We briefly discuss the development of optical telescopes from the age of astrology to those of today and the future. We include the rationale for changes in the design of telescopes during this time. We also discuss the cost drivers and how to reduce them.

Surface roughness data are presented for a matrix of diamond-turned electroless-nickel samples having a combination of six phosphorous contents and four heat treatments. Roughness measurements were conducted with commercial optical and stylus profilometers (Wyko and Talystep). The results are discussed in terms of the material composition and heat treatment, plus other factors having an observed influence on the surface roughness. For the optimum material properties, full-length (665 μm) restored 20X Wyko scans yielded values of better than 10A rms.

An infrared phase-shifting interferometric system has been integrated with a novel optical figure generator at the University of Arizona Optical Sciences Center. This unique generator facility can produce generalized axially symmetric surface figures in a timely and cost-effective manner. The success of this facility depends on both its ability to efficiently remove material while forming the surface figure, and its ability to monitor the surface figure during the generation process to provide feedback to the optician. The facility has been used on several occasions to custom generate off-axis parabolic segements. Figures to within 0.30 μm rms of the desired figure have been obtained. This paper discusses the usefulness of the infrared phase-shifting interferometric system for providing figure correcting feedback to the optician during the generation of the off-axis parabolic segments and how it is affected by the surface roughness produced by each generator tool.

Improved production of large optical mirrors may result from combining single point diamond crushing of the glass with polishing using a small area tool to smooth the surface and remove the damaged layer. Diamond crushing allows an accurate surface contour to be generated and the small area polishing tool allows the surface roughness to be removed without destroying the contour.

The ability to generate aspheric components to micrometer tolerances while leaving a surface relatively free of subsurface damage is quite recent. This paper describes the optimum parameters for generating high-quality aspherics.

One of the most useful tasks to be carried out at the space station will be the making of large precision telescopes. It will become possible to assemble optics bigger than can be launched in one piece. A further step would be to take advantage of extraordinarily favorable conditions in space for testing and even manufacturing optics. In this short paper we will consider these two aspects.

A brief description is given of the successful development of a large computer controlled machine for diamond turning aluminium alloy X-ray mirror substrates (1.5 m (60 in.) dia., 600 mm (24 in.) axial length). The in situ metrology facility for axial profile, round-ness and diameter measurement is described, together with the technique for automatic programming of the final tool path to compensate for errors of workpiece deflection, tool setting and edge radius, etc.. Examples are based on the ROSAT and SXT mirrors for which the axial and radial location/mounting surfaces are produced with precise reference to the reflecting surfaces. Adaptation of this type of precision machine/metrology facility to the production of glass ceramic optical components is also discussed.

The goal of this article is to describe the manufacture of lightweighted, 4 m diameter mirror blanks made of Zerodur in a fast and economical manner. Basic experiments have been performed involving production techniques and processes which may be usable to manufacture lightweighted mirrors in a mass-production format. On the basis of experiences gained through these experiments, a general consideration of the necessary equipment and further experimentation needed to develop the most promising of the processes will be described.

Initiation of the NASA Space Station project presents many challenging opportunities. One such challenge is the need to develop large, high specular reflectance, lightweight mirrors that can withstand thousands of deep heating and cooling cycles without degradation for virtually unlimited orbital operation. Further these mirrors, which are candidates for the solar concentrators employed as part of the electrical power generation system, would need to be produced at a rate equivalent to one 60-inch mirror per day as dictated by project schedules and anticipated power demands. This paper develops the requirements for, and an approach to, the design of these mirrors. Following the Introduction is a discussion of the concentrator design and performance requirements. Next a review of the principal degradation sources and their effect on various materials and material combinations is presented. The effect on cumulative mission, or life cycle, costs of different material choices is then presented and shows the potential advantage of glass. Finally concentrator segment design and producibility concepts are presented and applications to other projects such as long wavelength infrared imaging systems are suggested.

A program for the development and low temperature figure tests of Carbon Fiber Reinforced Plastic (CFRP) sandwich panels has been initiated as a possible solution to the requirements for a very lightweight primary mirror for a three-meter diameter balloon-borne far-infrared and submillimeter telescope. The requirements for the mirror are that it have a one to two micron rms surface figure accuracy, that it be very lightweight (approximately 10 kilograms per square meter), that it maintain its figure at an operating temperature of -40 to -50 degrees Centigrade, that it rapidly reach thermal equilibrium after a two hour ascent through the tropopause to the operating termperature at 30 kilometer altitude, and that it survive the acceleration forces during the launch, the parachute opening, and the landing. The CFRP sandwich panel readily meets the lightweight, thermal conductivity, .cp 11 and durability requirements. Figure accuracy achieved with current techniques replicating against a precision ground convex Pyrex mold are 2-3 microns rms, close to the required one to two microns. Optical figure measurements on the first two of a series of four .5 meter test panels replicated to a spherical surface with a radius of curvature 10 meters, show large-scale focus (radius of curvature) change and astigmatism down to temperatures of -60 degrees Centigrade which follow approximately the theoretical prediction for the composite. All other deformation at -60 degrees is less than 1 micron rms. These results indicate that the material and process control is excellent and that with appropriate design changes in the lay up, the large-scale changes can be compensated for. Further tests are being carried out to demonstrate this hypothesis.

By combining efficient fabrication processes and advanced testing methods, a real production capability exists for the manufacture of large aspheric optics. Currently, a five-axis, computer-controlled optical surfacing (CCOS) unit is used for the rapid grinding and polishing of elements up to 4-m in size. This paper describes the processes and equipment in use today and concludes with a scenario for meeting the projected future fabrication needs.

In March 1985 a 1.8m diameter, f/1 focal ratio, lightweighted honeycomb sandwich mirror blank was cast at the Steward Observatory Mirror Laboratory. The front surface curvature was formed by spinning molten borosilicate glass at 15.6 rpm. The purpose of the casting was to test the method to be used to make 8m honeycomb mirrors. The blank will be used to test a polishing method for f/1 paraboloids.

Accomplishments in the development of lightweight, honeycomb-core, sandwich mirror blanks made of borosilicate and high-silica glasses at the University of Arizona for the Large Deployable Reflector program are described. In this paper, work spanning the last 2 years is reported, highlighting a new mirror blank fabrication technique that permits the fabrication of the honeycomb core integrally with the front and back plates of the blank in a single furnace cycle. Two types of mirror blanks made by this method, an off-axis, aspheric segment and a smaller Vycor circular piece, are described. The fabrication of two off-axis, aspheric mirror segments is also described. Cryogenic test results are included on the test of a 38-cm diameter, lightweight, honey-comb core, sandwich mirror made of Pyrex.

There is a wide spectrum of optical systems contemplated for use in future astronomy missions. At Itek, the systems range from the large, lightweight, deployable optics required for the large deployable reflector (LDR) to the precision of grazing incidence x-ray optics for the Advanced X-ray Astrophisical Facility (AXAF). This paper describes some of the tech-nological initiatives that Itek has developed to enhance the manufacturability and the measurement of the surface quality of the spectrum of optical assemblies to be manufactured for the astronomical telescopes.

Under the best atmospheric conditions mountain based telescopes have the potential to realize images with 0.25 arc second resolution. However, close thermal control of the observatory is needed to prevent local seeing from spoiling this quality. In particular it is very desirable to control the primary mirror surface temperature by removing heat from behind. In this paper we obtain expressions for the temperature of simple solid and honeycomb mirrors under a realistic observatory thermal environment and with different rates of heat removal. Heat transfer within a honeycomb structure by ventilation with air at ambient temperature is characterized by an efficiency and the mass transfer rate. Experimental values for efficiency are presented for a simple flow geometry. It is shown that readily achievable flow rates can produce the required thermal equilibrium of an 8m honeycomb mirror.

Large multi-component mirrors are coming into widespread use as elements of optical systems. Initially used as light buckets, the introduction of precision mounting and control mechanisms allow for use in imaging systems. The optical parameters of "light buckets" are relatively straight forward: Light collection is a function of input cross section, and the diffraction effects are a function of individual component size and shape. The image parameters of multi-component mirror systems are a complex set of factors involving component size, shape, figure and lateral placement. Accurate longitudinal is required for coherent imaging. This paper discusses the image parameters of large, multi-component mirror systems both as excentricaly apertured segments of total mirror structure and as a summation of asymmetrical or tilted and decentered components. The two approaches yield comparable results.

In existing types of telescope lenses or mirrors elements with spherical and aspherical surfaces, which are expensive in design and manufacturing, are used. Manufacturing of telescopes with significant or adjustable magnification increases its complexity and cost. The usage of the prismatic telescope, allows to develop cheaper optical telescopes manufacturing, due to use of simple and similar elements in its design. Prismatic's telescope magnification may be easily controlled by deviating the prismatic elements. In existing telescope designs, magnification is proportional to objective focal length and in inverse ratio to eyepiece focal length. Owing to the fact that the eyepiece focal length cannot be changed significantly, magnification is considered proportional to objective focal length. For significant magnifications, it is necessary to use objectives featuring big focal length. This makes the telescope heavier and more complicated, because the weight of a telescope with invariable F-number, is approximately proportional to the objective's focal length cubic value. A prismatic telescope consisting of consecutively arranged prism rows, changes the relative angle between parallel ray bundles, entering the telescope, into two orthogonal directions. Light dispersion in it can mutually be compensated by locating prism wedges in opposite directions.

This paper describes a Fourier transform telescope designed to image solar flare X-rays and gamma rays at energies up to 1 MeV with arcsecond or subarcsecond resolution. The imaging technique makes use of a bigrid collimator divided into a number of smaller areas called subcollimators. The grids in each subcollimator consist of a set of linear apertures so configured that each subcollimator provides a measurement of a single Fourier component of the angular distribution of the source. The imaging concept is therefore a mathematical analog to aperture synthesis in radio astronomy. For X-ray and gamma-ray astronomy, this approach has significant advantages in terms of relaxed requirements for position sensitivity in the detector and for control of grid alignment in the large scale telescope structure. The concept of the Fourier transform telescope will be illustrated with numerical parameters of a version now under study for the Pinhole/Occulter Facility.

We discuss several concepts to keep track of small relative angular motions between a reference point and a remote platform when the two are located at separate points on a flexible structure. The goal is to accurately transfer knowledge of the orientation of the remote platform relative to the reference point. First-order equations and order-of-magnitude calculations are presented for image centroiding and interference fringe projection approaches for space station applications.

Four-mirror telescopes of the same configuration, once employing a spherical and the other time employing a parabolic primary are compared with respect to their performance, and the alignment sensitivities of the three correction mirrors. This is done for three different primary focal ratios, F/1, F/.8 and F/.6. The systems with parabolic primaries provide a far superior wide field performance, and are less sensitive to misalignments of the tertiary and quarternary mirrors. The spherical primary systems are highly corrected over a much narrower, though, for many applications possibly sufficiently extended field of view. The attractiveness of the spherical primary is the simplicity of its fabrication, and if segmented, also the largely simplified alignment procedure of the segments.

The Optical Surface Analysis Code (OSAC) is used to predict the optical performance of the Hubble Space Telescope. The analysis is described in detail and compared to previous performance prediction analyses. The mathematical foundations of the various analyses are compared and a description of the mirror data that was used as input is summarized. Finally, the analyses and their results are compared in detail. OSAC is shown to be a valuable tool for separating and analyzing the effects of misalignments, figure errors, and mid and high-frequency statistical mirror surface errors on optical performance and compares well with previous analyses.

Manufacture of the 2.4-meter diffraction-limited hyperboloidal primary mirror for the Hubble Space Telescope encompassed several state-of-the-art processes. These included computer-controlled polishing and interferometric test and evaluation. To advance the manufacturing processes to the 0.008μm rms surface figure level achieved on the primary mirror, full-aperture and sub-aperture interferometric test configurations were developed in conjunction with a precision interferogram analysis facility. Full-aperture interferometric testing of the uncoated mirror took place in air with the test chamber providing the necessary thermal and vibrational stability. A remotely-controlled, six-degree-of-freedom table was used to orient and position the mirror with respect to the fixed geometry metrology unit consisting of a Co-axial Reference Interferometer (CORI) and a reflective null corrector. The CORI generated a high quality spherical wavefront which, in turn, was converted to a hyperboloidal wavefront characteristic of the desired mirror surface by the reflective null corrector. Interferograms in the form of photographic negatives recorded the departures from the highly corrected null condition to provide surface contour and aberration data. The high spatial frequency surface defects, so important to the ultraviolet (UV) performance of the Space Telescope, were measured via sub-aperture interferometry. A laser interferoscope and precision test plates with reference surface radii appropriate to several zones on the primary mirror were employed to produce high fidelity measurements of "mid-frequency" defects at the 0.002μm rms level.

Preliminary engineering studies are in progress to define a telescope for the Far Ultraviolet Spectroscopic Explorer (FUSE) mission. General science objectives include high resolution spectroscopy in the 900A to 1200A region, low or moderate resolution spectroscopy in the 100A to 900A range, and limited long slit imaging at a spatial resolution of one arcsecond. Telescope design studies indicate that a one meter diameter Wolter-Schwarzschild type II glancing incidence telescope with an effective collecting area of 3000 square cm is required to meet the primary science objectives. A baseline optical design has been completed, and initial alignment sensitivities derived to begin the process of error allocation for the entire system. Various mirror fabrication methods are under evaluation, and include the incorporation of contemporary diamond turning technology with small tool computer controlled polishing.

The Solar Optical Telescope (SOT) is a shuttle-borne telescope that will point deep ultraviolet science instruments at the sun with sub-arc second stability. The telescope will be actively aligned in orbit while viewing the sun. This paper describes the unique active compensation techniques used on SOT to satisfy the on-orbit resolution requirements.

The University of Texas 7.6m telescope design has evolved through several stages, each bringing added strength and efficiency to a well-planned overall approach. The current design is based on an f/1.8-f/13.5 Nasmyth beam directed through either of the elevation bearings. Recent studies include consideration of a class of correctors defined for an f/4.0 internal Cassegrain focus by Charles F. W. Harmer, optical consultant. An implementation of a truss design suggested by Meinel and Meinel has the possibility of zero coma at the Nasmyth focus. In the area of primary mirror design and support, research toward a superlative design is still in progress, primarily to take advantage of the rapidly emerging techniques of high efficiency borosilicate spin casting developed at Steward Observatory. There exist designs of ribbed borosilicate mirrors of the 7.6m class which are very nearly passively supportable. The figure stability can be made considerably better than any mirror extant with a modest number of controlled supports in the axial support mechanism. Mirror support consists of a combination of pneumatic and counterweighted flotation systems with a low bandwidth tuning system added for figure improvement. Certain classes of external loads, to be expected in a realistic observatory environment, may be obviated by such a system. There are also designs for the ribbed mirror that lend themselves to laboratory-grade fabrication, where modern NC cutting machines are not common, but are somewhat more difficult, yet not impossible, to support. One such design is a candidate for the 7.6m mirror.

The W. M. Keck Observatory is a joint project of the University of California and the California Institute of Technology to build and operate a new ground-based astronomical telescope. The telescope will be used for observations at both visible and infrared wavelengths. One innovative feature of the telescope is the ten meter diameter primary, mirror (76m2). The mirror is a mosaic of thirty-six hexagonal segments. The fabrication of this large mirror requires 1) fabrication of the individual segments, 2) support of the segments, and 3) active control of the pistons and tilts of the segments. The glass-ceramic segments are 75mm thick and 1.8m in diameter. They will be polished using the technique of Stressed Mirror Polishing. First forces and moments are applied around the edge of the blank to deform the surface. Next a sphere is polished into the stressed blank. Finally the forces and moments are released and the blank elastically deforms to the desired off-axis section of a paraboloid. In the telescope, the segments are supported by whiffietrees which take axial loads and by a post attached to the center of mass with a thin diaphragm which takes radial loads. The relative pistons and tilts of the segments are electronically stabilized by an active servo system. We describe the status of the design and development work and the plans for fabrication.

A study was carried out at the Jet Propulsion Laboratory during the first quarter of 1985 to develop a system concept for NASA's Large Deployable Reflector (LDR). The primary scientific requirements are met and the cost and development time are minimized using this new system concept. The LDR requirements were investigated to determine whether or not the major cost drivers could be significantly relaxed without compromising the scientific utility of LDR. In particular, the telescope wavefront error is defined in a way that maximizes scientific return per dollar. Major features of the concept are a four mirror, two-stage optical system, a lightweight structural composite segmented primary reflector and a deployable truss backup structure with integral thermal shield. The two-stage optics uses active figure control at the quaternary reflector located at the primary reflector exit pupil, allowing the large primary to be passive. The lightweight composite reflector panels limit the short wavelength operation to approximately 30 μm but reduce the total primary reflector weight by a factor of 3-4 over competing technologies. System optical performance is calculated including aperture efficiency, Strehl ratio and off-axis performance. On-orbit thermal analysis indicates a primary reflector equilibrium temperature of less than 200 K with a maximum gradient of ≈5°C. Weight and volume estimates are consistent with a single shuttle launch and are based on space station assembly and checkout.

Numerous astrophysical objectives could be achieved with an astrometric instrument able to measure the angular separation of a pair of widely separated stars with an uncertainty of a few microarcseconds. An instrument that could make tens of measurements per day would form the basis for a multifarious scientific program. Although more complex, an astrometric interferometer offers a substantially larger information rate than a comparably-sized astrometric telescope for most applications. POINTS (Precision Optical INTerferometry in Space) is a design concept for an astrometric interferometer originally conceived as a means of performing the light deflection experiment of general relativity to second order. The present "strawman" version, which has a pair of 2 m baselines and four 25 cm telescopes, would fit fully assembled with a support spacecraft in one-third of the Shuttle bay. For a pair of tenth-magnitude stars about 90° apart, POINTS would yield the separation uncertain by 5 μas after a 10-minute observation. We consider both the design of the instrument and aspects of a mission.

Graphite fiber reinforced glass matrix composites are being developed for a variety of structural applications requiring excellent thermomechanical stability. These materials are ideally suited for lightweight, high strength, thermally stable infrared mirrors because of their low density, low thermal expansion, high strength and stiffness, and their ability to be machined, replicated and figured using standard polishing techniques. These properties are particularly promising for applications such as a three-meter balloon-borne far-infrared and submillimeter telescope mirror which must be both very lightweight and able to retain its figure accuracy when cycled between room temperature and its operating temperature of -50°C. This paper presents the results of a set of low temperature optical tests conducted to determine the figure stability of a 30 centimeter diameter frit bonded graphite-glass mir-ror in the +20 to -60°C temperature range using a 10.6 micrometer laser interferometer. The results indicate that the residual change in figure was less than 0.3 micrometers rms.

Large beryllium (Be) mirrors are being proposed for several applications including NASA, SDI and others. Traditional fabrication methods produce anisotropy and inhomogeneity of properties that may result in thermally-induced dimensional instabilities. In addition, the current methods are slow and costly and may be limited to mirror sizes that can be machined from billets of about 1.7-meters diameter. This paper discusses hot isostatic pressing (HIP), the preferred fabrication method for large Be mirrors since it can produce dimensionally stable mirrors in less time and at lower cost than with conventional methods.