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The scanning mirror system described is being developed as the primary mirror in the spectroheliometer. The system consists of a gimbaled mirror mount that is rotated through extremely small angles by the use of frictionless pivots and a closed-loop servo system. Design, fabrication, and test experience are discussed.
Work is described which was performed on debris that adhered to the Surveyor 3 camera mirror after it was returned from the moon by the Apollo 12 mission. The chemical and morphological natures of the debris are described and some fine scale features of the mirror surface are discussed. Supplementary information was provided on the adhering debris so that the causes of the optical degradation of the mirror could be determined more accurately.
The results are presented of a detailed study of the discrete control of linear distributed systems with specific application to the design of a practical controller for a plant representative of a telescope primary mirror for an orbiting astronomical observatory. The problem of controlling the distributed plant is treated by employing modal techniques to represent variations in the optical figure. Distortion of the mirror surface, which arises primarily from thermal gradients, is countered by actuators working against a backing structure to apply a corrective force distribution to the controlled surface. Each displacement actuator is in series with a spring attached to the mirror by means of a pad intentionally introduced to restrict the excitation of high-order modes. Control is exerted over a finite number of the most significant modes.
This paper considers the problem of designing a control system to achieve and maintain the required surface accuracy of the primary mirror of a large space telescope. Control over the mirror surface is obtained through the application of a corrective force distribution by actuators located on the rear surface of the mirror. The design procedure is an extension of a model control technique developed for distributed parameter plants with known eigenfunctions to include plants whose eigenfunctions must be approximated by numerical techniques. Instructions are given for constructing the mathematical model of the system, and a design procedure is developed for use with. typical numerical data in selecting the number and location of the actuators. Examples of actuator patterns and their effect on various errors are given.
Mirror refurbishment processing is described, and the results of processing 251 mirror assemblies are reported. The mirror replica bonding, optical tests, electrical discharge machining, and vacuum coating are discussed.
The primary mirror of a large (26-inch diameter aperture) solar telescope was made of glass ceramic and designed with an integral hub on the back of the center of the mirror. This permits heat from the mirror to radiate off its back to a nearby cold plate. To permit mounting without high stresses, the hub was ground down to a smooth cylindrical surface 3.5 inch in diameter. The ground surface was then acid-etched to remove 0.007 inch (on the diameter) by immersion for five minutes in a mixture of four parts 92% sulfuric acid and three parts 50% hydrofluoric acid. The acid etching removes microcracks from the ground Cer-Vit surface. An Invar sleeve was fabricated to fit over the hub with about 0.010 inch radial (0.020 inch diametral) clearance.
A method follows for applying a mirror reflecting layer to the surfaces of parts, instruments, apparatus, and so on. A brief analysis is presented of the existing methods of obtaining the mirror surface and the advantages of the new method of obtaining the mirror surface by polymer casting mold are indicated.
A study has been conducted of a configuration resembling that of a Cassegrain telescope with two nested mirrors in the plane of the primary and two nested mirrors in the plane of the secondary. The focal plane was located in the plane of the first and the third mirror. The four surfaces available in the device for corrections made it possible to correct for third-order spherical aberration, coma, astigmatism, and distortion.
A ray-trace analysis is described of aberrations and extreme rays of a Cassegrain-type telescope with a tilted secondary mirror. The work was motivated by the need to understand the factors limiting image quality and to assist in the design of secondary mirrors for three telescopes with oscillating secondary mirrors (OSM) used at Ames Research Center for high altitude infrared astronomy. The telescopes are a 31-cm-diameter Dall-Kirkham (elliptical primary, spherical secondary) flown aboard a Lear jet, a 71-cm balloon-borne Dall-Kirkham flown on the AIROscope gondola, and a 91-cm true Cassegrain (parabolic primary, hyperbolic secondary) flown aboard a C-141 jet transport. The optics for these telescopes were not designed specifically for OSM operation, but all have OSM's and all must be used with various detector configurations; therefore, a facility that evaluates the performance of a telescope for a given configuration is useful. The analytical expressions are summarized and results for the above systems are discussed. Details of the calculation and a discussion of the computer program are given in the appendices.
Calculations are carried out to investigate the effects of obscuring the central portion of the secondary mirror. Two cases are treated: (1) a Gaussian distribution of reflectivity, and (2) a sharp-edged obscuration. Substantial reduction in reflection from the secondary mirror is found achievable, but the reduction was found highly sensitive to the form of the radial distribution of reflectivity. The total power reflected from the secondary mirror that is incident on the detector is estimated. Techniques for experimental testing of alleviation schemes are suggested.
Uniquely identifiable artificial landmarks were generated in LANDSAT imagery by making use of specular solar reflection from small, easily transportable mirrors. The results are impressive in well-registered color composite enlargements, where the mirror returns appear as bright white spots. The signatures are unique enough that returns can be readily located in the computer compatible tape records either manually or by machine search. As embodied in experiments to date, the system is entirely passive. Mirrors must be carefully positioned prior to each satellite pass, but the required angle calculations are simple enough to be made on a hand calculator and the required input data are easily available. Possible applications of the technique include providing site identification and geodetic control in remote regions of the earth, enhancement of small targets such as offshore oil derricks, providing atmospheric transmission data coincident with other LANDSAT experiments, and providing a nearly perfect optical point source for overall system checkout.
The dependence of the charge exchange lifetimes on the mirror latitude for ions mirroring off the geomagnetic equator was re-computed using the improved hydrogen distribution models. The Chamberlain model was used to define the spatial distribution of the neutral hydrogen environment through which the ring current ions traverse. The resultant dependence of the charge exchange lifetime on mirror latitude is best fitted by the approximation that contains the charge exchange lifetime for equatorial particles.
The prospect of placing large astronomical telescopes in earth orbit, free of atmospheric degradations, has created a need for ultrahigh-quality large optics. Large mirrors, several meters in diameter, must be produced and maintained with a surface quality that is an order of magnitude better than that typically used in terrestrial telescopes. This paper describes the experimental development of automatic control techniques for correcting optical surfaces to a precision of 0.01 microns rms. The approach consists of interferometrically measuring the mirror figure error and then, using servomechanisms, push or pull on the back surface of the mirror to reform the optical surface
A fused-quartz mirror is attached to a conductive metal substrate by the following steps: tinning one surface of a fused-quartz mirror with a solder of substantially pure indium; tinning a metallic substrate with an indium eutectic alloy consisting essentially of indium bismuth, lead and tin having a melting point substantially below that of indium; heating the eutectic alloy to a temperature substantially above its melting point, but below that of the solder; floating the mirror into place, and subsequently cooling the alloy to a temperature substantially below its melting point.
Among the different proposed mechanisms for generating parallel electric fields, magnetic mirroring of charged particles seems to be the most plausible. In the present paper, it is suggested that magnetic mirroring is the basic cause of parallel electric fields in the magnetosphere and that the magnetic mirroring effect may be able to form the basis of an auroral theory that can remove a major portion of the ambiguity of observations. In the model proposed, the parallel electric field is due to a magnetic confinement of a negatively charged hot collision-free plasma. A transfer of electron gyroenergy into wave energy tends to weaken this confinement; if this energy transfer becomes too strong, the parallel potential gradient will break down. Hence, from this model, in contrast to certain other models of parallel electric fields, only a small fraction of the total auroral particle energy may be expected to be transformed into electromagnetic wave energy during the acceleration process.
The dependence of the charge exchange lifetimes on the mirror latitude for ions mirroring off the geomagnetic equator has been re-computed using improved hydrogen distribution models. The Chamberlain model, with the input parameters determined by recent satellite observations, has been used to define the spatial distribution of the neutral hydrogen environment through which the ring current ions traverse. The resultant dependence of the charge exchange lifetime, tau, on mirror latitude, lambda-m, is best fit by the approximation tau-m = tau-e cos 3.5 lambda-m, where tau-e is the charge exchange lifetime for the equatorial particles.
A three-mirror telescope arrangement that can be corrected for four aberrations (coma, spherical aberration, field curvature, and astigmatism) is described. Geometric spot size is less than 0.1 sec of arc, in an easily accessible flat field of 1.5 deg. The primary-secondary configuration is Cassegrain-like, with the real image formed closely behind the primary. This secondary image is re-imaged by a tertiary mirror at roughly unit magnification. A small, flat mirror at the exit pupil, located between primary and tertiary, folds the light rectangularly away from the telescope axis. The shape of the large image area is helpful in space applications where several instruments may share the focal plane.
Mechanical aspects in the development of an oscillating scan mirror mechanism were reviewed, that featured a remarkably low level of structural vibration for the impact energies involved in mirror oscillation. Another feature was that energy lost during impact was returned to the mirror by applying torque only during the instant of impact. Because the duration of impact was only about 0.010 second, it was critical that energy losses be minimal because there was not much time to restore them.