Ramsey K. Melugin

Alignment And Evaluation Of The Cryogenic Corrected Infrared Astronomical Satellite (IRAS) Telescope

Room temperature alignment and evaluation techniques for the Infrared Astronomical Satellite (IRAS) telescope which has a primary mirror figured to correct for surface distortions at the 2 K operating temperature will be discussed. Interferometric cryogenic testing of the 0.6 meter, f/1 .5 light-weighted beryllium primary mirror at its intended operating temperature revealed surface distortions that could be modeled with Zernike polynomials. With this model, it was possible to derive the -inverse- of the cryo wavefront error (ideal cryo mirror) and figure the cryo correction into the primary mirror using Perkin-Elmers Computer Controlled Polisher. It was recognized that during room temperature assembly of the system, misalignment of the secondary mirror could introduce additional unwanted aberrations that would cancel or distort the wavefront errors purposely intro-duced by the cryo figuring. To avoid this possible degradation and to ensure optimum telescope performance, the system Zernike polynomial coefficients and wavefront maps generated from the in-process alignment interferograms were monitored and compared to Zernike coefficients and wavefront maps for the cryo corrected primary mirror. Using Zernike polynomials to monitor the optical quality of the telescope enabled figure and alignment errrors to be monitored, and demonstrated that the alignment tolerance was achieved.

Optimum Shapes For Lightweighted Mirrors

Two types of monolithic lightweight mirrors with arched backs are discussed: the center-supported single arch and the ring-supported double arch. The theoretical deformations of a 20-in.-diameter double arch mirror are compared with the actual deformations. A mirror of this size weighs about 50% less than an equivalent conventional mirror. The double arch design may be scaled up to 144 in. where the mirror weighs less than 40% of the eauivalent conventional mirror. Further weight savings are possible due to the reduced size and simplicity of the support required by the double arch mirror design.

Space Infrared Telescope Facility (SIRTF) Observatory Design

The Space Infrared Telescope Facility (SIRTF) is a one meter class cryogenically cooled infrared observatory under study by NASA for launch and operation in the mid 1990s. It operates in the spectral range 2-700 µm and represents a factor of over 1000 increase in sensitivity over the first NASA IR space mission, the Infrared Astronomical Satellite (IRAS) . SIRTF will be the first true infrared observatory in space and is complementary in wavelength coverage and comparable in sensitivity to the Hubble Space Telescope (HST), Gamma Ray Observatory (GRO) and the Advanced X-Ray Astrophysics Facility (AXAF) missions. such, it is considered part of the great observatories program being developed by NASA .

Fused Silica Mirror Evaluation For The Shuttle Infrared Telescope Facility (SIRTF)

The SIRTF optics are intended for operation at 20°K (or less); it will be extremely inconvenient, expensive, and time consuming if it becomes necessary to accomplish all of the optical element testing, assembly, and alignment at comparable temperatures. The thermal strain behavior, including potential anisotropies and inhomogeneities, of a mirror substrate between room temperature and 20°K thus becomes a major factor in the selection of the substrate material, structural configuration, and joining methods for lightweight structures. With support from Space Projects, NASA Ames Research Center, Itek Optical Systems is executing an optical figure evaluation of a 0.65-meter, lightweight, fused silica mirror at a low-temperature goal of 20°K. The design details of a thermal shroud, provisions for extracting heat from the low-conductivity mirror, and wavefront error sources other than the mirror surface are discussed and preliminary test results presented.

Compensation for 6.5-K cryogenic distortion of a fused-quartz mirror by refiguring

A 46 cm diameter, lightweight, Amersil TO8E, fused-natural-quartz mirror with a single-arch cross section was tested at the NASA-Ames Research Center Cryogenic Optical Test Facility to measure its cryogenic distortion at 6.5 K. Then the mirror was refigured with the inverse of the measured cryogenic distortion to compensate for this figure defect. The mirror was retested at 6.5 K and found to have a significantly improved figure. The compensation for cryogenic distortion was not complete, but preliminary analysis indicates that the compensation was better than 0.25 waves P-V if edge effects are ignored. The feasibility of compensating for cryogenic distortion by refiguring has thus been verified.

Ultra Lightweight Mirror Performance At 8 Degrees Kelvin

In response to technology needs for infrared (IR) telescopes operating at cryogenic temperatures, Eastman Kodak Company has developed a 0.5-meter (m), ultra lightweight, frit bonded, fused silica mirror capable of being scaled to a larger size that would provide a fast aspheric, smooth, low scatter optical surface. This mirror has been evaluated by Kodak at a temperature of 100 degrees Kelvin (°K). This paper reports on a continued evaluation of the mirror jointly by Kodak and Ames Research Center (ARC) to a temperature of 8°K. Analysis of common interferograms by independent processing hardware and software has been carried out by Kodak and ARC. The results of both processes are compared and reported.

A Mirror Mount For Cryogenic Environments

The finite element method was used to study the effect of mount-induced aberrations on the optical surface of a lightweight double arch mirror subjected to cryogenic temperatures. The mount design was controlled by the requirements imposed on the optical surface quality and stress levels. The finite element analysis was used to define the feasible range of mount parameters and the selection of a design within the feasible region. The final design consisted of three spring-loaded Invar T-clamps that uniquely define the location of the mirror, three radially compliant parallel spring guides that remove the effect of radial contraction of structure in cryogenic temperatures, and a flexible baseplate that was used to reduce the effect of temperature-induced baseplate tilt errors. The experimental results from the applica-tion of this system to an existing 20-inch fused silica double arch mirror are shown, and possible improvements in system performance are discussed.

Infrared Telescope Design: Implications From Cryogenic Tests Of Fused-Silica Mirrors

A brief review of results from recent cryogenic tests of fused-silica mirrors is given with consideration of the implications for the design of cooled infrared telescopes. Implications include optical performance with a discussion of the top-down optical error budgeting for the Shuttle Infrared Telescope Facility (SIRTF), thermal properties of the mirrors, and mirror mounting.

Cryogenic surface distortion and hysteresis of a 50-cm-diameter fused-silica mirror cooled to 77 K

A 50 cm diameter, lightweight, Amersil TO8E, fused-natural-quartz mirror with a single arch cross section was tested at the NASA/Ames Research Center Cryogenic Optics Test Facility to measure cryogenic distortion and hysteresis. The mirror was cooled to 77 K in four serial tests and the mirror figure was measured with a phase-measuring interferometer. On the basis of the repeatability of room temperature and cryogenic optical measurements, it was determined that the Single Arch Mirror had no measurable hysteresis and displayed repeatable cryogenic distortion. The Cryogenic Optics Test Facility, optical and thermal test methods, test results, and measurement accuracy are described.

Primary mirror and mount technology for the Stratospheric Observatory for Infrared Astronomy (SOFIA) telescope

Studies have been conducted at the Ames Research Center on the feasibility of concepts for Stratospheric Observatory for Infrared Astronomy (SOFIA). This paper summarizes studies of the feasibility of the SOFIA telescope primary mirror and its mounting. The primary mirror is required to be very lightweight (areal density approximately 100 kg/m2), have an f/ratio near 1.0, and have surface quality that permits imaging in the visible as well as the infrared. Data on large mirror technology is presented to represent the space of areal density and size defined by current and projected technology. The desired subspace for the SOFIA primary mirror is identified. Also described in the paper are the results of the design study conducted to assess the feasibility of designing a suitable mounting system for the primary mirror. The requirements for the mount design are given both in terms of the environmental conditions and the expected optical performance. The mirror and mounting was modeled using Programs PATRAN and NASTRAN. The mirror studied was a sandwich-type made of Ultra Low Expansion (ULE) silica with square cells in the core, and with a continuous edge band. The mirror is modeled using equivalent solid elements for the core. Several mount designs were evaluated, having different numbers of axial supports. The design study produced primary mirror surface deflections in 1 g as a function of mirror elevation angles. The surface was analyzed using an optical analysis program, FRINGE, to give a prediction of the mirror optical performance. Results from this analysis are included.