There is an increasing need for precision large aspheric optics with small focal ratios for astronomical and space
applications. However, testing such optics presents a challenge. Interferometric testing of aspheric surfaces often
requires the use of null lenses. Many of these null lenses are tested using a certification computer-generated hologram
(CGH) for better error calibration. We present a method that will measure large aspheres to a greater level of accuracy
than is presently possible. We use segmented and superposed CGH elements to certify and calibrate null lens errors
absolutely to a high degree of accuracy. In such holograms two different phase functions are encoded on the CGH by
means of aperture division. One subaperture generates a spherical wavefront that is used to determine the pattern errors
of the hologram while the second subaperture reconstructs an aspherical wavefront used to calibrate the wavefront errors
of the null lens. This careful calibration process involves the removal of both axisymmetric and non-axisymmetric
errors in the null test. Once this is accomplished, the null lens may be used to test the asphere to a high degree of
accuracy. Our initial results show that we can test 4-meter class aspheric mirrors to better than 1nm rms surface error.
In current experiments we have set a goal of measuring such mirrors to better than 1nm rms surface error.
Precise manufacturing of optical flats requires precise characterization of the surface. A scanning pentaprism system is ideal for use as an absolute and precise test for an optical flat. Such a system was built and used to test a 2-m diameter flat mirror. This system uses light from an autocollimator that is reflected from two 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-m flat, characterizing the errors and their sources. There is enormous potential for our system to be used to test larger flats and even curved surfaces, made of either a glass or a liquid.
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.
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.
Dynamically refocusing the Rayleigh backscatter of a modestly powered laser beacon is a concept for increasing LGS brightness by 10 times. Dynamic refocus will allow for high photon return from multiple Rayleigh beacons enabling MCAO for wide field correction of the MMT and Magellan telescopes. 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 the design, laboratory tests and preliminary field tests of a dynamic refocus system for 351nm Rayleigh beacon laser guide stars. The purpose of dynamic refocus is to increase the beacon signal from a pulsed laser, by maintaining focus in a fixed plane while the laser pulse travels through the atmosphere over an extended height range. The focusing element in our system is a moving concave mirror. The optics have been designed and built to focus on a ring of 5 beacons at 1 arc minute radius at the 6.5 m MMT, covering the range 18 through 40 km. Laboratory tests of image quality resulted in 0.5 arcsec refocused images corresponding to the height range 22 through 28 km, free from spherical aberration. Preliminary field tests were performed on the Mt. Bigelow Observatory 1.5 m telescope, with a frequency tripled, Q switched YLF laser beam projected from a 25 cm telescope. To simulate an off axis sub aperture of the MMT, the laser and telescope axes were set 3 m apart and reimaging optics were placed ahead of the refocus unit to image at the same plate scale as the MMT (500 μm/arcsec). Returns from different heights were selected by gating the detector with a Pockels cell. Returns over a 10 km height range from 8km to 18km were brought into focus for a total mirror motion measured to be 900 μm. The system is now ready for testing dynamic refocus, which will be accomplished by attaching the mirror to a metal resonator tuned to the laser pulse frequency. The range from 23 to 35 km to be used will require a motion of 500 μm.