Segmented Adaptive Optic Mirrors have been developed, fabricated, and demonstrated in real time atmospheric compensation systems. Until recently, most Segmented Adaptive Optic Mirrors have been designed for single wavelength applications and have not required more than 1.5 (mu) of surface motion since absolute phasing of the surface is not required for very narrow bandwidth compensation. Requirements for astronomical and imaging systems have required the design and fabrication of long stroke (6 - 10 (mu) ) segmented mirrors capable of absolute phasing of the segments, optical response from 0.4 to 3.5 (mu) and bandwidths above 2.5 KHz.
The MMT consists of six comounted 1.8 m telescopes from which the light is brought to a combined coherent focus. Atmospheric turbulence spoils the MMT diffraction-limited beam profile, which would otherwise have a central peak of 0.06 arcsec FWHM, at 2 microns wavelength. At this wavelength, the adaptive correction of the tilt and path difference of each telescope beam is sufficient to recover diffraction-limited angular resolution. Computer simulations have shown that these tilts and pistons can be derived by an artificial neural network, given only a simultaneous pair of in-focus and out-of-focus images of a reference star formed at the combined focus of all the array elements. We describe such an adaptive optics system for the MMT, as well as some successful tests of neural network wavefront sensing on images, and initial real-time tests of the adaptive system at the telescope; attention is given to a demonstration of the adaptive stabilization of the mean phase errors between two mirrors which resulted in stable fringes with 0.1 arcsec resolution.
An optical wavefront propagating through the atmosphere will be perturbed by local variations in the refractive index ofthe atmospheric gases. When accumulated over long optical path distances they will impart a spatial and temporally random distortion to the wavefront. These distortions
have a characteristic spatial coherence length r0 and an atmospheric decorrelation time T0. In directed energy applications, atmospheric distortions can reduce the peak target energy densities of larger diameter laser beams by orders of magnitude. The problem is not solved through the use of larger apertures; once the aperture size increases beyond one
or two r0, the far-field spot remains constant in size. Hence, for large aperture systems, the overall performance is set by the spatial coherence of the atmosphere and not by the system's exit pupil.
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