Steward Observatory is completing the manufacture of a deformable f/15 secondary mirror for the 6.5m Multiple Mirror Telescope conversion that will, along with the wavefront sensing system, compensate for atmospheric turbulence. A potential difficulty of an adaptive secondary mirror is the ability to verify the commanded mirror shapes of a large convex deformable surface. An optical design is presently being implemented to test the deformable mirror's closed loop control system by optically projecting an artificial star to simulate starlight in the actual telescope. The test system has been designed to verify the control system by fitting into both a laboratory test structure as well as the telescope support structure itself. The optical design relies on two wavelength computer generated holograms used to remove spherical aberration as well as aid in the alignment of the test system optics by projecting alignment patterns.
We will present a system to perform closed-loop optical tests of the 64 cm diameter, 336 actuator adaptive secondary made at the Steward Observatory Mirror Laboratory. Testing will include Shack-Hartmann wavefront sensing and modal correction of static and dynamic aberrated wavefronts. The test optical system is designed so that experiments can be made with both the focal plane instrument and secondary installed in their normal configuration at the MMT, or with the same 9 m spacing in a laboratory test tower. The convex secondary will be illuminated at normal incidence through two 70 cm diameter lenses mounted just below. The artificial, aberrated star is projected from near the wavefront sensor in the Cassegrain focus assembly. Computer generated holograms correct for spherical aberration in the really optics at the test wavelengths of 0.594 and 1.5 micrometers . Atmospheric turbulence is reproduced by two spinning transmission plates imprinted with Kolmogorov turbulence. The Shimmulator will give us the opportunity to test fully the adaptive optics system before installation at the new MMT, hence saving much precious telescope time.
Recent experiments with four 10x EUV imaging systems provide the first direct comparisons of visible-light and at- wavelength EUV interferometers performed using the state-of- the-art measurement tools that will be used to assemble and align the next generation of EUV imaging systems. Measurements from four individual multilayer-coated Schwarzschild objectives are discussed. Favorable agreement has been achieved between EUV and visible-light system wavefront measurements in all four optical systems. Measurements made in the presence of surface contamination and multilayer thickness variation, however, do show expected localized differences between the two measurements.
The phase-shifting point diffraction interferometer (PS/PDI) has recently been developed and implement at Lawrence Berkeley National Laboratory to meet the significant measurement challenge of characterizing extreme UV (EUV) projection lithography systems. Here progress on the characterization of the PS/PDI accuracy is presented. Two major classes of errors affect the accuracy of the interferometer: the first being systematic effects arising from the measurement geometry, and the second being random and systematic errors caused by an imperfect reference wave. In order to characterize these contribution and calibrate the interferometer. Experimental results demonstrating a systematic-error-limited accuracy of 0.004 waves is reported.
Modifications made to the Long Trace Profiler (LTP II) system at the Advanced Photon Source at Argonne National Laboratory have significantly improved the accuracy and repeatability of the instrument. The use of a Dove prism in the reference beam path corrects for phasing problems between mechanical errors and thermally-induced system errors. A singe reference correction now completely removes both error signals from the measured surface profile. The addition of a precision air conditioner keeps the temperature in the metrology enclosure constant to within +/- 0.1 degree(s)C over a 24 hour period and has significantly improved the stability and repeatability of the system. We illustrate the performance improvements with several sets of measurements. The improved environmental control has reduced thermal drift error to about 0.75 microradian RMS over a 7.5 hour time period. Measurements made in the forward scan direction and the reverse scan direction differ by only about 0.5 microradian RMS over a 500 mm trace length. We are now able to put 1-sigma error bar of 0.3 microradian on an average of 10 slope profile measurements over a 500 mm long trace length, and we are now able to put a 0.2 microradian error bar on an average of 10 measurements over a 200 mm trace length. The corresponding 1-sigma height error bar for this measurement is 1.1 nm.
Modifications made to the long trace profiler system at the Advanced Photon Source at Argonne National Laboratory have significantly improved the accuracy and repeatability of the instrument. THe use of a Dove prism in the reference beam path corrects for phasing problems between mechanical errors and thermally-induced system errors. A single reference correction now completely removes both error signals from the measured surface profile. The addition of a precision air conditioner keeps the temperature in the metrology enclosure constant to within +/- 0.1 degree C over a 24 hour period and has significantly improved the stability and repeatability of the system. We illustrate the performance improvements with several sets of measurements. The improved environmental control has reduced thermal drift error to about 0.75 microradian RMS over a 7.5 hour time period. Measurements made in the forward scan direction and the reverse scan direction differ by only about 0.5 microradian RMS over a 500mm trace length. We are now able to put 1- sigma error bar of 0.3 microradian on an average of 10 slope profile measurements over a 500mm long trace length, and we are now able to put a 0.2 microradian error bar on an average of 10 measurements over a 200mm trace length. The corresponding 1-sigma height error bar for this measurement is 1.1 nm.
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