A scanning pentaprism system may be used as an absolute test for an optical flat. Such a system was built and used to test a 2-meter flat mirror. This system uses light from an autocollimator that is reflected from 2 pentaprisms to project reference beams of light onto the flat mirror. The light reflected from the mirror back through the pentaprisms provides information on low order optical aberrations in the flat mirror. We report results of the test on a 2-meter flat.

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Dynamically refocusing the Rayleigh backscatter of a modestly powered laser beacon is a concept for increasing LGS brightness by 10 times 1,2,3 . Dynamic refocus will allow for high photon return from multiple Rayleigh beacons enabling MCAO for wide field correction of the MMT and Magellan telescopes 2,3,4,5,6 . In a system without dynamic refocus, light from a beacon integrated from 20 to 30km is blurred to a length of 14arcsecs. In a system with dynamic refocus, the bow tie is restored to a spot limited only by atmospheric seeing. The dynamic refocus system has been designed to deliver images with <3/4arcsec of induced aberration. This paper reports on field tests performed on the Mt. Bigelow Observatory 61" telescope, optically configured to appear as an off-axis sub-aperture of the 6.5m MMT. In these tests the Rayleigh backscatter from pulses of a Q-switched doubled Nd:YAG operating at 5kHZ was dynamically refocused. These preliminary tests present an uncorrected 7 by 3arcsec beacon image. The 7arcsec length is a result of using a field stop as the range-gating mechanism and the 3arcsec limit is due to double pass imaging (projecting and imaging) through the atmosphere in less than ideal seeing conditions. Upon correction, this 7x3arcsec image is dynamically refocused to a 3arcsec FWHM diameter spot.

We present a method for a cascading null test using twin computer-generated holograms to calibrate errors in null correctors. This will allow us to test large aspheres an order of magnitude better than current limits. We discuss various sources of CGH errors and how to calibrate them. We also mention some ways to measure and calibrate the errors in the test optics.