Large optical systems, those in the regime of two-foot aperture and larger, have represented an important branch of the optical sciences for a considerable time. Such systems have been improving continually in design concept and performance for more than two hundred years. The science of astronomy provided the driving force behind early developments in the field, beginning in the late eighteenth century, and has continued to stimulate improvements since that time. Prior to 1800, Sir William Herschell had built a number of astronomical telescopes, including one with a four-foot aperture.
A large, ultralightweight Cer-Vit mirror of oblong shape has been optically polished to a surface irregularity of 0.060 rms, where = 0.633 urn. Although its major dimension is 193 cm, its finished weight is only 97 kg. The nonconventional fabrication methods required by its light weight, unusual aperture shape, relatively steep curvature, and excep tionally large diameter-to-thickness ratio are described. A 64-point fluid piston mount employed to support the mirror both for optical fabrication and for interferometric test ing is also described. Techniques specially developed to produce both the mirror blank and the final optical surface demonstrate significant advances in the manufacture of large, very lightweight glassy mirrors of high optical quality.
The optical fabrication and testing of the optical components for two large optical telescopes is described. These systems are the 88-inch aperture telescope for the University of Hawaii, and the 85-inch telescope for the Universidad de La Plata. A brief historical account of events leading to the large optics facility is included.
An optical test method for determining the optical quality of convex paraboloidal surfaces is described. Several test methods are described briefly; however, the optimum method using a collimator-reflector is described in more detail.
A major scientific goal of the Large Space Telescope (LST) is to observe very faint astronomical objects. Such observations require extreme attenuation of light entering the end of the telescope from bright objects in the celestial sphere. Operation on the daylight side of the orbit is required, hence portions of the internal structure of the telescope may be illuminated by the earth, moon, or sun. The intensity of light from these sources reaching the focal plane must be small compared to the intensity of the object of interest. A computer program for straylight suppression system design and analysis was developed for MSFC under contract to the University of Arizona Optical Sciences Center. MSFC is using this program to analyze various proposed LST straylight suppression systems. Simultaneously, experimental measurements are being made on a simplified LST straylight suppression system in a unique facility at MSFC. The experimental measurements are being used to verify and improve the computer program. The facility represents the state-of-the-art in straylight suppression measurements, and transmission factors of 10-12 have been measured.
A highly corrected three-mirror telescope has been designed for space application. It is free of spherical aberration, coma, astigmatism and field curvature. The rms-spot size within a 40 arc min field is equal to or better than .025 arc sec. The full field cannot be utilized in its entireness. A number of subfields for different instruments can be folded out radially.
In a feasibility study the system analyst must assimilate many conflicting mission goals before selecting preliminary parameter values. If the analyst uses a computer to handle the data he may lose sight of some important interrelationships. It is therefore often preferable to keep to a paper-and-pencil level in the preliminary stages of a system design. In this paper such a level of analysis is developed for relating spatial and radiometric resolution to aperture and f/number in a large optical system. This "asymptote analysis" has been applied in a recent study of NASA's SEOS/LEST concept.
The purpose of this investigation was to determine the feasi-bility of producing large diameter, high quality optical surfaces similar to those that will be required for some of the planned orbital observing units such as the Large Space Telescope (LST). Current requirements for the Large Space Tele-scope require that a A /20 rms wavefront be produced at the Cassegrain focus. A reasonable distribution of error sources would, therefore, demand that the primary mirror be figured to a tolerance of A/64 rms on the surface. NASA has funded the initial step toward demonstrating the ability to meet that requirement. An optical fabrication demonstration program was undertaken using an available 1.8-meter-diameter ULE monolithic mirror blank weighing on the order of 1,200 pounds. The mirror that was used had no central perforation and was approximately 12 inches thick with 1-inch front-and hack-plates. In order to accomplish this task, a variety of support and test equipment was designed and fabricated and the actual optical fabrication and test were conducted over a period of approximately eight months. The results of this effort yielded a mirror with a surface quality of 0.015 wave rms or A/65.3. = 632.8 nanometers.)
The Multiple Mirror Telescope, a joint University of Arizona and Smithsonian Astrophysical Observatory project, was first reported in this journal in 1972. The optics are in production, some complete, others near completion. Some of the ideas we used worked, others did not. This paper discusses the techniques actually used and how well they succeeded. The Multiple Mirror Telescope i(MMT) was described previously in the March-April 1972 issue of this journal. For a quick review, Figs. 1 and 2 present the optical design of this instrument. It consists of six independent Cassegrainian telescopes placed in one optical support structure (OSS) and mounted in an altitude over azimuth mounting. The six telescopes will be collimated to a mutual axis and maintained by the use of an active optical alignment system. The alignment signals, as well as stellar tracking information, are gen erated by a seventh (30" diameter) guide-alignment telescope located in the center of the large array. Each of the Cassegrain telescopes has a primary aperture, 72" in diam-eter, and either a 9.5 or 10.5" diameter secondary. The reason for two secondary mirror diameters is that the telescope will be used in both the IR and visible regions of the spectrum. The instrument is designed primarily for the 1R, but differences in secondary coatings and design requirements dictated that two independent secondary configurations be fabricated. The primary mirrors are of lightweight fused silica eggcrate construction. Each of these mirrors weighs approximately 1200 lbs. and had to be slumped from their original flat configuration to the required radius of curvature of 388.8". When placed into the telescope their completed fino. will be f/2.7 yielding a total telescope Casse-grain f/no. of f/31.7. The total array f/no. is determined by the beam combiner (second folding flat) and will be approximately f/9 initially.
As spaceborne optical systems increase in diameter to achieve improved resolution, the stability requirements imposed on structures approach values which were unthinkable only several years ago. To achieve the capabilities of these apertures optical path errors must not exceed a specific fraction of the wavelength of light, and this fraction, typically x/20 rms in the focal plane, is independent of system size. Thus, from a percentage error basis, large optical support structures represent a far more formidable development task than do smaller systems. The Large Space Telescope (LST) sponsored by MSFC/NASA is a case in point. The Optical Telescope Assembly (OTA) is shown (Fig. 1) installed in the LST spacecraft. The vertex-to-vertex spacing of the primary and secondary mirror is 193 inches. To achieve satisfactory optical performance, this spacing must be maintained constant to a precision of -Â±11.1 for observation periods up to 10 hours. During this time it may be necessary to alter the spacecraft attitude with respect to the sun, which would change the temperature levels and gradients within the structures. It is believed that by exploiting the use of graphite-epoxy in a novel manner, the stringent alignment require-ments can be satisfied with a nominally passive structure.
The state of the art in manufacturing large one piece metal mirrors has advanced dramatically in the past 10 years, following important pioneer work by NASA's Jet Propulsion Laboratory in 1965. Since that time, several additional large solar simulator mirrors have been manufactured. Careful mechanical and thermal design analysis and structure material selection are important elements in a successful finished product. Manufacturing techniques for mirror structure fabrication borrow from the heavy metal industry, while optical processing still follows relatively classical methods. Requirements for electroless nickel plating, sea transportation, and improved long term optical surface protection are challenges that have resulted in innovative practical solutions. The recently completed 5.5 meter (18 foot) diameter mirror for the Japanese National Space Development Agency's new space center at Tsukuba, Japan, exemplifies much of the technical advancement achieved over the last 10 years.
This column is a review of the seminar on Modern Utilization of Infrared Technology Civilian and Military held in San Diego, California on 1920 August 1975, of which! was the general chairman. (Otherwise this review belongs in Barry Johnson's column on Inflared, but a telephone call to him brought agree ment that I should write on infrared in this instance.)