Astronomy has taken a significant step forward with the success of NASA's Orbiting Astronomical Observatory designated OAO-A2 (Figure 1). This 4400 pound satellite, the heaviest and most com-plex unmanned observatory developed by the United States was launched on December 7, 1968, from Cape Kennedy. It has now entered its fourth year of operation which represents over 15,000 orbits of which 90 percent have been available for experimentation . During this period of time astronomers have made over 12,000 observations of stars, gal axies, planets, and nebulae. Over one million com-mands have been executed.
Studies conducted during the past 2 years at Itek have identified many of the major system level considerations leading to the design of a diffraction limited 3-m-aperture astronomical telescope for NASA's proposed large space telescope (LST) program. Studies in the following areas are described: telescope configuration; primary mirror and system focal ratios; primary mirror configuration, materials, and mount; secondary mirror mount; telescope structural materials; operating temperatures and thermal control; fine pointing; and system maintainability. A tentative system level wave-front error budget is presented for a 3-m-aperture diffraction limited LST, and the practicality of achieving this goal in an opera-tional system late in this decade is commented on.
A method of modal control for active optical systems, suggested by Creedon and Lindgren (Ref. 1), has been implemented on a small scale for real-time automatic figure control of a large spaceborne telescope mirror'-'. The system configuration includes a set of actuators placed behind a 30-inch diameter, 1/2-inch thick mirror to deform its reflecting surface. The control sig-nals are derived from an interferometer uging a Helium-Neon Laser for a refer-ence source. The object of the method is to select a minimum set of actuators at strategic locations so that while the lower order bending mode amplitudes of the mirror's figure error are nulled, the higher order mode amplitudes are not excited. In this way, modal control suggests a strategy for an implementation to effect diffraction-limited performance while minimizing the number of active mechanical control elements required. This paper reports the validation of theoretical conclusions by comparison with experimental results for a seven point system. The results have yet to be applied to larger multi-point systems which may be encountered in practice due to the additional complexity.
Marechal's criterion for diffraction limited performance, based on a Strehl definition of 0.80, is extended to include the effects of image motion and a central obstruction, as well as wavefront error. The properties of the extended Marechal criterion are studied to show how it can be used to establish tolerable levels of wavefront error and image motion, and the maximum permissible value for the central obstruction diameter ratio. The limitations of Marechal's criterion are examined by comparing it briefly to other image quality criteria. The properties of the Ritchey Chretien telescope are described in a form permitting analysis of its image-forming characteristics. The manner in which the choice of telescope design parameters can influence the amounts of wavefront error and image motion and the size of the central obstruction is described. These properties are then used in conjunction with tolerance levels established using Marechal's criterion to show how an image quality criterion can limit the choice of telescope design parameters. Tentative boundary diagrams are presented indicating the probable range of design parameters that can be used for the large space telescope, based on the use of Marechal's criterion.
To achieve the high degree of pointing accuracy required for a large space telescope, and to maintain this accuracy over an observa-tion time that may be hours in length, it has long been obvious that the guiding optics must include the main telescope system. Insurmount able boresighting problems would result from using auxiliary pointing optics.
The Large Space Telescope(I) will have a 10-foot diameter diffraction-limited primary mirror with theoretical spatial resolving capability of .04 arcsecond in the visible light range. Its secondary mirror, approximately 3 feet in diameter, will be some 20 to 30 feet forward of the primary depending on the final optical configuration. The alignment of the secondary to the primary is critical to the performance of a diffraction-limited optical system and is an important problem area in development of the instrument. The tube structure defining the primary-secondary relationship will not be sufficiently rigid to maintain the required alignment precision in the face of thermal and mechanical disturbances. Accordingly, the secondary will be articulated relative to the structure to compensate for tube deformations.
As a requirement for the Small Astonomy Satellite-B (SAS-13), a. star sensor was designed to provide satel lite positional data to an accuracy of 3 are minutes for satellite spin rates of 1/12 rpm. and 1 rpm. The star sen-sor is required to detect stars having a +4 visual magnitude during the sun lit portion of the orbit. The space-craft orientation, can be anything achievable in its near equatorial 500 Km, orbit. The design of the star sensor re sulted in an instrument which weighs 6.5 lbs., consumes 0.4, watts and op-erates at spin rates ranging from 1/12 rpm to 3 rpm. The instrument design incorporates optical features provid-ing excellent uniformity of resLonse across the 5° X 10° field of view. The basic design is intended as a building block, which can be utilized throughout the SAS series of space craft. Fig. 1 is a photo of the star sensor package.
The design of a unique, self deployable light shade is discussed. This shade was specifically designed for the Small Astronomy Satellite (SAS-B), of the National Aeronautics and Space Administration. While there are numerous boundary conditions imposed upon the design of the light shade by the physical limitations of the SAS-B satellite and the payload envelope, the basic design concepts can be utilized for other applications. A number of candidate light shade designs are presented with a discussion of the functional concepts. The final design selection was predicated upon its photometric properties, e.g., very high attenuation of scattered light, its rugged, light weight physical properties, and its compressibility or fold-ability along with its self deployment features within the restrictions of the satellite payload envelope.
This paper is a two-stage study of the photo-graphic aspects of the Apollo optical bar camera. The first part, prepared and presented in advance of the flight of Apollo 15, deals with the analysis of the lunar surface characteristics; defines the film, filter, and exposure for optimum camera performance; and investigates lunar object detection. These studies led to the choice of the photographic camera parameters incorporated in the mission. The second part is an analysis of the quality of the optical bar photography with particular attention given to the correlation between the predictions of part one and the actual results derived from measurements of the imagery. These findings are presented (1) to demonstrate the image quality of the photography and (2) to substantiate the value of photographic predictions as a means to ensure optimum camera performance.
For the past several years a group at Perkin-Elmer Corporation has been developing techniques for the manufac-ture of Solid Fabry-Perot Etalons. The etalons are suitable for use as very nar-row band-width isolating filters when combined with a conventional all-dielectric interference filter and some form of temperature stable environment.
In the usual Babcock magnetograph
the sun f s image is scanned by a single
exploring aperture which admits light
to a spectrograph for subsequent photo-
electric detection and analysis.
We replace this aperture by a slit,
and in the focal plane of the spectrograph
a fiber-optic probe dissects the
spectral and spatial image, feeding
light to an array of 80 photomultiplier
tubes. Magnetograms taken with
the instrument show a resolution comparable
to that achieved by photographic
methods, but with the advantage
of lower noise and direct measurement
of magnetic flux.
The University of Arizona and Smithsonian Astrophysical Observatory are jointly undertaking the development of the concept of a multiple-mirror telescope for astronoical uses. The system consists of six 1.8-m-aperture casse-grainian telescopes combined to produce a common focal surface in the center of the telescope. The early form of the telescope will provide for automatic alignment of the six independent systems. The telescope will be constructed to facilitate later extension of the alignment subsystem to include aperture synthesis (phasing) by methods proposed by Meinel and Shannon. The light-gathering power of the combination is equal to a standard telescope of 180-in. aperture, but the construction will cost very much less. The mounting will be of the alt-azimuth type. The telescope will be located at an elevation of 8,600 ft at the SAO's Mt. Hopkins Observing Station near Tucson.
The design of a multiple-mirror telescope (MMT) presents opportunities to explore new techniques for the control and alignment of the individual optical systems. This paper discusses the optical, electronic, and mechanical subsystems, with some description of the techniques to attain the tolerance required for image combination. Also, some consideration is given to the optical and alignment fabrication techniques, with emphasis on the methods and problems that are most critical to successful completion of this novel instrument.
Two low-dispersion scanning spectrophotometers were recently delivered to the Kitt Peak National Observatory (KPNO) for the astronom-ical community engaged in photometric observations there and at the Cerro Tololo Interameri can Observatory. Developed under a contract between the Associated Universities for Research in Astronomy (AURA, Inc.) and Harvard College, Observatory (HCO), these. two-channel units are designed to permit simultaneous observations of an object and the adjacent sky. Further, they were designed primarily to derive their control and data handling functions from small general purpose digital computers in the observatory domes to maximize the efficiency of the observing programs and the flexibility of operation.
The 1975 Mars Viking program represents this country's first effort to directly explore the surface of another planet. During the 90- day design life of the Viking Lander, a variety of scientific experiments will be conducted to examine the planet and search for signs of life ('ef. 1). Several aspects of the Martian surface and atmospheric features will be explored with a pair of facsimile cameras designed and built by Itek Corporation for Martin Marietta. These cameras are lightweight and compact instruments designed to provide stereoscopic views of the Martian surface in both color and black and white. They will visually characterize the landing site, and support biology, geology, and meteorology experiments. The Lander Camera System consists of a stationary mast with a rotating sensor head. The stationary mast contains all the electronic components necessary to support camera operation. The rotating head contains a scanning mirror, focusing optics, and a photosensor to convert the image into an electronic signal. The two cameras are shown mounted to a Lander in Fig. 1.
Space-borne astronomical observations are often limited by the detectors employed, instead of the telescope itself (mirror, pointing system, transfer optics, etc.). Electro-optical sensors are continually being developed, and it is important to apply detectors with improved characteristics as soon as possible. It is also important to consider what devices could be developed in the near future that would increase the efficiency of astronomical observations.
The rapid and intensive development of the field of electro-optics over the last decade has resulted in the production of a wide range of sensitive, photoelectronic imaging devices for applications in astronomical research. It is five years since Livingston (Ref. 1) reviewed the applications of image intensifiers to astronomy but since that time existing devices have been further developed and new devices have been introduced. A wide range of devices is currently in use - this review will describe those devices which have found a wide measure of acceptance amongst observational astronomers and will mention some recently developed devices which show considerable potential.
Magnetically-focused electrono-graphic cameras have been under development at the Naval Research Laboratory for use in far-ultraviolet imagery and spectrography, primarily in astronomical and optical-geophysical observations from sounding rockets and space vehicles. Most of this work has been with cameras incorporating internal optics of the Schmidt or wide-field all-reflecting types. These cameras have flown on a number of sounding rocket missions for stellar ultraviolet studies, and are planned for addition-al missions on sounding rockets and space vehicles in the near future. More recently, we have begun development of electronographic spectro-graphs incorporating an internal concave grating, operating at normal or grazing incidence. We also are developing electronographic image tubes of the conventional end-window-photo-cathode type, for far-ultraviolet imagery at the focus of a large space telescope, with image formats up to 120 mm in diameter. Our work so far has been with air-exposable alkalihalide photocathodes sensitive in the wavelength range below 2100 A. We are presently under-taking to extend the utility of the electronographic Schmidt and end-window devices toward longer wave-lengths, by the use of appropriate photocathode materials.
A desirable goal in experimental astronomy would be to find a detector that can individually record the time and coordinate position of each input photon, regardless of its wavelength -- subject, of course, to unavoidable quantum-uncertainty restrictions. For certain spectral regions, photocathodes would come close to meeting these basic objectives if satisfactory techniques could be developed to individually record each emitted photoelectron. While perhaps not widely recognized, the first practical television camera tube, the image dissector, does an excellent job of detecting individual photoelectrons, since each electron which enters its aperture is multiplied by a comparatively large factor, typically 105-106, and appears almost instantaneously as a large readily-counted burst of charge in the output circuit.
A sensitive polarimeter-photometer has been constructed for solar studies by the Institute for Astronomy on Mt. Haleakala. It can be used to study either the corona or the Sun's disk, and operates over a wide range in photon arrival rates. Since significant polarization as small as 10-5 is observed on the Sun's disk, noise and other sources of spurious modulation become of great importance. In this paper we present our measure-ments of the magnitude of these effects.
One of the most important techniques used in astronomy is spectroscopy, The receivable photons from the currently most interesting stellar objects are typically quite low. The investigation of such spectra emphasizes consideration of noise, information density, and quantum efficiency. The two major detector techniques are photographic and electronic. The photographic methods are superior for information density and are lacking in quantum efficiency and low noise; whereas, the electronic methods typically have comparatively low information density, but are capable of significantly superior noise and quantum efficiency performance. Considerable improvement in many areas in applying the electronic methods is possible. We will discuss here an electronic device which significantly improves the spectroscopists' capability to detect and measure very faint spectra with comparatively higher inf ormation density and higher accuracy than is usual with similar techniques.
This paper describes the design and per-formance of an image intensification and integration television system (13 TV) that has been developed primarily for use in guiding astronomical telescopes on star images that are too faint for visual detection.
Television, as a scientific recording device, is less well known than its entertainment cousins. However, the continuously improving state of the art in electronic imaging sensors has vastly increased sensitivity, resolution, and other characteristics. Furthermore, improvements in electronic data recording and data transmission have enhanced television's greatest boon to space investigation: its capability of realtime or quasi realtime data transmission. This paper summarizes the results of a study to determine if an elec-tronic imaging system could compete with film as the data gathering technique for the NASA/California Institute of Technology's Photoheliograph program.
Since the development of reliable polyethylene balloons, astronomers have had available to them, at a moderate cost, a vehicle capable of carrying payloads of a few thousand kilograms to altitudes from which observations could be made at wave-lengths inaccessible to the ground-based observer. To the designer of such an airborne observatory, the problems associated with the pointing and stabilization of telescopes and detectors are formidable. Since each new experiment requires a unique set of pointing specifications, there is little standardization in either the design techniques or the hardware.
The purpose of this paper is to de-scribe a balloon-borne telescope-spec-trometer, designed to perform spectro-photometric measurements of the Mg II doublet at 2795 Å and 2802 Å in stars of visual magnitude 5 or brighter, with a spectral resolution of 0.5 Å. These experiments are part of a research program conducted by the Manned Spacecraft Center, Houston, Texas. The scientific goals, and results obtained in two flights in 1971 have been described elsewhere by Kondo e. a. (Ref. 1).
The advent of rocket borne instruments opened many new areas of research such as the study of far ultraviolet spectra of stellar sources. If much information is to be gained concerning stellar processes it is imperative to obtain high resolution spectra from which line profiles and separations can be accurately determined.
We describe in this paper a Hadamard-Transform Spectrometer (HTS) designed for airborne infrared astronomical observations, and, specifically, for airborne observations of the 2.8μ to 3.5μ water-of-hydration absorption band in the reflection spectrum of Mars. This instrument, the Spectral Imaging, Inc., Model HTS-19-1, was designed and construct-ed for Cornell University and flew as one of the three primary experiments on board NASA's Convair 990 observatory aircraft during the 1971 Mars Opposition observations last July and August. As such, it represented the first operational use of Hadamard-transform spectrometry. We describe the design and operation of the Airborne Mars-Observation HTS below, and briefly discuss further applications of this technique to infrared astronomical observations.
A new attempt is being made at MIT to develop a completely 'automated telescope. The Ealing Corporation 24-inch telescope and the computer control system were designed together. The computer system, involving two central processors (a Datacraft and a Data-general NOVA), should be fast and versatile enough to operate the telescope, auxilliary instrumentation and on-line process the data. The cost of the system is well below that of other systems which attempt to accomplish these goals. The telescope and the computer are operational. The interface and software are under development with completion scheduled for summer, 1972.
Those astronomers who develop novel or specialised instruments invariably reach a point at which they must move this equipment for one reason or another. The typical development cycle being: Development Lab ÃƒÂ· Test on small telescope -* Large telescope Other Hemisphere When the latest development was a new photographic emulsion this presented few difficulties. However, in the present era of such instruments as photon-counting multi-channel (Ref. 1,2 & 3) spectrometers where a computer is a necessary part of the equipment one is invariably faced with the alternatives of building the computer into the instrument or using the facilities provided by the observatories.
The advantages of a photon counting scheme over a current measuring scheme in the case of a single point or zero dimensional detector is obvious. For a current measuring scheme the circuit noise introduces more and more error as the light level goes down and any error in determination of the circuit gain shows up as an error in the luminosity measurement. On the other hand, photon counting schemes are immune to circuit noise as long as there is a good pulse height distribution, and an error in determination of the gain of the circuit has no effect on the error in the measurement. The error instead is set by the laws of statistics so that the accuracy approaches the ultimate and depends only on the number of photons detected.
The purpose here is to present a simple theoretical model for predicting the signal-to-noise ratio possible with various tube types at different light levels with a raster scan at nearly standard rates.