF on the science instrument. The design makes extensive use of computer generated holograms (CGH) and strong aspheres to meet the specified performance of a residual wavefront error small enough not to compromise the achievable correction of the AO system.