An Interferometric Hartmann Wavefront Analyzer for the 6.5m MMT, and the First Results for Collimation and Figure Correction

The theory, optical, mechanical, and software design for an interferometric Hartmann wavefront analyzer for the 6.5 m MMT is presented. The instrument is modular so it can be used at the prime focus or any of the three unique Cassegrain foci. We discuss the first results at the f/9 Cass focus where the detected wavefront error was used to correct both collimation and the primary mirror figure distortion. Despite the fact that neither M1 nor M2 were thermally controlled at the time, it is found that two correction iterations produce an image psf near 0.1 arcsec p-p. I. Overview The 6.5m MMT will have 3 Cassegrain foci -f/9, f/5, and f/15. The MMTO will provide facility wavefront sensors for the f/9 and f/5 focal modes. The f/15 instrumentation will have dedicated wavefront sensing provided by Steward Observatory’s CAAO group tailored for the use of adaptive secondary mirrors. The facility wavefront analyzers consist of two types. The first is a relatively high resolution modular wavefront analyzer whose configuration can be changed for use at either the prime, f/9, f/5 or f/15 foci. This provides over 30 phase apertures across the pupil diameter. It is primarily intended for stand-alone opto-mechanical studies of the optics, mirror support systems, telescope support structure, and the construction of elevation and temperature-dependent look-up tables. The instrument is based on the interferometric Hartmann (or Korhonen-Hartmann) technique invented at the Nordic Optical Telescope (NOT) and also used at the Vatican Advanced Technology Telescope (VATT) [1-6]. The second type of wavefront sensor resides permanently at the f/9 and f/5 Cassegrain foci. They have not yet been constructed, but will most likely be Shack Hartmann or curvature sensing units. They will provide nightly routine refinements to the look-up table collimation and figure correction. Section II presents a brief overview of the theory of the interferometric Hartmann technique and contrasts it to a Shack Hartmann device. Section III outlines the design and optomechanics of the interferometric Hartmann analyzer. Section IV explains how the wavefront error is determined from the phase-differences detected by the analyzer. Section V summarizes the first results of active figure correction and collimation at the f/9 focus of the 6.5m MMT. Section VI explains our development of interactive software for collecting and analyzing wavefront data. II. The Interferometric Hartmann Technique The interferometric Hartmann analyzer directly measures wavefront phase differences in contrast to the Shack-Hartmann which measures wavefront gradients. Instead of a lenslet array, the interferometric device uses a simple Hartmann aperture mask array placed at the collimated re-imaged pupil. A single converging lens focuses the Airy patterns produced by the apertures. The relatively large Airy patterns overlap each other in the extra and intra focal areas adjacent to the focus. Groups of 4 adjacent apertures (a quartet) produce sharp interference in these overlapping regions. The position of the interference spots depends upon the phase differences in the corresponding 4 apertures. The size of the diffraction spot from the Shack Hartmann lenslet is inversely proportional to the diameter of the lenslet (2.44/D), while the size of the interference spot from the Korhonen device is inversely proportional to the separation between apertures (1/d). Therefore, the interference spot can be made up to 7 times smaller than the corresponding spot from a Shack Hartmann device. Insofar as the accuracy of the centroiding algorithm increases with decreasing spot size, the accuracy in detected wavefront errors increases. The position of the m=0 diffraction spot depends upon the phase differences between the 4 apertures. It is sensitive to all sources of phase difference (including piston, to which the 1. Now at the Univerisity of Arizona Center for Astronomical Adaptive Optics.