Specifications for optical surfaces have traditionally been given in terms of low frequency and high frequency components, often with a separate classification for surface slope. Low spatial frequency components are commonly referred to as figure errors and can be described by the standard 37-term Zernike polynomial set. High spatial frequency errors are commonly referred to as finish and are quantified using rms roughness. Specification with the qualitative scratch and dig classification is done usually for cosmetic or aesthetic purposes. Mid-spatial frequency errors such as waviness, ripple, and quilting can be important and are not explicitly covered by such traditional figure and finish specifications. In order to bridge the gap to cover mid-spatial frequencies, in terms of quantifying surface characteristics, Power Spectral Density (PSD) can be utilized. For such usage, it is important for the greater optics community to understand the metric, how to calculate it, and how to use it. The purpose of this paper is to provide an overview of PSD, its application in optics, and an outline of calculations needed to effectively apply it to specify optical surfaces.
Manufacturers of optical glass strive to make a product that is
homogeneous, isotropic, and free of any bubbles or mechanical
strain. Glass used in forming images is very good, but the
process of mixing the constituent materials, and melting them into
a glass is limited. As uniform as the mixtures are, they are not
perfect, and the effects can be seen anytime light must propagate
through several centimeters of glass. One method for measuring
the three dimensional inhomogeneities in a piece of glass will be
shown. Interferometry and computed tomography will be used to map
the bulk refractive index variations. Having three dimensional
information on the refractive index is the first step in
compensating for errors in an imaging system.
Large field of view, high power, lithography, and laser fusion systems are all limited by refractive index variations of their constituent glass elements. To estimate how the image degrades, a model ofthe bulk index inhomogeneity must be fonned.
Exploration of faint distant objects in space has been limited by the power of telescopes. Currently our only option for studying these remote objects is to build larger and better telescopes. These giant telescopes are often constrained by system mass, which is dominated by the primary mirror. It appears that the evolutionary path of using conventional technology to build giant mirrors will not be sufficient to meet the small areal density of approximately 1.5 kg/m2. Therefore the development of large primary mirrors for space is dependent on innovative approaches and new technology. One approach to building a large primary reflector is to use smaller individual segments and place them along a curve approximating a paraboloid. These smaller segments could be comprised of either flat or curved thin membrane mirrors. These thin membrane mirrors have the potential of meeting the small areal density requirement.
We have started development on a thin membrane mirror. We have built and are testing a 6 inch stretched membrane mirror prototype that uses electrostatic pressure to pull the nominally flat mirror to a 32 m radius of curvature and adaptively correct for aberrations. Preliminary test results of the flat membrane are promising. The surface error for the flat membrane was measured to better than λ/10 rms for the center four inches and λ/20 rms over the central three inches. The interferograms for the curved membrane show a residual figure-eight pattern of high order astigmatism, most likely due to tension anisotropy in the mirror. Analysis on the fully curved mirror is still on-going. This paper discusses the SMEC design, development, test results, and current on-going activities.
Very lightweight mirrors can be constructed by stretching a membrane to form a flat surface. Adding tension to the membrane, making it flat, can be done by discrete attachment points, or by using a continuous boundary. Such lightweight mirrors are very attractive for space telescopes where a 100-m aperture can be made up of smaller mirror segments. Adding a slight curvature to each segment simplifies the optical train. This article looks at the making of a curved membrane mirror, and demonstrates its use. Measurements of the flat membrane, and the curved figure will be shown.
Current work concentrates on making optically flat mirrors using stretched membranes. Very lightweight mirrors can be made that only require a rigid support at the perimeter of the membrane. Contact with the membrane need not be continuous, only discrete attachment points are required to tension the material. Initial results of useable area as a function of the number of attach points will be given. Experimental fixtures demonstrating methods of forming a flat membrane are shown. The potential for nearly flat mirrors is mentioned including one method of implementation. Surface measurements are also contrasted for different materials.
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.
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