The objectives of the Far Infrared Sky Survey Experiment (FIRSSE) are to obtain unbiased measurements of the position and radiance of infrared celestial sources in five spectral bands between 8 and 120 µm. The superfluid helium cooled sensor will be flown on an Aries sounding rocket sometime in 1980. The status of the sensor is presented in this paper.
The Infrared Astronomy Satellite (IRAS) is a project undertaken by NASA jointly with the space agencies of the Netherlands and of Great Britain. It will be launched into a sun synchronous polar orbit early in 1981 and will survey the entire celestial sphere in four broad infrared bands centered at 11.8µm, 24.4µm, 58.6µm and 101µm. The 60 centimeter diameter telescope is constructed entirely of Be and will be maintained at 2K by 65Kg of superfluid He. During the expected lifetime of one year, the sky will be surveyed with a high degree of redundancy to permit an unbiased, highly reliable mapping of the celestial sphere down to a flux level on the order 19-19 W/cm2. Both extrinsic silicon and germanium will be used in an array of 62 detectors to provide a positional accuracy of 1 arcmin or better for point sources. Because these detectors are used in a direct coupled circuit, both extended sources and diffuse infrared background will be measured directly. Additional experiments will be carried out to expand the sensitivity in selected regions of the sky and to provide additional spectral data on selected objects.
The infrared telescope (IRT) on Spacelab 2 will be the first cryogenically cooled telescope operated from the Orbiter. The principal objectives, consistent with those of the second Spacelab mission, are to measure the induced environment about the Orbiter and to demonstrate the ability to manage a large volume of super-fluid helium in space. The prime astrophysical objectives are to map extended sources of low surface brightness infrared emission, including the zodiacal light, the galactic plane and extragalactic regions. The IRT consists of a 250-liter helium dewar and an articulated cryostat containing the telescope which scans ±45 degrees about a single axis orthogonal to both the local vertical and the Orbiter pitch axis. The telescope is an f/4 15.2 cm highly baffled Herschelian telescope cooled to 8K which may scan to within 35 degrees of the sun. The focal plane cooled to 3K consists of nine discrete photoconductors covering the wave-length 4.5-120 microns in five bands, each having a 0.6 x 1.0 degree field of view. A single stellar detector is used for aspect determination. A cold shutter provides a zero flux reference. Overlapping scans, contiguous orbits, and a six degree per second scan rate permit rapid redundant coverage of 60% of the sky.
SIRTF, the Shuttle Infrared Telescope Facility, will be a versatile astronomical telescope that can accommodate photometric, spectroscopic and polarimetric instruments. It is expected to be 100 to 1000 times more sensitive than any existing infrared telescope over much of its 2 to 1000 micrometer spectral range. A study that demonstrated the feasibility and provided a tentative design of such a telescope was completed by Hughes Aircraft Corp. in 1976. Since then, more detailed designs of cooled IR telescopes have been carried out for the Infrared Astronomical Satellite (IRAS) and the Small Helium Cooled Infrared Telescope for Spacelab 2. Analyses of off-axis rejection of these new systems have suggested improvements to the SIRTF optical and baffle designs. Rocket tests have verified the capability of using superfluid helium as a cryogen in zero gravity. Constraints on funds for Shuttle payloads favor an evolutionary approach to the development of the full potential of SIRTF. All these factors require a consideration of design alternatives involving the optical configuration, the cryogen, the mechanical structure, and size of SIRTF. Studies are currently underway at Perkin-Elmer Corporation and at Ames Research Center to examine the alternatives in terms of performance, cost and reliability. At this time it appears that the baseline optical configuration will be changed to an aplanatic Cassegrainian (Ritchey-Chretien) system.
The 40cm - telescope is equiped with an it camera, a photometer - polarimeter, an Ebert - Fastie - spectrometer, a Michelson - interferometer and a Fabry - Perot - Etalon. These focal plane instruments, covering the wavelength region between 3 and 500/μm, are under development at different institutes in Germany. Numerous observations and experiments are planned in the fields of astronomy, atmospheric physics and helium II physics. The hardware phase of the project started in November 1978, with MBB, Dornier and Linde as industrial contractors. The first flight is scheduled for the D 4 - Mission in 1983/84.
We are developing a cryogenic helium system to provide cooling to a scanning infrared telescope for the Spacelab 2 mission in 1982. The, infrared optical/detector system and related electronics are being developed by the Smithsonian Astrophysical Observatory and the University of Arizona. A superfluid helium dewar and porous plug phase separator permit gas cooling of the infrared focal plane assembly to about 2.5K, and of the two telescope sections to 8K and 60K respectively. In this paper the design of the cryogenic system, including a commandable vacuum cover, and the prelaunch liquid helium servicing and maintenance approach will be discussed. The system can be readily adapted to other types of cryogenic experiments on later Spacelab missions.
In today's telescopes, especially the infrared telescopes the importance of controlling diffraction effects impacts directly on the form of the optical design chosen for a given mission. It is of paramount importance that the designer understand the diffraction processes involved. It has been found that the geometrical theory of diffraction, developed over the last 20 years, provides a lucid picture of the diffraction processes and provides a convenient computational tool for evaluating them. The nature of the geometrical theory of diffraction is briefly reviewed. Application of the concepts to typical telescope diffraction problems is discussed. It is shown that the geometrical theory of diffraction provides a useful conceptual picture of the diffraction process especially multiple diffraction processes. The importance and sizes of telescope stops required to minimize diffraction effects in telescopes are discussed.
The 2.4-meter Space Telescope (ST), to be launched in 1983, will provide the first opportunity for astronomers to fully exploit the observing environment above the Earth's atmosphere. The absence of atmospheric turbulence and scintillation, air glow, and absorption in the ultraviolet and infrared spectral bands will permit the observation of a greatly increased volume of space, and studies of nearby objects with finer angular and temporal resolution, higher photometric accuracy, and broader spectral coverage than has been possible heretofore. A group of five powerful and versatile scientific instruments will be used to conduct the initial observations from the ST observatory --a wide-field and planetary camera, a faint-object camera, a faint-object spectrograph, a high-resolution spectrograph, and a high-speed photometer. Also, astrometric observations will be obtained with the ST fine-guidance sensors. This paper briefly describes the ST observatory, outlines the design concepts of the five scientific instruments, and summarizes their astronomical capabilities.
The Faint Object Spectrograph (FOS) is being designed and built for use with Space Telescope to provide digitized spectra of faint astronomical objects over the wavelength range from 115 to 700nm at resolving powers of 1000 and 100. A variety of concave gratings and prisms is employed to form nearly stigmatic spectra on either of two Digicon photon counting detectors which are optimized for two different but overlapping spectral ranges. The science goals are summarized, and the optical and detector designs and their predicted performance are discussed.
Measurements of angular scattering due to surface roughness are taken from a 24-layer dielectric mirror and compared to theory. In addition, the top surface roughness of the multilayer stack is analyzed from Talystep profilometer measurements. This roughness data is used to obtain a spectral density function to be used in a vector multilayer scattering theory. When this is done, it is found that the theory, using the experimentally obtained roughness spectral density function, agrees remarkably well with the measured angular scattering data. This is especially true if care is taken to differentiate between particulate and roughness scattering. The theory uses three multilayer stack models to incorporate possible effects of different levels of correlation between interfaces of the stack. The stack was produced by Optical Coating Laboratory Incorporated.
Proc. SPIE 0183, Bidirectional Reflectance Distribution Function (BRDF) Measurements Of Stray Light Suppression Coatings For The Space Telescope (ST), 0000 (27 September 1979); https://doi.org/10.1117/12.957400
A knowledge of the bidirectional reflectance distribution function (BRDF) of black coatings used on stray light suppression systems is extremely important for the Space Telescope (ST). The ST stray light suppression requirement is to reduce Earth, Moon, and Sun light in the focal plane to a level equivalent to one 23 My star per square arcsecond, an attenuation of 14 orders of magnitude. Because of the large size of the ST, it is impractical to verify the performance of a proposed baffle system design by full scale tests, so a computer analysis is used to select the design. To insure an accurate computer analysis, a knowledge of the diffuse scatter at all angles from the surface of the coatings, for all angles of incident light, is required. During the early phases of the ST program, a BRDF scanner was built at the Marshall Space Flight Center to study the scatter from black materials. This paper describes that apparatus and gives the result of measurements on samples proposed for use on the ST.
Perkin-Elmer has been actively involved in the development of the Computer Controlled Polishing (CCP) technology for over 8 years. It was apparent that the process would be a good candidate to polish large mirrors if adequate performance goals could be demonstrated.
A facility for interferometric test and evaluation of large mirrors has been developed to aid in the fabrication of a 60- inch diameter f/2.3 hyperboloid. Testing takes place in a test chamber which provides vibration isolation and thermal stabili ty in a clean room environment. The uncoated mirror is floated on a calibrated metrology mount located upon a six-degree-of-freedom table used to align and orient the mirror to a fixed geometry metrology unit. The metrology unit consists of a Co-axial Reference Interferometer (CORI) and a reflective null corrector which produces interferograms in the form of photographic negatives. Interferograms of this concave hyperboloidal primary mirror are evaluated in the Interferogram Analysis Facility (IAF) which employs a scanning microdensitometer, and fringe extraction and analysis software specially developed for high performance applications. The IAF routinely provides surface figure analyses in the form of contour and aberration data. This includes multiple measurement statistics as well as aberration calculations based on polynomials orthonormalized over an annulus.
NASA's Space Telescope calls for a concave-convex 98"-diameter lightweight mirror substrate. The material chosen was Corning's Code 7971 ULE glass, which was fabricated into a 10"-thick lightweight core faced with 1"-thick front and back plates. ULE glass is manufactured by a very unusual process, and has some equally unusual properties. The manufacturing method is known as flame hydrolysis, in which silicon and titanium tetrachloride vapors are reacted in a flame to produce tiny molten glass particles. These particles are collected on rotating tables to form clear glass discs 60" in diameter and 5-6" thick, from which the substrate must be fabricated. Perhaps the most unusual property of ULE material is its weldability. Discs are sawed into thin slabs that are welded together in an open room to form inner and outer rings and a square-celled monolithic honeycomb core. Other discs are heated to 1600°C and allowed to flow out to the needed diameter to produce the face plates. The entire assembly is sealed together at 1600°C and sagged to curvature to produce the rough blank, which is then annealed. Another unusual property is the linear correlation of the coefficient of thermal expansion (C.T.E.) and ultrasonic velocity. This permits non-destructive evaluation and documentation of the actual material used for the component parts, which assures that the final blank will meet the demanding requirements for homogeneity of C.T.E. as well as near-zero C.T.E.
There are two basic approaches to obtaining satellite radiance measurements used in retrieving vertical atmospheric temperature and constituent profiles: (1) to make the measurements in a number of different spectral intervals (frequency scanning) or (2) to make the measurements at a number of different zenith angles (angle scanning). This paper describes a study of the effect of combining these two approaches into a single sounding system, which is accomplished by viewing a fixed target at several frequencies and at several zenith angles as the satellite moves along its orbital track. If there are M spectral intervals and N angular views, a total of MxN essentially independent measurements are possible. Instrumental and geometric details,and results of a numerical simulation study of this dual scanning approach are presented, along with a discussion on how one can scan across the orbital track at the same time to obtain two-dimensional horizontal coverage.
Interferometry of optical surfaces, components, and systems is used extensively by the optical industry in fabrication, quality control, and system assembly and alignment. The use of digital techniques to analyze interferograms has increased in the past few years. Sophisticated analysis techniques using micro-computers may be used by even the smallest optical shop. Digital instrumentation can be used to evaluate large numbers of interferograms. Optical quality of diffraction-limited systems is bounded by the root-mean-square wavefront departure from the appropriate reference sphere. The geometric ray properties are bounded by the maximum slope of the wavefront. The specification and evaluation of space optics often rest on these properties. Both the rms wavefront and the maximum slope of the wavefront can be determined from interferometric measurements.
The design of ultra-lightweight mirrors for large optical space systems is necessitated by spacecraft payload capabilities. The weight of supporting structure is in turn lessened as the optical element weights decrease. Lightweight mirror design, however, must be shown capable of inducing minimal distortion error response from the severe environment of a typical orbital mission. Such includes on-board vibration, gravity release, and a cryogenic operational temperature often below 100°K. Further, the optical elements must withstand the extreme loadings of a launch environment. As such, the mirrors must exhibit excellent stiffness, strength, and thermal expansion characteristics, particularly so in a passive (non-correctable error) system. A symmetrical, fused silica glass lightweight design, composed of fused facesheets and core, which meets these design requirements is presented. The fabrication and manufacturing processes utilized to accomplish the design and its objectives are also discussed.
When a rigid polishing tool is moved in a radial direction on an aspheric surface, it will not fit the surface throughout the stroke. The nonuniform pressure under the tool as a result of this misfit will result in a nonuniform removal of material and thus alter the shape of the aspheric surface. This result may be avoided by providing for some flexibility in the tool. A tool consisting of a rigid backing, a soft resilient layer, a thin layer of selected flexural rigidity and finally a thin layer of polishing pitch will provide such flexibility. For a given aspheric the material and thickness of the controlling layer may be selected, depending on tool size and shape and the stroke desired so that the variation of pressure on the optical surface may be made acceptably small. However, sufficient rigidity is retained to provide rapid smoothing action. Thus, the use of such tools result in smooth aspheric surfaces. Some large optical mirrors are constructed of thin face plates separated by a ribbed core structure in the interests of light weight. Because the face plate deflects under the polishing pressure, the areas over the ribs are polished away faster than other areas. The composite tool described here reduces this effect as the resilient layer is made softer and the tool larger. Properly designed tools then permit the polishing and smoothing of large aspherics with thinner face plates and thus, lighter than has been required by previous polishing methods.
As early as 1927, methods of rendering glass and glassy materials more susceptible to mechanical destruction have been investigated. Considerable literature has been published in the fields of diamond drilling, the scribing and identing of glass as well as other nonmetallic materials. Little significant progress has been made in either determining the conditions under which this can be accomplished or any sound description of the underlying mechanisms. This literature led us, some months ago, to attempt to apply these concepts to the possibility of applying precision machine technology and diamond turning to processing glass used in optical applications. Initial findings indicate that material can be removed from glass components without large tool wear, and that experiments relating surface character to machine parameters reveal near theoretical surfaces in the machined area. In this paper, criteria for determining what constitutes a machined "optical quality" or "easily polished" surface will be described. Also, experimental results relating machined surface character to machine parameters will be presented.
The value of direct nuclear-pumped lasers (DNPL's) for space application has been appraised in a weight scaling analysis. The analysis showed that the weight of closed-cycle space-based DNPL systems tends to be dominated by cooling facilities. DNPL cooling was investigated using a heat sink, by boiling water with steam exhausted to space, by using a refrigeration cycle, and by using passive radiators. The passive radiator coupled with a reactor constituted an excitation system with no significant expendables. The lack of expendables began to yield significant weight advantages over conventional pumping schemes when longer operating times were approached. The area of the passive radiator scales inversely with the fourth power of temperature. It was concluded that DNPL's which can operate with high efficiency, at high temperature and with low flow requirements are best suited for space applications.
The essential fluid flow processes associated with the Solar and Jovian atmospheres will be examined in a laboratory experiment scheduled for performance on Spacelab Missions One and Three. The experimental instrumentation required to generate and to record convective fluid flow is described. Details of the optical system configuration, the lens design., and the optical coatings are described. Measurement of thermal gradient fields by schlieren techniques and measurement of fluid flow velocity fields by photochromic dye tracers is achieved with a common optical system which utilizes photographic film for data recording. Generation of the photochromic dye tracers is described, and data annotation of experimental parameters on the film record is discussed.
In the past few years a family of solid state sensors called Charge Transfer Devices (CTD's) have been developed for the television industry. These devices show promise of being superior to the Image Dissector as a stellar sensor and a number of technology programs have begun to develop around the devices. Inherent advantages of these devices are: low voltage requirements, insensitivity to magnetic fields, good linearity, and low weight and power. Two basic types of CTD's have been developed; the Charge Coupled Device (CCD) and the Charge Injection Device (CID). This paper discusses the stellar tracking advantages of the CID over other devices and the work done by the General Electric Co. in developing a CID particularly suited for this application.
The capabilities of conventional optical systems and detectors have proved inadequate for many measurements in space astronomy. In order to overcome these limitations a family of photoelectric photon-counting array detectors has been developed for use in instruments on space-borne telescopes. The Multi-Anode Microchannel Array (MAMA) detector system can be operated in a windowless configuration at extreme-ultraviolet and soft x-ray wavelengths or in a sealed configuration at ultraviolet and visible wave-lengths. MAMA detectors with up to (512 x 512)-pixels are now under evaluation. In this paper the construction and modes-of-operation of the MAMA detectors are described and the designs of spectrometers utilizing the array detectors are outlined.
Optics for spacecraft presently use antireflection (AR) coatings whose space environment performance has only been measured indirectly via overall system performance. New AR coating techniques, chemically prepared coatings, are not readily accepted for use due to a lack of qualifying space environment data. We describe an experiment to be carried out on the NASA Long Duration Exposure Facility (LDEF). The experiment will utilize multiple samples, both coated and uncoated, with non-flight samples being maintained as controls. Conventional vacuum deposited coatings will be utilized as well as the new chemically prepared coatings. The present LDEF mission will provide 6 to 9 months of low earth orbit environment with future missions providing several years of exposure both in low earth orbit and in higher orbits.
We discuss the feasibility of a coherent optical telescope array, operated as a free-flying satellite, and capable of observing faint astronomical sources with very high angular resolution. In many ways, the coherent optical telescope array is directly analogous to the large radio astronomy arrays and very long baseline interferometers. A prototype instrument is discussed in detail, comprising four mirrors on a 10-meter baseline. This instrument could operate from the UV to the near-IR with one-dimensional resolution of 0.006 arc sec in the visible and achieve a faint object limit of about +25m in the visual. The field of view would be about 3 arc sec. In this article, we identify the principal technical problems associated with telescope arrays for astronomical observing and address certain of them. There is a long list of astronomical objects which could profitably be studied with milli-arc-sec resolution. Certainly one would include Seyfert nuclei, quasars, globular clusters, x-ray binaries, recent novae, binary stars, individual stars, asteroids, and planetary satellites. The ability to quickly produce an image, and to go to very faint objects, ensures that nearly all areas of astronomy will benefit from such an instrument.
A 1-meter, high-resolution, wide field-of-view telescope (Starlab) to be used on board the Space Shuttle is described. The objective of Starlab is to obtain optical astronomical observations in the ultraviolet (UV) portion of the spectrum. Scientific investigations will be conducted that require high-resolution wide-field imaging, far-ultraviolet spectroscopy, precise spectrophotometry and polarimetry, and synoptic planetary observations. The facility will provide scientific flexibility by permitting the use of photographic film and by readily accommodating a wide range of focal-plane instrumentation that can be supplied by a variety of observational groups. The Starlab uses an f/15-modified Ritchey-Chretien telescope, followed by an instrument selector that gives access to the conventional Cassegrain focus or, by inserting a diagonal mirror, to a radial focal plane. The results of five critical subsystem-design studies reveal the feasibility of using state-of-the-art design practices in meeting the Starlab science and engineering requirements.
A new variation on an older optical form, the three-mirror anastigmat, has been designed. Using the form off-axis in both aperture and field allows for an unobscured system that can be packaged in a small volume without any auxiliary folding mirrors. Significant extensions both in speed and in field of view have been made. Two examples are shown, i. e. , (a) a 3° x 6° field of view, f/2.6 design with 0.17 mrad resolution, and (b) a 0.5° x 10° field of view f/3.0 design with approximately 0.10-mrad resolution. Both designs have flat focal surfaces and good shielding from stray radiation. The performance of these two systems has been enhanced by tilting and decentering the mirrors by small amounts during the final optimization stages. This technique, which makes it possible to put the conic surfaces to the best optical use, is applicable because of the off-axis nature of the design.
A reimaging three-mirror anastigmat has been developed to cover fields of view of up to 2 degrees by 9 degrees. By using the optical system on-axis with an offset field of view, a significantly larger field of view than has been reported up to now can be used with low obscuration. The concept of tilting and decentering optical elements for a field view 2 degrees square, which yields a significant improvement in image quality, is discussed.
A conventional optical system having a low f-number and a wide field of view contains many reflecting and refracting elements. Such a system poses problems in alignment and mechanical stability, especially when operating in sever environments over a wide range of temperature. A fiber bundle designed in the form of a cone with a larger diameter at the entrance than the diameter at the exit transmits and reduces the image size. This permits the use of a simple lens of longer focal length and larger effective aperture. Since there is a net gain in flux per unit area, this system intensifies as well as transmit an image. It also enables the use of detector of smaller size to achieve a higher signal-to-noise ratio.
The new generation of satellite-borne earth resources scanners, the Thematic Mapper, is being built for launch on the Landsat-D spacecraft. It will gather data for applications such as crop inventory, land use planning, forest management, and geology. This paper gives an overall design description, further discussion of principal design features, performance achievements where data are available, and system performance predictions.
The Department of Interior (DOI) and the National Aeronautics and Space Administration (NASA) have entered into a joint program to provide a digital image processing system in support of the Landsat 3 mission. NASA will provide the data reception and pre-processing facilities, while the DOI provides the production image processing system that generates film products and computer compatible tapes for users. The system procured by the DOI's U.S. Geological Survey was installed at the Earth Resources Observation System Data Center (EDC) in Sioux Falls, South Dakota. The EDC Digital Image Processing System (EDIPS) is capable of operating in standard or special formats. During 13 hours of operation, the system can process up to 200 five-band Multispectral Scanner (MSS) scenes and 160 Return Beam Vidicon (RBV) subscenes in the standard mode plus up to 60 scenes/subscenes in the special-order mode. The input media for the system are High Density Tapes (HDT's) generated by NASA's Goddard Space Flight Center (GSFC). These tapes contain two types of data: those that have been radiometrically and geometrically corrected with resampling of the data to fit a known map projection, and those that have been radiometrically corrected but have not been geometrically corrected or resampled. The system can provide 185 mm (MSS) and 198 mm (RBV) black and white film products for each MSS band or RBV subscene via a high resolution laser beam recorder. The CCT's generated by the system are compatible with the HDT format, and are available either in Band Sequential (BSQ) or Band Interleaved by Line (BIL) data. This paper will present both an overview of the total NASA/DOI system and a detailed discussion of the EDC Digital Image Processing System that is installed at the USGS facility in Sioux Falls, South Dakota.
An experimental pushbroom scan sensor, called the Multispectral Resource Sampler (MRS), is being developed by NASA for an earth orbiting spacecraft flight in the mid-1980's. This sensor will provide new and unique earth survey research capabilities beyond those possible with current sensor systems, and is designed with flexibility to provide a research facility for a number of preselected experiments. The sensor will have a ground resolution (IFOV) of 15 meters over a swath width of 15 kilometers, in four bands, or 30 kilometers in two bands. A data rate limitation of 15 megabits/second controls the permitted swath width. Each of the four arrays will have five separate spectral filters that will be selectable by command while in orbit. The basic sensor uses four 2000 element detector arrays in the focal plane of a 70 cm focal length (F/3.5) telescope. The four arrays are aligned on a common focal surface; thus no beamsplitters are required. This causes a spatial separation on the ground which requires computer processing to register the bands. A 2.2 ms dwell time of the pushbroom array allows bandwidths as narrow as 20 nanometers over the spectral range from 0.35 to 1.0 micrometers. Response in each band will be quantized into eight bits. The MRS can be pointed at ± 40° in the across track direction and ± 55° in the along track direction. Along track pointing permits stereo coverage at variable base/height ratios and atmospheric correction experiments, while across track pointing will provide repeat coverage, from a Landsat-type orbit, of every 1 to 3 days. A number of significant experiments which could be performed with the MRS include experiments in crop discrimination and status, rock discrimination, geobotanical mineral exploration, land use classification and forestry.
The preceding paper by Schnetzler and Thompson describes the Multispectral Resource Sampler, a visible and near visible sensor system for satellite borne assessment of earth resources which can be built using established technologies. In this paper, we will show how new technologies will make possible other design choices for future use in the 1985-1990 time frame. To make this example concrete, the characteristics of a suggested future earth resources sensor system are outlined in table 1. Inertialess pointing, i.e., the selection of a preferred ground area beneath the track, is provided without need for mechanical motion, figure 1, by selecting a short segment of a long detector array. The system provides a 10m x 10m detector footprint with 18,500 detectors in the visible array to cover a 185Km swath width across the vehicle track. The near and far infrared arrays use proportionally fewer larger detectors to maintain for all three spectral regions the same ratio between detector dimensions and the diffraction limit of the 30 cm aperture f:3.5 optical system.
This paper is concerned with the optical portion of the Voyager spacecraft imaging science subsystem (ISS). After a brief description of the Voyager mission and its photographic aspects, the functional requirements for the television optics are outlined. Environmental considerations that constrained the design are summarized. One section of the paper is devoted to radiation testing and its impact on the design of the wide-angle optics. The main portion of this report, however, is devoted to a description of the Voyager television optics. Finally, Voyager imagery results that are current with the preparation of this paper are presented.
We have analyzed the feasibility of measuring the global wind field from orbital altitudes to 800 km using a coherent infrared lidar. A one-meter diameter telescope is assumed on the satellite, collimating the 10 J pulses that are 3-7 µs is in duration from a CO2 TEA isotope gas laser. The lidar scans in a conical pattern around the nadir point in 11 s while pulsing at an average rate of 8 Hz. A comprehensive computer simulation that includes lidar, platform, and processor characteristics and atmospheric effects indicates that it should be possible to measure global winds at 1 km height intervals throughout the troposphere with an accuracy of 1-2 m s-1, so long as dense clouds do not obstruct the line of sight. A conceptual design for a Space Shuttle feasibility demonstration lidar has been completed.
The multichannel infrared radiometer on Pioneer Venus Orbiter is similar in concept to Earth weather satellite instruments. Its main function is to measure the thermal emission from the atmosphere at seven pressure levels above the Venus clouds, allowing a determination of the vertical temperature structure. In addition to these temperature sounding channels there are two channels operating in the visible and near infrared to study the structure of the upper clouds, and a far infrared channel sensitive to water vapor in and above the clouds. The instrument can operate in four distinct modes including a calibration sequence. By utilizing the spinning action of the spacecraft and the relatively short integration times (200 msec in global mode and 30 msec in local mode) a substantial portion of the planet can be mapped within a 90 minute data taking period centered about periapsis time. During the course of the mission to date, many thousands of temperature profiles have been retrieved covering a latitude range from the pole to the equator. These profiles, coupled with extensive mapping of the structure of the cloud tops over a sufficiently long time span, will produce considerable insight into the dynamical processes of the upper atmosphere.
The Orbiter Cloud Photopolarimeter (OCPP) onboard the Pioneer Venus Orbiter spacecraft serves both as a spin-scan imaging system providing 30-km resolution images of the ultraviolet (365 nm) cloud features and as a polarimeter yielding low resolution maps in the four spectral bands: 270, 365, 550 and 935 nm. Early images show numerous cloud features and apparent circulation patterns similar to those observed by Mariner 10 in 1974. The polar regions exhibit pronounced brightening, with the polar "ring" features located between 45° and 65° latitude usually having the highest intensity values in each hemisphere. Polarimetry measurements indicate that the main visible cloud layer of 1-μm radius sulfuric acid droplets is covered at least in the morning terminator region with a thin haze layer composed of submicron size particles. The performance of the OCPP to date has been excellent.
The goals, design and performance characteristics of the Solar Flux Radiometer flown on the Large Probe of the Pioneer-Venus Multiprobe spacecraft launched on 8 August 1978 are described. Radiance measurements of the Venusian atmosphere in several spectral channels between 400 and 1800 nm as a function of altitude were made to further understand the role of solar radiation in the thermal balance of the atmosphere. Elevation and azimuthal measurements on the radiation field were made with five optical channels. Twelve filtered Si and Ge photovoltaic detectors were maintained near 30°C with a phase-change material. The detector output currents were processed with logarithmic transimpedance converters before being multipliexed and digitized with an 11-bit A/D converter. Atmospheric sampling in both elevation and azimuth was done according to a Gaussian integration scheme. The data output was serial digital at an average rate of 20 bits/sec and included housekeeping (sync, spin period, sample timing and mode). The received data were used to determine the deposition of solar energy in the atmosphere of Venus along with upward and downward fluxes and radiances with an altitude resolution of several hundred meters between 67 km and the surface.