Manufacturing and testing of a convex aspherical mirror for ASSIST

摘要

ASSIST is the testbed for the ESO Adaptive Optics Facility. The main objective of ASSIST is the characterization of the1.2 meter deformable mirror which will replace the existing secondary of one of the 8 meter VLT telescopes. A largeconcave 1.65 meter diameter aspherical primary mirror combined with a 140 mm diameter convex aspherical secondaryforms the main optical system of ASSIST. Two additional optical units provide the light sources and the wave frontanalyzing optics. Without having the possibility for checking the entire optical system as a whole, each individual mirrorhad to be manufactured and tested using reliable techniques. The secondary mirror for ASSIST (AM2) is made of anoptically transparent material (BK7) with a specific and accurate backside radius in order to achieve a null test intransmission. Furthermore, not only the overall RMS surface error of AM2 is important, but due to the fact that it will beused in a setup that measures specific spatial frequencies, also the spatial frequencies of the surface error of AM2 isimportant. The aspherical surface is tested in double pass using an optical flat and an interferometer with a transmissionsphere. Manufacturing of this asphere is mainly done by hand at the optical lab of NOVA-ASTRON. The final accuracyof the reflecting surface is within the required 50 nm RMS with a surface roughness of less than 2 nm RMS. This paperreports in more detail on manufacturing and testing of the a-spherical convex mirror.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.