The quest for knowledge and understanding of the universe has consumed almost everyone at one time or another. Because of this, modern history is full of events in the parallel search for instruments and technology that will better allow this search to continue, that will allow us to seek even further and more faint celestial objects. The progress of this technological search has led to steadily larger telescopes. It has now reached a point of demand for primary objective elements that are so large and accurate that their specifications typically exceed the capabilities of the available materials and manufacturing processes. With apertures in excess of 10 m in diameter now being specified, only multiaperture or segmented mirror techniques appear viable from the point of view of cost and material.
Large beryllium mirrors are being proposed by NASA, the SDI Organization, and others for a number of space applications. Traditional fabrication methods produce anisotropy and inhomogeneity of material properties that can result in thermally induced dimensional instabilities. In addition, current methods are slow and costly and may be limited to mirror sizes that can be machined from billets with a maximum diameter of about 1.7 m. This paper discusses hot isostatic pressing, the preferred fabrication method for large beryllium mirrors since it can produce dimensionally stable beryllium mirrors in less time and at lower cost than conventional methods.
The ability to generate large aspheric optical components to micrometer tolerances while leaving surfaces relatively free of subsurface damage is quite recent. This paper describes methods for determining the optimum parameters for generating high quality aspheric surfaces.
Surface roughness data are presented for a matrix of diamond-turned electroless nickel samples having a combination of six phosphorus contents and four heat treatments. Roughness measurements were conducted with commercial optical and stylus profilers (WYKO and Talystep). The results are discussed in terms of the material composition and heat treatment, plus other factors having an observed influence on the surface roughness. For the optimum material properties, full-length (665 itm) 20x WYKO scans yielded values of better than 10 A rms after correction for instrument roll-off.
Faster production of large optical mirrors may result from combining single-point diamond crushing of the glass with polishing using a small-area tool to smooth the surface and remove the damaged layer. Diamond crushing allows a surface contour accurate to 0.5 Am to be generated, and the small-area computer-controlled polishing tool allows the surface roughness to be removed without destroying the initial contour. Final contours with an accuracy of 0.04 pm have been achieved.
The optical surface analysis code (OSAC) is used to predict the optical performance of the Hubble Space Telescope. We describe the analysis in detail and compare some of the results with those of a standard optical transfer function analysis. Also, we discuss the mathematical foundation of the OSAC code and describe in detail the error data that were used as input. OSAC is shown to be a valuable tool for separating and analyzing the effects of misalignments, figure errors, and mid and high frequency statistical mirror surface errors on optical performance.
Quasi-Cassegrain-type four-mirror telescopes are compared to conventional two-mirror Cassegrain telescopes for use as high performance, very large aperture space telescopes. Spherical and parabolic primaries with continuous as well as segmented surfaces are considered. Imaging characteristics and misalignment sensitivities serve as the principal criteria of comparison. The evaluation shows that parabolic primaries yield superior wide-field performance, whereas spherical primaries hold distinct advantages regarding manufacturability and regarding certain alignment aspects in the case of segmentation.
Preliminary engineering studies are in progress to define a telescope for the Far Ultraviolet Spectroscopic Explorer (FUSE) mission. General science objectives include high resolution spectroscopy in the 900 A to 1200A spectral region, low or moderate resolution spectroscopy in the 100 A to 900 A range, and long slit imaging over a 1 arcmin field at a spatial resolution of 1 arcsec. Telescope design studies indicate that a 1 m diameter Wolter-Schwarzschild type II glancing incidence telescope with an effective collecting area of 3000 cm2 is required to meet the primary science objectives. A baseline optical design has been completed and initial alignment sensitivities derived to begin the process of error allocation for the entire system. Various mirror fabrication methods are being evaluated, including the incorporation of contemporary diamond turning technology with computer-controlled polishing.
A study was carried out at the Jet Propulsion Laboratory during the first quarter of 1985 to develop a system concept for NASA's Large Deployable Reflector (LDR). This new system concept meets the primary scientific requirements and minimizes the cost and development time. The LDR requirements were investigated to determine whether or not the major cost drivers could be significantly relaxed without compromising the scientific utility of LDR. In particular, the telescope wavefront error is defined so as to maximize scientific return per dollar. Major features of the concept are a four-mirror, two-stage optical system; a lightweight structural composite segmented primary reflector; and a deployable truss backup structure with integral thermal shield. The two-stage optics uses active figure control at the quaternary reflector located at the primary reflector exit pupil, allowing the large primary to be passive. The lightweight composite reflector panels limit the short wavelength operation to approximately 30 pm but reduce the total primary reflector weight by a factor of 3 to 4 over competing technologies. System optical performance is calculated including aperture efficiency, Strehl ratio, and off-axis performance. On-orbit thermal analysis indicates a primary reflector equilibrium temperature of less than 200 K with a maximum gradient of =°C across the 20 m aperture. Weight and volume estimates are consistent with a single Shuttle launch and are based on Space Station assembly and checkout.
The effect of UV preionization in a discharge-excited high power nitrogen laser is presented. A maximum energy of 7.7 mJ in a 4.7 ns pulse is reported for the case of a N2/He mixture. An increase of the output energy by 65% with the addition of SF6, an electron quencher, is also observed.
Light-sensitive liquid films are used to produce cross modulation between light beams in an all-optical setup. Two light beams are superimposed in the same region of the film. The samples are black oils whose physical properties strongly depend on the temperature. The liquid film is deformed by heating with the first light beam, thus changing the divergence of the other one. In the first setup presented in this paper the power concentration per unit area of a laser beam (HeNe, cw, 1 mW) is switched from a maximum to a minimum value and conversely) following the intensity variations of another laser beam (HeNe, cw, 10 mW). The time required to change the power concentration from 0.1 mW/mm2 to 10-3 mW/mmz is about 100 ms. In another setup, large-scale deformations of the liquid film (diameter about 5 mm ) are induced by heating with a white-light beam. The deformed region behaves as a mirror whose curvature radius depends on the intensity of the light beam. This allows one to form the real image of a 3-D object through the mirror and to switch it on and off by chopping the light beam. A typical switching time is about 1 s.
Separable median filters over 3 X3 neighborhoods can be realized by applying in succession logical operations in the horizontal and vertical directions. We show how to take advantage of the architectural resources of commercially available pipelined image processing systems in implementing these filters. The required logical operations are decomposed into a succession of image additions, subtractions, and translations that can be executed very efficiently. The proposed method showed a 70-fold improvement of processing speed over the well-known histogram updating method for 3 X3 filters over 512 X512 X8 bit images.
The simplified band transport model of the photorefractive effect in which the material responds linearly to local changes in light intensity modulation ratio (fringe contrast) is examined in detail. The validity of the model for cosinusoidal grating formation is first discussed with reference to frequency and time dependence. The "hopping" model of the photorefractive effect is then considered. It is shown that, although physically distinct from the band trans-port model, the hopping model may be considered as a special case of the band transport equations. Results of a numerical simulation of the band transport model are presented that illustrate the validity of the linear in modulation approximation. The band transport model is then extended to describe non-plane-wave interactions and to include a tensor static permittivity. Features of this extended model are emphasized by some experimental results. These results show the effect of nonuniform beam intensity profiles on apparent time constants as measured in four-wave mixing experiments using photorefractive Bi12SiO20-
I thought that the readers of Optical Engineering might be interested in a brief look at the status of the Journal, and with that in mind, I prepared the following chart. The figures under the 1985 column are actual figures, whereas those under the 1986 column are estimates based on the publication data available for the first nine issues of this year.
One of the most successful workshops on
tunable solid state lasers held in recent years
is documented in this book. The Workshop
on Tunable Solid State Lasers for Remote
Sensing was held October 1-3, 1984, with
NASA as its sponsor. Nearly 120 scientists,
leaders in their respective fields, met in Stanford,
California, to assess the status and progress
in tunable solid state sources for remote
sensing and to learn of the requirements for
remote sensing from a space platform.