The University of Wisconsin-Indiana University-National Optical Astronomy Observatories ('WIN') 3.5-m aperture telescope project's design concepts and development status are assessed. The WIN telescope employs a wide field of view in order to take advantage of recent advancements in multiobject fiber-optic spectroscopy. A novel support system is under development for the borosilicate honeycomb primary mirror blank which acts solely on the mirror's rear surface; mirror temperature will be actively controlled. The WIN telescope's control system will use a distributed, easily expanded and upgraded network of microprocessors connected to a master computer via serial bus.
Image-quality data are presented which were gathered at the University of Arizona's Multiple Mirror Telescope (MMT) using a very high quality 1.8-m mirror, for median image sizes of 0.72 arcsec in 8-sec integrations. The measured image is judged to have been degraded by 0.48 arcsec from that allowed by the free atmosphere by such factors as errors in telescope focus, collimation, mirror support, mirror seeing, and the combined optical figure of the five mirror surfaces required to bring the light to the image-analysis CCD. These data imply the appropriateness of a total optical error budget of 0.2 arcsec for 8-m diameter telescopes currently being designed.
The two 8-m aperture telescopes planned by the National Optical Astronomy Laboratories for Mauna Kea and Cerro Pachon will specialize in high-quality imaging in the visible and IR ranges. Spin-cast lightweight honeycomb monolithic mirror blanks will be employed, in conjunction with active primary support structure and thermal control. The focal-plane instrumentation will encompass a CCD imaging system, a multiple-aperture Cassegrain spectrograph, a multiobject spectrograph, high angular resolution IR imagers, a medium-resolution IR spectrograph, and a cryogenically cooled high resolution IR echelle spectrograph.
The British astronomical community is currently engaged in the development of an 8-m aperture visible/IR telecope whose structure is based on the successful Herschel 4.2-m telescope. The primary mirror will be of meniscus type, with a 40:1 aspect ratio and active support; two secondaries will be used, of which the first (f/7) will furnish a corrected 40-arcmin field, while the second (f/35) can be chopped for use in the thermal IR. Enclosure design options under consideration encompass a carousel, a conventional hemispherical dome, and a lightweight octagonal enclosure.
The Magellan Project has as its goal the construction of an 8-m aperture optical telescope at Las Campanas, Chile, whose f/1.2 parabolic primary mirror is of borosilicate honeycomb type. The principal configuration of the telescope will be with Cassegrain focus at f/6.26; image sizes are expected to be of the order of 0.25 arcsec rms over the entire field, from 0.33 to 1.10 microns without refocus. An IR Cassegrain focus is planned at f/15, with a chopping secondary of low emissivity. Despite the fast primary-mirror f-ratio, spherical aberration will be acceptably low when chopping is accomplished via rotation of the mirror about its vertex.
The W.M. Keck Observatory and its Ten-Meter Telescope are nearing completion at the summit of Mauna Kea. The 10-m diameter primary mirror has a 17.5-m focal length and is composed of 36 hexagonal segments. There will be seven Ritchey-Chretien f/15 foci: two of them at Nasmyth foci, one at Cassegrain focus, and four at bent Cassegrain foci on the elevation ring. There will also be an f/25 IR focal plane at the intersection of the optical and elevation axes, whose focus will be chopped by a beryllium secondary mirror. Image quality with a FWHM of the order of about 0.25 arcsec, and an 80-percent enclosed energy diameter of about 0.40 arcsec, are anticipated.
The Japanese National Large Telescope's 7.5-m diameter mirror will be of 20-cm thickness, thin-meniscus type. In conjunction with the use of a low thermal expansion coefficient material, this mirror concept is anticipated to have a lowest eigenfrequency of about 15 Hz; a floating support system will be employed which uses 264 contacts with the mirror. An altazimuth fork mount in conjunction with an oil mount-supported truss structure. The design of the structure is such as to allow ready access to the Cassegrain focus for polarimetric and IR bandpass studies. Pointing accuracy is projected to be better than 1 arcsec.
The ESO's Very Large Telescope will employ an array of four 8-m aperture telescopes which can be operated in either independent or combined modes, as well as in an interferometric mode. The primary mirrors used are of thin, Zerodur glass-ceramic meniscus type, figured for f/1.8; the overall design is optimized for f/15 at the Nasmyth foci. Active optics are employed to compensate for slowly varying deformation and thermal distortion effects, as well as wind buffeting. Wavefront sensing for active figure maintenance is accomplished by means of a Shack-Hartmann CCD wavefront sensor which is integrated with the imaging CCD used for field acquisition and tracking. Two types of enclosure are under consideration for the telescope which attempt to maximize natural ventilating flows.
Up to now telescope optics were usually specified in terms of geometrical errors which cannot be linked to the actual performance under atmospheric turbulence limitation. A more realistic approach is proposed which takes into account atmospheric seeing and diffraction. The main advantage of the method is that at the same time it describes the final performance of the telescope, and gives to the optical manufacturer the maximum freedom to define and possibly modify its own manufacturing error budget.
The Columbus Project's double 8-m diameter binocular (11.3-m aperture) telescope optical configuration encompasses (1) two Cassegrain foci at f/5.4 that are optimized for wide-field optical observations, (2) an additional two Cassegrain foci at f/15 which are optimized for the thermal IR, (3) a combined f/33 focus optimized for interferometry, and (4) several additional stations derived from either the folding or redirection of the first three types. Attention is presently given to the telescope's error budget, whose goal is the achievement of a wavefront structure function equivalent to images of 0.23 arcsec FWHM; the wavefront of the combined telescope, atmosphere, and instrumentation should be equivalent to a detected image of 0.34 arcsec FWHM.
The Very Large Telescope Interferometer (VLTI) is one of the operating modes of the VLT. In addition to consisting of the four stationary 8-meter-diameter telescopes, it includes a number of movable Auxiliary Telescopes which both complement the (u,v) plane coverage of the large telescopes and provide a powerful interferometric facility by itself (available 100 percent of the time). The current plans for the implementation of the VLTI are described. These plans will be finalized after the choice of the VLT site in 1990.
At the primary focus of JNLT, three-lens correctors with two aspheric surfaces will be used. They are designed for the wavelength region of 320-500 nm and 440-1100 nm, respectively, and provide images of less than 0.2 arcsec rms radius for a field of 30 arcmin diameter. The corrector for the blue region will include an atmospheric dispersion corrector. Characteristics of aberrations that arises in JNLT primary corrector are discussed.
This report considers the optics of single versatile telescopes, rather than MMTs and interferometers. There are many new points to consider in the optical design which make the problems of the eight-meter different from those of previous telescopes. A wide-field focus is required at 6-8 ft, but should not conflict with the need to switch between Cassegrain and Nasmyth positions; this can lead to a new type of design. It is not necessary to use either of the textbook optical systems, Cassegrain or Ritchey-Chretien (R-C), at the f-number, but all possibilities must be considered. The present paper describes detailed possibilities for systems of the R-C type, and further work in progress on conventional Cassegrain and intermediate systems is outlined for completeness and comparison.
The adaptive optics techniques currently under development for telescopic mirrors of 8-m aperture class are presently projected to serve as the bases for mirrors of the order of 32-m diameter, in virtue of the rapid increase in the sensitivity advantage of diffraction-limited imaging with increasing aperture size. If the technology of atmospheric wavefront error sensing by means of artificial starlight is perfected, diffraction-limited imaging will be possible over the entire sky at even optical wavelengths. The corresponding resolution of 0.004 arcsec, in conjunction with the light-gathering power of a 32-m aperture, will constitute a milestone advancement for astronomical research prospects.
The application of full adaptive optics to astronomical telescopes in the foreseeable future is likely to be limited to infrared wavelengths greater than 1 micron both because of the limited number of bright enough wavefront sensing objects at visible wavelengths and because of the complexity and expense of making an adaptive optics system for large telescopes with the large number of elements required at visible wavelengths. Adaptive optics designed for infrared wavelengths do, however, improve the image quality at wavelengths shorter than the design wavelength, thus improving the sensitivity of interferometric imaging at those wavelengths.
It is generally thought that the resolution of large ground-based telescopes is limited by atmospheric turbulence rather than by diffraction from the telescope aperture. However, longer wavelengths are less affected by atmospheric turbulence than shorter wavelengths and, conversely, longer wavelengths are more affected by diffraction from the telescope aperture. An optimum wavelength exists where these two counteracting effects balance. At this wavelength, maximum (diffraction-limited) resolution is obtained. In night seeing conditions at typical telescope sites, the optimum wavelength is in the range 1-2.5 microns. For a 5-m telescope, it should be possible to obtain resolution of the order 0.05-0.15 arcsec routinely at these wavelengths. However, to facilitate such precise resolution the telescope must be diffraction-limited.
As applied to the 4.2-m William Herschel Telescope, the multiaperture real-time image-normalization system presented implies a wavefront whose size requires a mask of six optimally-scaled subapertures. These subaperture images are separated and examined on a single image photon detector which yields x, y, and t coordinates for each recorded photon. The motions of these images feed back to six independent piezoactuated active mirrors which act to null the image motions at a CCD focus. Data are presented from two image normalization runs, with and without active mirrors, which illustrate the size and variation behavior of the coherent seeing length, characteristic seeing times, and power spectra.
Since liquid mirrors are potentially useful in science (e.g., astronomy, atmospheric sciences, and optical testing), work has been undertaken to determine whether they are technologically feasible. A testing tower has been equipped with a scatterplate interferometer interfaced with a CCD for data acquisition and a microcomputer for data analysis. This equipment was used to test a 1.5-m-diameter f/2 liquid mirror, showing that it is diffraction limited; interferometric measurements give Strehl ratios of order 0.8. A 2.7-m-diameter liquid mirror and astronomical observatory presently under construction is briefly described.
General design considerations of objective-mirror coronagraphs are presented. A 1-m-focal-length prototype reflecting coronagraph based on a 5.5-cm aperture spherical superpolished silicon mirror objective is described. The design is simple off-axis reflection from the objective to a conventional coronagraph optical system (occulting disk, field lens, Lyot stop, and imaging system). This instrument has produced the first images of the emission corona using a ground-based reflecting coronagraph. A second prototype instrument based on a 15-cm aperture superpolished fused-silica mirror is also described.
The design concept of a wide-field astronomical imaging telescope for use as a payload on an unmanned space platform, a Space Station attached payload, or a Delta-class Explorer is described. The instrument is based on a space Schmidt telescope concept studied by NASA and ESA (1979) for Spacelab missions. The astrophysical objectives include all-sky surveys in the UV and NIR ranges. Objects of interest include very hot and very cool stars and the interstellar medium. The UV range is inaccessible from the ground, and large-area surveys and sensitive imagery of diffuse sources are impractical with current or planned UV space telescopes. The NIR range is severely compromised in ground-based observations, particularly of diffuse sources, by airglow emissions, and no wide-field NIR space telescopes are currently approved for flight.
A 1.6-meter diameter f/0.95 all-reflecting telescope was designed to observe orbital debris particles as small as 1 mm from the shuttle payload bay. The telescope was specified to have a flat focal surface without the imposition of refractive elements. Two design configurations involving three mirrors were evaluated - a reflective Schmidt-Cassegrain and a modified Paul corrector. The Paul system was found to be more compact and appropriate for this application.
The 2dF project aims at giving the AAT Prime-focus a 2-deg diameter corrected field of view geared primarily toward multiobject fiber spectroscopy. The combination of the 4-meter aperture of the AAT, a 400-fiber autopositioner and high throughput spectrographs should make the 2dF a powerful facility for statistical spectroscopic studies in the years to come. The design study for the corrector, fiber-positioner and spectrographs is now complete and the main conclusions are reported.
It is very significant to develop the fiber-optic telescope that makes use of the large cross-section (i.e., large information) image-transmitting bundle and to realize the system combined 'rigid' with 'soft', in order to achieve the observation of remote objects in some difficult conditions. This paper discusses the key technical problems by which simpler structure, higher performance indices (magnification, field of view) of systems, finer resolution and better results of observation are realized under the condition of the definite image-transmitting bundle. These problems are: to choose and use high performance LCSITB; to select a reasonable eyepiece to restrain the mosaic pattern of the image; to make use of the objective lens that has long focal distance and large aperture to obtain optimal coupling between the objective lens and LCSITB and to obtain the higher resolution and sufficient brightness of the image; and to select the simplest erecting system. Using the fiber-optic telescope which is designed and developed, it is observed and photos show the remote objects from about one kilometer and good results are achieved.
An improved chopping secondary system for a 300 mm balloon-borne IR telescope (BIT) has been developed. BIT is planned to be flown for the first tine in May 1990 in China. Design and overall properties of the chopping secondary system are presented. The important feature of this chopper is its control driver, which is a new type of vibrator. The whole system including the control circuit is very simple, lightweight, and low-cost. A 2-msec rise time of the 93 mm secondary mirror motion was achieved at a frequency of 10 Hz.
In order to achieve a high speed/high precision/low power consumption IR telescope chopping system which excites no mechanical resonances, in conjunction with a device size smaller than the secondary mirror diameter, a chopping servo has been developed which uses feedforward control with optimized driving torque as well as feedback control guided by robust stability theory. The compactness of the device has been achieved through the use of an Nd-Fe-B magnet motor and the balancing of the mirror against the mass of the electromechanical system. The chopper has a 10-msec transient time, and stability in excess of 1 arcsec, for square-wave chopping with amplitudes of as much as 300 arcsec.
Telescopes designed for non-conventional imaging of near-earth satellites must follow a unique set of design rules. Costs must be reduced substantially and the design must accommodate a technique to circumvent the atmospheric distortions of the image. Apertures to 12 meters and beyond are required along with alt-alt mounts providing high tracking rates. A novel design for such a telescope has been generated which is optimized for speckle imaging. Its mount closely resembles a radar mount and it does not employ the conventional dome. Costs for this design are projected to be considerably reduced compared to conventional designs. Results of a detailed design study will be presented. Applications to astronomy will be discussed.
The factors contributing to fringe contrast decrease for telescopes working near their diffraction limits are summarized. These factors include variations with time, such as atmospheric variations, vibrations, pathlength drift, and fringe tracking noise; and variations accross the pupil; variation with wavelength and factors relating to polarization effects; unequal beam intensities; detector resolution; and pupil transfer geometry. The effects on multispeckle images are also considered. The resulting error budget for the Very Large Telescope Interferometer (VLTI) is derived. It is concluded that the total random error in the fringe contrast is 4.2. The total calibratable systematic error amounts to 34 percent (27 percent due to the instrument, 9 percent due to the atmosphere).
The development of generic optical designs for supporting wide-field imagery from an array of individual telescopes is presented. These systems will be employed to gather imagery over a total field of 0.5 to 1.0 degree. Various design possibilities are studied assuming the use of a square field, with the option of utilizing a slit field. A general optical design concept that can be transferred to a reasonable number of fairly closely packed arrays is examined, evaluating the promising three- or five-mirror arrangements.
This paper discusses the requirements posed on the ESO Very Large Telescope (VLT) Interferometer by the applications that require a field-of-view larger than the Airy disk of the individual telescopes. The most essential requirement for such wide field-of-view use of interferometric arrays is the maintenance of the pupil configuration, which applies to all the details of this configuration. Not meeting this requirement leads to path-length differences among the rays of each of the telescopes composing the array. An error budget for the optical design parameters of the VLT Interferometer is derived.
The sensitivity of image quality to various system and subsystem parameters has been studied in order to determine the utility of imaging phased telescope arrays to wide field of view (FOV) applications. An error budget tree is developed to include optical design errors, assembly and alignment errors, optical fabrication errors, and environmental errors. Trade studies, parametric analyses, and previous engineering experience permitted the derivation of design and engineering tolerances from error budget allocations based on known state-of-the-art performance characteristics. The FOV limitations of the residual optical design errors (off-axis aberrations) are investigated in detail. It is shown that the somewhat benign (for conventional optical systems) aberration of field curvature results in field-dependent relative phase (piston) and pointing (tilt) errors, which rapidly degrades the image quality of phased telescope arrays as the FOV is increased. Thus extremely light tolerances on residual field curvature are needed for telescope diameters larger than one meter.
A new geometric and physical optics simulation code for predicting the performance of phased array telescopes is described. A skew aspheric ray trace routine computes a composite spot diagram and an array of optical path differences for the entire telescope system. This includes the four nearly identical afocal telescopes and the single combiner telescope. A second routine then computes subaperture Zernike aberration coefficients. A wave optics code computes point spread functions, and a final code computes optical transfer functions. The simulated performance of Air Force's Multipurpose Multiple Telescope Testbed (MMTT) is then presented and discussed. All optical surfaces of the telescope plus in situ measured aberrations are simulated. The results show that the telescope is nearly diffraction limited at small field angles, but suffers from phase and tilt differences among the telescopes at field angles above two milliradians.
A key feature of the Multipurpose Multiple Telescope Testbed (MMTT) is its relatively wide field of view -- up to 30 arcminutes total. A thorough evaluation of the telescope array necessitates some form of image analysis over this field. System designers chose the star test, here modified to simltaneously display point spread functions (PSFs) at several locations in the image plane. Deviations of the image structure from ideal indicate control system deficiencies. Diagnostics hardware was designed and built to both simulate numerous unresolved light sources throughout the field of view and to display several of the corresponding images for detailed analysis. The resultant multiple target generator is an array of precision pinholes complete with beam-converting optics that require minimal alignment. PSFs are displayed with modern solid-state video equipment. Image irradiance cross-sections can be readily compared with theoretical predictions.
A phased-array telescope is formed by combining a number of optical telescopes to yield the resolution of a single much larger telescope. One such telescope, the Multipurpose Multiple Telescope Testbed (MMTT), consists of four 20-cm-aperture telescopes phased together with a 30-arcmin field of view. This paper illustrates the model identification of the MMTT system. Each of the 4 channels of the system consists of over 30 subsystems and/or components. Various experimental tests were set up for the MMTT components using a white Gaussian noise source on a spectrum analyzer. The input/output signals for each experimental set up were measured and an ASCII file was created on a personal computer. The transfer function of each subsystem was identified using either the spectrum analyzer and/or through standard system identification software on an AT-class PC. The model thus identified can be used to study system's behavior by simulation as well as designing various controllers for tilt, piston, and pupil geometry control purposes.
The control system for the Multipurpose Multiple Telescope Testbed (MMTT) is described. Performance of the control system is measured against the goal of maintaining lateral pupil geometry to within one micron of the ideal. The MMTT control system consists of four channels. A discrete-time-varying Kalman filter processes the sensor measurements to estimate lateral pupil geometry and piston errors in each channel. The controller integrates the Kalman filter estimates along with the x- and y-tilt measurements, which are treated deterministically. Multiplication by an axis separator matrix converts the control input commands into appropriate piezoelectric transducer actuator commands. The commands from each control channel remove piston, tilt, and pupil geometry error between each telescope and the telescope denoted the reference. The entire control algorithm is implemented on high-speed array processor boards. Preliminary test results show that the control system is accurately controlling lateral pupil geometry for small field angles. For larger field angles, the system is unstable due to parameter variations.
The Multipurpose Multiple Telescope Testbed is described, and initial tests are discussed. After the optical quality of individual telescopes was established with interferometric tests, the cophasing and image superpositioning accuracy of the array were measured using star tests. Point spread functions were calculated with a physical optics code. Preliminary star tests using two of the four telescopes are presented and compared with the predicted pattern. It is concluded that the preliminary results obtained lend credence to the given method of controlling lateral pupil geometry and to previous calculations of optical aberration and optical alignment tolerances.
A new wavefront sensing technique called curvature sensing is described. It maps the wavefront total curvature rather than its slope and has been applied to an experimental seeing monitor which detects turbulence induced fast focus fluctuations. Some of the advantages this monitor presents, as compared to DIMM's, are: (1) sensitivity is increased by the use of a circular pupil, (2) the cost is lowered by the use of a photomultiplier, (3) the loss of signal is prevented by the system's fast run, (4) the system runs continuously, and (5) the noise bias is continuously measured and subtracted out.
Astronomical observatory site testing programs in the USA and USSR have used a variety of stellar image motion monitors in the selection of the best sites for the construction of large (6 to 10 meter) telescopes. While there appears to be a reasonable agreement between microthermal and sodar results for the better sites in both countries, there remain unexplained inconsistencies in measured seeing, especially at Mauna Kea, Hawaii and Mount Sanglok. The photoelectric seeing monitor built by Scheglov (1984) of the Moscow Sternberg Institute, and the National Optical Astronomy Observatories site-survey intensified CID seeing monitor have been mounted on the same telescope. Simultaneous image motion data recorded are
During two short campaigns intensive coordinated measurements have been performed to determine the various contributions to image degradation on Mauna Kea. Some of the results already obtained are presented here.
Interferograms of the front surface of a zenith-pointing 1.8 m mirror for a range of surface-to-air temperature differences were analyzed using two different methods to show that significant mirror seeing effects do not occur in the range of + or - 0.5 C, and may be acceptably small up to + or - 1 C. Mirror seeing was shown to comply with Kolmolgorov turbulence laws up to limits that are apparently due to the test setup enclosure. Small seeing effects were discernible despite much larger image error sources.
The project to enlarge the Multiple Mirror Telescope (MMT) to a 6.5 m single primary mirror telescope is described. The goal is to provide a telescope which is competitive with the existing MMT in tracking and pointing performance (0.2 and 1.0 arcseconds, respectively) but has more than twice the light gathering power and 15 times the angular field of view. The existing mount and building will be used with minor modifications so that the cost of the project is relatively modest. Casting of the 6.5 m mirror is scheculed in early 1991 and first light in late 1993.
The seeing degradation in the Japanese National Large Telescope (JNLT) caused by its hemispherical dome is investigated. A possible plan of the JNLT site layout and the thermal control concept are introduced in order to attempt to reduce the seeing degradation induced by the dome to the 0.1 arcsec FWHM range budgeted. A three-dimensioal compressible fluid analysis of the inside and outside the dome, including heat transfer effect, is developed and used to understand the seeing degradation mechanism as well as the wind buffet effect on the telescope and the primary mirror.
A site testing telescope (STT) was placed for a period of 3 months outside the building, and for a period of 4 months inside the building of the Multiple Mirror Telescope (MMT) located on Mt. Hopkins, and measurements of the astronomical seeing were carried out with both the STT and MMT. A comparison of the simultaneous and interleaved measurements with the two telescopes reveals a tight correlation and a well-defined relationship between the seeing image sizes determined with each telescope. The STT predicts very well the size of a long-exposure image obtained with the MMT. There exists for the MMT an optical blur component of about 0.47 arcsec that is revealed in image size data obtained with only the MMT and that is also revealed and must be accounted for in comparisons between the STT and MMT. Also discovered is a downward component of seeing that is caused by a trailing downwind plume of cold turbulent air that is shed off the radiatively cooled exterior building surfaces. The interior dome component of seeing is remarkably small (upper limit about 0.2 arcsec). The median site seeing is determined to be 0.55 arcsec at an effective wavelength of 7165 A for the MMT observations, or about 0.59 arcsec at 5000 A. The 10 percentile value is about 0.28 arcsec (0.30 arcsec at 5000 A). The median seeing observed with the MMT from one and a half years of data is 0.72 arcsec (0.75 arcsec at 5000 A), and is degraded from the site value virtually entirely by the optical blur component.
Results are presented on the evaluation of the site for the Nordic Optical Telescope (NOT) at its site at the Roque de los Muchachos Observatory. Basic design features of the NOT are described together with the parameters that define image quality, with special consideration given to the role of atmospheric turbulence and the factors taken into account in the selection of the telescope site. Attention is given to the optical elements of the NOT and to its mechanical structure as well as to the thermal control of the telescope, the enclosure, and the ancillary instrumentation. Results from first observations at the NOT point to excellent observing conditions in terms of transparency and extinction stability as well as image quality.
A 3.5 m telescope is under construction at Apache Point near Alamogordo, New Mexico, at an elevation of 2800 m. A thermal model of a telescope enclosure is described. The model evaluates various strategies for minimizing local sources of image degradation (dome seeing). Direct and diffuse insolation, radiation to the sky, conduction, and the thermal inertia of the walls, interior air, roof, and structural steel are included. It is observed that highly reflective surface coatings reduce heat absorbed during the day, but are not very effective in reducing heat transfer in the telescope chamber at night, assuming that components with large heat capacities or thermal time constants are insulated.
The preservation of image quality is one of the most important design criteria for a telescope enclosure. Image quality is adversely affected by the presence of fluctuations of air temperature in the light path. In an ideal telescope enclosure, the air inside the telescope chamber is continually replaced by ambient air before it has the opportunity to be warmed or cooled by the surfaces of the telescope enclosure. In the present study, dye in a water tunnel is used to trace flow around and through four models of 8 m telescope enclosures. The models are constructed of transparent acrylic and are 1:172 scale. Flow attributes are compared for the four enclosure designs under consideration. The enclosure models are oriented at various angles with respect to the flow direction. Additionally, each design contains apertures in the exterior walls which can be opened to improve flushing and other flow characteristics. For the octagonal and rectangular designs, flushing of the telescope chamber through the open vents is good in all orientations. Flushing of the cylindrical concept, which has no vents in its side walls, is poor when the slit is oriented from 60 to 120 deg with respect to the flow direction. Flushing of the hemispherical enclosure is poor at angles larger than 60 deg. With the vents closed, flushing is poor for all designs, especially at angles greater than 60 deg.
The cost of an 8-m telescope is still too high for most national observatories. Cost reductions must involve new technology for the primary mirror: its material and figuring, its mass, handling and the aluminizing plant. The present scheme addresses these problems using some features of the Keck technology, but is simplified. Consideration is given to a segmented mirror in which the segments are radially cut sectors. The sectors for an 8-m aperture will fit inside a 4-m aluminizing plant (which already exists on some sites, and in any event is cheaper than an 8-m plant). Zero-expansion material (glass-ceramic or fused silica) may be used. The model thickness is about 10 cm. The proposed method of production is to figure the whole set assembled as a single mirror. No cutting takes place after figuring.
The fabrication of high aspect ratio mirrors up to 8-m in diameter presents numerous new challenges in the field of optical manufacturing technology. These mirrors, with aspect ratios in the region of 40:1, represent a new class of optics which requires unique handing, figuring, polishing and metrology approaches. A facility conceived explicitly for the manufacture of high-quality large-aperture primary mirrors for astronomical telescopes is described.
While coefficient of thermal expansion (CTE) differences between the facets of a large (8-m diameter) telescope's hexagonal-mosaic mirror generate optically significant deformations in the midspatial frequency regime, when the average temperature of the mirror undergoes substantial change, it is presently established that the magnitude of these deformations lie within acceptable limits for the active figure-control system envisioned for such telescopes. It is nevertheless recommended that CTE discontinuities within such mosaics be consciously planned for at the outset of construction planning.
The National Optical Astronomy Observatories (NOAO) have been working for several years to develop the technology for 8-m telescopes using structured borosilicate glass primary mirrors. In March 1989 the final stage in this technology development program began with the delivery of a 3.5-m mirror blank, cast under NOAO contract at the University of Arizona Mirror Lab. The project will have four phases: (1) initial fabrication, (2) testing of support and thermal systems, (3) aspherizing the mirror and rework of the support and thermal systems, and (4) final acceptance test. At the conclusion of this effort there will be a finished 3.5-m f/1.75 mirror, which will then become the heart of the new WIN telescope on Kitt Peak.
It is presently noted that the availability of CCD cameras, in conjunction with state-of-the-art computer hardware and software, render Hartmann testing in optical shops a viable alternative to, or checking procedure for, other types of tests. An evaluation is made of data reduction methods for this test. While the reduction of data-fitting to derivatives of Zernike polynomials filters out high-frequency surface information, and runs the risk of yielding incorrect coefficients, it may be able to find special-purpose applications. The least-square integration of a surface is reliable, but time-consuming in the case of large data sets. Subaperture Hartmann results can be assembled to obtain the entire surface; rastering can double the high spatial frequency spatial information of the surface.
A modification to the usual phase-shifting interferometry algorithm permits measurements to be taken fast enough to essentially freeze out vibrations. Only two interferograms are time critical in this 2 + 1 algorithm; the third is a null. An error analysis has been performed for this new algorithm. The implemented system acquires the two time-critical interferograms with a 1-msec separation, on either side of the interline transfer of a standard CCD video camera, resulting in a reduction in sensitivity to vibration of one to two orders of magnitude. The required phase shift is achieved via frequency shifting.
In order to reduce polishing costs and correct unexpected errors in fabrication and polishing, the support of very large optics can be actively enlisted in telescope mirror optical figure adjustment. A set of leaf springs is used by the Keck Ten-Meter Telescope to apply moments about the pivots of the mirror mosaics' whiffletree support. The springs successfully reduce the polished rms surface error by a factor of 6 to 15, while reducing the 80-percent enclosed energy diameter by a factor of 2.5-6.0. Additional current limitations on figure improvement include the difficulties of polishing higher spatial frequencies and predicting warping during mirror fabrication.
We are in the process of polishing a 1.8-rn f/i ellipsoid with an actively stressed lap. As a preliminary
exercise, we have polished the mirror as a sphere using a rigid subdiameter lap. The overall surface error
was 25 nm rms, and the surface met a specification corresponding to i/8-arcsec image quality. A stressed
lap 600 mm in diameter was designed and built to polish the mirror as an f/i ellipsoid. It consists of an
aluminum disk which changes shape continuously under the influence of 12 moment-generating actuators.
These actuators are programmed to produce the shape changes necessary to make the lap fit the mirror
surface as it moves across that surface and rotates. In this paper we describe the principles and design
of the lap, test results, and progress to date in polishing the 1.8-rn mirror.
Technology developed for computer-controlled mirror manufacturing is described. The polishing tool is equipped with
electromagnetic force actuators to regulate the local polishing forces according to the measured mirror errors. Optical
testing with the interferometric modification of the Hartmann test and a CCD-camera as a detector allow accurate and fast
measurement with high sampling frequency. Air turbulence and vibration effects are minimized in the workshop which is
blasted into the bedrock and equipped with good thermal insulation.
The numerical control device for zonal figuring of aspherical surface according to polar coordinates has the advantage of simplicity and compactness in structure. The method to predict the surface profile after figuring is presented. Effort has been made to develop some methods for automatic generation of the zonal figuring procedure by computer. An example of making a Schmidt correcting plate by zonal figuring is given. The results of various figuring procedures are compared and discussed.