A review of the present technology status of wavefront sensors and deformable mirrors is presented. As components in adaptive optics systems these elements have been developed for specific requirements. Future applications will require advanced capabilities for visible wavelength operation. Some forecasts of these requirements are provided.
Analytical and experimentally determined performance characteristics of the Integrated Imaging Irradiance (0) Sensor are presented with emphasis on features relevant to closed-loop operation. Hardware components which were used to develop data on ultimate S/N performance of the sensor are described. Experiments performed on the 13 Sensor breadboard under computer control in which optical tilt variations in an input wavefront, induced either by an electronically controlled jitter mirror or by artificially generated atmospheric turbulence, are compensated using a Type 1 digital servo, are detailed. Other sensor features reviewed include operation in the presence of partial detector failure, and with either negative or positive contrast sources.
By modeling a wavefront sensor and deformable mirror system as a high pass spatial and temporal frequency filter, its correcting ability in terms of a transfer function can be determined. Since the characteristics of a particular sensor/mirror system, as well as the uncorrected incident wavefront, affect the form of the transfer function, both must be used to describe the system's correcting ability. In this paper a mathematical formalism is presented that is based upon a linear systems approach and which relates the transfer function to the characteristics of a deformable mirror system. Three methods of introducing system characteristics into the filtering process to correct for random and deterministic errors are presented. In situations where spatiotemporal coupling becomes important, a three dimensional filter in frequency space is employed.
A new actuation scheme for mirror figure control, introducing a complete set of membrane shear and bending stresses onto the edges of the mirror, is presented. Thermoelastic response of both segmented and continuous mirrors is analyzed from a closed form solution of the "exact" Love theory of thin spherical shells. There are significant departures from Reissner's theory of shallow shells. Very low residual figure error is possible by edge control alone, lower than with interior point loads. Minimum rms residual error stress profiles are computed. Engineering applications are discussed.
A discrete-actuator, deformable mirror has been characterized for use in the infrared spectral region. This device, with a clear aperture diameter of nine inches, achieves a surface deformation of 8.5 μm at 1500 volts. Conformity of the reflective surface with a desired figure has been determined by interferometric analysis and residual error computation for focus, tip, tilt and two orders of astigmatism.
During the last 3 years under the High Altitude Large Optics (HALO) Technology Program, we have been developing computer-controlled, optical surfacing technology for application to the manufacture of large lightweight, odd-shaped, infrared-quality aspheric mirrors that will be needed in the next generation of space telescopes. The program has consisted of developmental work on our small optical surfacing machine, to support on-going production of a large oblong demonstration mirror with our large machine. The problems addressed have been the measurement of the aspheric surface, the material removal model, the data reduction machine control algorithm, and lap designs for producing aspherics rapidly. To date, flats and spheres have been produced with rigid laps, control of absolute sag to 0.1 μm has been demonstrated, and a compliant lap concept has been developed. Controllability of this lap has been verified in polishing. The proof test, to start shortly, will be a full grind/ polish fabrication sequence to produce an eccentric ellipse. The program plan calls for later fabrication of a large two-panel eccentric ellipse, using the present large oblong demonstration mirror as the inner panel. A new concept for a self-calibrating, absolute measuring machine is being developed for use in the production of the large ellipse. It should be accurate enough for all of the grinding and perhaps for the polishing and final figuring too.
Large lightweight deployable optical systems are highly sensitive to structural changes from material aging and from solar heating. Optical tolerances are a fraction of a wavelength of the radiation the system is designed for and the tolerances remain constant regardless of system size. These large three-mirror infrared systems are to be manufactured at normal temperatures and for use at cryogenic temperatures. Even the most uniform optical material known, fused silica, becomes one of the quality limitations at these temperatures. The quality maintenance problem is compounded when many mirror panels must be assembled to provide a single large mirror area. The support structure must keep individual panels accurately located on one mathematical surface to a small fraction of a wavelength of light. The structural materials that are available to mount the mirrors are an order of magnitude poorer in stability than the fused silica mirror panels, thus large structural warpages with time and with the position of the sun must be accommodated by system actuators between the mirror panels and the support structure. A sensing system is described that measures mirror panel-to-panel mismatch and that determines system wavefront as a function of sensor location in the image field. These data are accepted by a central computer control system that deconvolves the control signals. Examples are given of computer simulation of the sensing and control process, showing the number of iterations required to bring a system into optical adjustment.
Two sharpness functions are considered for the problem of dynamic phase estimation and correction of aberrated images. One is based on the integral of the square of the image irradiance distribution. The other uses integration of the irradiance distribution over an area smaller than the diffraction-limited resolution area. Examples of phase retrieval using these sharpness functions are given, and laboratory experiments on their verification are described. Applications of this technique to high-energy laser systems, thermal blooming, imaging through atmospheric turbulence, and large optical systems are discussed.
A basis for making absolute distance measurements to an accuracy of 0.025 μm over 0-1.5 m intervals is reported. Extensions of this technology will permit distances of 50 m and greater to be measured to the same accuracy. Two-color, synthetic Michelson interferometry using a CO2 laser source capable of generating four sets of R- and P-line pairs is employed. This allows reduction of the very large ambiguity exhibited by conventional Michelson interferometers as well as the resolution of difficulties which would otherwise arise due to instabilities in the measurement arm of the interferometer. This latter effect is a practical rather than a fundamental consideration, but is nonetheless important if the interferometer is to emerge from the laboratory as an effective, workable instrument. Distance is determined in terms of a denumerable number of precisely known wavelengths and fractions thereof.
A simple CO2 laser can be caused to exhibit a specific sequence of five rotational lines or "colors" with cavity length tuning. The sequence, useful in multicolor interferometric distance measurement, is predicted to appear only near certain discrete laser cavity lengths. Three different length breadboard lasers are described in which a desired sequence is found.
In this paper a control engineer's point of view of the Large Space Structure (LSS) problem is presented. A definition of a "large" space structure from a control engineer's standpoint is presented along with a discussion of the various factors which drive the control problem. The interaction of the controller with the structure to be controlled is explored to point out the role of each disipline in solving any practical LSS problem. The ways in which they can contribute to a successful system are presented. Finally, the role of the optics engineer as both system user and technology provider is discussed.
Following an overview of the role of space observations in contemporary astrophysics the next generation of space-based observatories (the Space Telescope, the Gamma Ray Observatory and the Advanced X-Ray Astrophysics Facility) is described. Possible new directions which may be pursued in the 1990s are also discussed. These include the development of large flux collectors for use in astronomy in the ultraviolet, optical, infrared and millimeter wave portions of the spectrum and the development of space-based interferometers to carry out a variety of astrophysically important measurements. Many of these longer term programs require substantial advances in optics, structures, and control technology.
The Advanced X-Ray Astrophysics Facility (AXAF) will be a national observatory designed for the observation of galactic and extragalactic X-ray sources. The observatory is currently planned to be launched by the Space Shuttle, maintainable in orbit and retrievable, if necessary, during its 10 to 15 years of operation. The heart of the AXAF is an X-ray telescope made up of six Wolter type I mirrors, with the diameter of the outermost mirror 1.2 m. The focal length will be 10 m, and the AXAF will allow for interchanging and (in-orbit) replacing of focal plane instruments. The optics are being designed to provide a spatial resolution of 0.5 arc second over a several arc-minute field and somewhat reduced angular resolution over the entire 1-degree field of view. The energy bandwidth will be 0.1 to 8 keV. These design goals place severe requirements on the materials, tolerances, construction, and alignment of the telescope. These will be discussed and compared to previous work in this area.
Large apertures improve the capability of astronomical telescopes in two ways: increased spatial resolution (linearly dependent on aperture size) and increased sensitivity (dependent on aperture size squared). For the purpose of a technology assessment, the Large-Aperture Telescope (LAT) for infrared and submillimeter astronomy is envisioned to be 10 to 30 m in diameter, operating in the 2-μm ≤ λ ≤ 1000-μm wavelength range. It would be carried to orbit with a single launch of the Space Transportation System and semi-automatically deployed as a free flyer with a nominal 10-yr mission duration. Periodic revisits at 2-yr intervals would allow servicing and instrument change. LAT must be placed above the Earth's atmosphere to avoid the absorption that occurs through much of the infrared and submillimeter, and to avoid turbulence which limits spatial resolution. Important technical considerations for LAT include: (1) telescope optical form, (2) primary mirror material, (3) figure control techniques, (4) deployment techniques, (5) pointing and stabilization, and (6) thermal control. This paper discusses the science objectives and rationale for LAT and describes different hardware techniques and concepts for its implementation.
A concept is presented for a Ritchey-Chretien optical-UV telescope to be deployed in orbit using a preassembled segmented primary mirror of 8 meter aperture, carried into orbit inside the modified interstage of the Space Shuttle External Tank. Shuttle revisits allow major assembly tasks to proceed such as the installation of the light shield, fine alignment of the system and attachment of scientific instrument modules to the instrument adapter. Prime technology requirements for the VLST are assessed in this paper.
The conceptual basis of an orbiting observatory designed for high angular resolution optical astronomy is presented. Operating above the atmosphere, an array with a 15 meter baseline could achieve a one-dimensional angular resolution of 0.005 arc sec in the visible. Broad-band detection from the UV to the near infrared should be possible. To obtain two-dimensional images at high resolution we expect to employ tomographic reconstruction techniques, followed by sidelobe removal if necessary. Active optical elements will be needed in order to maintain coherence of beams in the image plane. The instrument is an optical analog to the Very Large Array in radio astronomy and a space analog to the Multiple Mirror Telescope. Prototype designs consisting of four to six mirrors are discussed for the case of a single shuttle payload.
A class of space telescopes for astronomical observations with a resolution and collecting capability more than one order of magnitude better than what is expected from the 2.4 m Space Telescope is discussed. To this purpose aplanatic two-mirror systems of coplanar primary/secondary mirror arrangements with approximately 45° angles of incidence and an overall diameter of about 100 m have been designed and analyzed. The main advantages of these systems are their compactness and the associated minimization of the moment of inertia in two axes. Two opposing secondary arrangements, one forward-reflecting and the other backward-reflecting are analyzed and compared.
Some of the optical and mechanical aspects of a proposed laser gravitational wave antenna in space are discussed briefly. The proposed concept consists of a free-mass antenna with the test masses separated by 106 km. A laser heterodyne technique is used to measure the distance change between test masses resulting from gravitational wave interaction. The proposed scheme appears to offer the necessary sensitivity to detect gravitational radiation from binary stars predicted by General Relativity Theory.