The Multiple Mirror Telescope (MMT) serves a dual function. It is a very large astronomical telescope with state-of-the-art instrumentation which is now being used for 85% of the time for astronomical research. It fills an equally important function as a testbed for new concepts in making large aperature telescopes at a modest cost. The MMT outperforms other telescopes in a number of areas, thus making it the benchmark against which future astronomical facilities will be measured. This paper summarizes the performance of the MMT.

A brief review of current technical activities related to the 15-Meter NNTT is given along with a listing of tentative design goals specified by astronomers. A general comparison is made of filled versus unfilled aperture characteristics. These characteristics will influence the final NNTT concept selection to be made in 1983 between a Segmented Mirror-Telescope (SMT) and a Multiple-Mirror-Telescope (MMT).

The history and background of LDR is reviewed. The results of the June 1982 Asilomar Workshop are incorporated into the LDR science objectives and telescope concept. The areas where the LDR may have the greatest scientific impact are in the study of star formation and planetary systems in our own and nearby galaxies and in cosmological studies of the structure and evolution of the early universe. The observational requirements for these and other scientific studies give rise to a set of telescope functional requirements. These, in turn, are satisfied by an LDR configuration which is a Cassegrain design with a 20 m diameter, actively controlled, segmented, primary reflector, diffraction limited at a wave-length of 30-50 pm. Technical challenges in the LDR development include construction of high tolerance mirror segments, surface figure measurement, figure control, vibration control, pointing, cryogenics and coherent detectors. Project status and future plans for the LDR are discussed.

The conventional feedback of multidither outgoing-wave adaptive optical systems is inoperative in the presence of a featureless target resolved by the transmitting optics. This paper proposes to define an artificial glint by imaging the target laser pattern on a pinhole aperture. The concept is somewhat obvious but, in turbulence, the existing theory on resolution and coherence states that it is restricted to small scintillation strength. It is shown, through correlation measurements between the target irradiance and the artifificial-glint irradiance, that the range of validity is much broader than predicted. Operational tests with a two-degree-of-freedom system demonstrate that the artificial-glint technique allows effective tracking and tilt compensation at propagation distances reaching far into the scintillation-saturation region, in disagreement with recent models.

Experimental verification of the NEPSPP nonlinear finite-element mirror model and stable closed-loop correction of an optically aberrateel wavefront have been demonstrated. These results were obtained using a single 60-deg segment of a 45-cm diameter, 1-inch-thick Zero-dur spherical mirror (f/1.7) having a 25-percent central obscuration. Seventeen holographically produced Hartmann sampling gratings were used for the figure sensing in the closed loop experiments and 30 electrodynamic actuators for figure control. The entire system was under computer control. The mirror model predictions of the surface were verified by analysis of interferograms of the segment taken with a Point Diffraction Interferometer (PDI) and analyzed by the FRINGE program. Details of the experimental configuration and auantitative results are presented.

A theoretical treatment of the influence linear intracavity phase aberrations have upon the transverse mode structure of an unstable cavity and the ideal compensation of such aberrations by an intracavity adaptive optic element is provided. Numerical calculations of the aberration sensitivity and the corrective capabilities afforded by such an ideally configured adaptive optic element demonstrate the accuracy of this analysis. A numerical analysis of the system performance obtained in a recent adaptive laser experiment shows that the multidither approach does not yield the optimum correction. As a consequence, it is suggested that the best approach to applying adaptive optics to intracavity mode control is to calculate the precise mirror surface profile from measurements of the outcoupled phase structure weighted by the appropriate theoretical aberration sensitivity coefficient.

A deformable mirror in the compacted beam region of a HSURIA (half symmetric unstable resonator with internal axicon) is used to conduct intracavity adaptive optics experiments. The gain cell is a convection cooled CO₂ electric discharge located in the compacted beam region. The closed loop performance of the resonator is optimized using multidither hillclimbing servo techniques to control the figure of the discrete actuator continuous surface deformable mirror which is driven zonally. Residual aberrations in the resonator conical optics (the w-axicon) are shown to be the dominant factor governing the mode for the open loop configuration. The extent of optimization of the resonator upon closure of the adap-tive optics control loop is determined by both the spatial frequency of the aberration with respect to the correction capability of the deformable mirror and the ability of the control to converge to the global rather than a local performance maximum. In addition to simple phase correction, mode modification by the intracavity deformable mirror must be considered as a probable response of the system especially in the presence of intracavity aberrations which are beyond the spatial frequency correction capability of the deformable mirror.

Intracavity adaptive optics (ICAO) studies were performed using a deformable mirror in the annular beam region of an HSURIA (half symmetric unstable resonator with internal axicon). Figuring of the deformable mirror to remove the residual aberrations of the conical optics results in good resonator mode quality, however, convergence of the multidither control to this optimum configuration occurs only for small perturbations of the resonator from its optimum operating condition. For example, w-axicon tilt aberration correction studies show control convergence to the global maximum only up to 5 Irad tilt, but the deformable mirror is shown to have the capability to correct for at least 30 Arad w-axicon tilt. Control convergence to a local maxima will occur whenever a barrier such as a resonator mode crossing point exists between the initial state and the global maximum of the resonator. The limited range of global maximum convergence by the hill-climbing servo controller is directly tied to the experimentally verified extreme perturbation sensitivity of the HSURIA.

The problem of using an adaptive optics system to correct for nonlinear effects like thermal blooming is addressed using a model containing nonlinear lenses through which Gaussian beams are propagated. The best correction of this nonlinear system can be formulated as a deterministic open loop optimal control problem. This treatment gives us a limit for the best possible correction. Aspects of adaptive control and servo systems are not included at this stage. We ask for that control in the transmitter plane which minimizes the time averaged area or maximizes the fluence in the target plane. The standard minimization procedure leads to a two-point-boundary-value problem, which is ill-conditioned in our case. We were able to solve the optimal control problem using an iterative gradient technique. An instantaneous correction is introduced and compared with the optimal correction. The results of our calculations show that for short times or weak nonlinearities the instantaneous correction is close to the optimal correction, but that for long times and strong nonlinearities a large difference develops between the two types of correction. For these cases the steady state correction becomes better than the instantaneous correction and approaches the optimum correction.

A system performance model is developed to predict the performance of a high-energy laser system taking into account the propagation through the beam control system and the atmosphere. The performance model essentially shows how the various subsystems, such as the laser device, intermediate transfer system, beam expander, target, etc., will interact with each other and how a closed-loop model for the atmospheric correction can be achieved. Such a model aids in the system design by identifying and optimizing the adaptive optic requirements for the laser beam cleanup and for the atmospheric compensation correction. The heart of such a model is the Perkin-Elmer Physical Optics Propagation (POP) code. The POP code has been successfully used in many high-energy laser programs. The general capabilities of the POP code and the performance model will be highlighted. Results from a test case using a chemical laser wavefront will also be presented.

Modal wave front estimation is reviewed. A set of functions are derived which are orthogonal to the gradients of the modal functions, such as the Zernike polynomials. These functions can be used to project out any mode without cross coupling. Cross coupling is due to undermodeling and not due to a lack of orthogonality of the gradients. Cross coupling can be beneficial in that it provides a means of obtaining the best fit to a wave front with a given number of modes. Best fit estimation is distinguished from modal projection estimation.

Intercomparison of C_n² measurements using a stellar scintillometer, UHF radar and ballon-borne microthermal probes(thermosonde) demonstrate near identical results. The radar provides data at improved altitude resolution (≈1 km) with a long time base. The thermosonde provides even higher altitude resolution (≈ 20 m) but over a short time base The results of the intercomparison measurements conducted in 1981 at Westford, MA are presented. A comparison of the coherence length and isoplanatic angle are shown using the different altitude resolution data.

An edge actuated mirror figure control experiment is described. Manual operation for evaluation of mirror controllability and local actuator attachment effects has demonstrated the edge actuation concept potential. Preliminary results of closed-loop computer-controlled experiments are presented.

Segmented mirrors with four and six segments over the full aperture are discussed with regard to the figure correction capability for both moment-and piston-type actuators. The number of actuators, ranging from 48 to 480 DOF over the full aperture, is analyzed for residual error for specific Zernike polynomial input errors. Tradeoff study results show that a very thin mirror will reduce required actuator forces, but if the mirror becomes a membrane and loses its bending stiffness, the figure correction capability degrades rapidly. In general, moment and piston actuators each have regions where one is superior to the other, but no generalization can be made for the cases presented in this paper. The results for a 65-cm-diameter mirror show that 50 to 100 actuators will provide very good wavefront correction capability for a rather broad spectrum of Zernike polynomial input errors.

A focusing sampling grating can be synthesized by combining a linear grating with a computer plotted zone plate. Phase front distortion figures below λ/20 can be achieved for sampler geometries calling for any diffraction angle and f-numbers down to one. The technique can be generalized for curved substrates.

A deformable annular flat mirror was developed for performing adaptive corrections of low temporal frequency aberrations in cylindrical lasers. Construction of the mirror is described, including the aluminum. faceplate, 4ifferential ballscrew actuators, and servomotor drivers. The annular deformable mirror aperture, having two free edges, presents unique problems for optical finishing due to edge roll-off as well as actuator print-through. These problems were successfully overcome by diamond turning the deformable mirror rather than employing conventional grinding and polishing techniques. It is believed that this is the first successful demonstration of diamond. turning a deformable mirror structure to optical tolerances.

The Fox and Li iterative method is used to find solutions for a three-dimensional phase conjugate resonator with a saturated gain, a random aberrator and a pinhole spatial filter. The performance of such a resonator is excellent for resonator Fresnel numbers as small as five and for phase error standard deviations as large as 2π. The use of a spatial filter to control the transverse mode shape is demonstrated.

It is shown that Brillouin enhanced 4 wave mixing can produce high reflection coefficients under certain conditions, and that this may be maximised by the presence of a non zero phase mismatch. However Rayleigh-Wing induced 4 wave mixing (which is associated with the optical Kerr effect) shows no such enhancement, and any phase mismatch only causes a further decrease in the reflectivity.