Progress in Diffraction-Limited Imaging at the Multiple Mirror Telescope with Adaptive Optics

Low spatial frequencies of atmospheric turbulence are specially troublesome to astronomers because the phase distortions they cause have large amplitude. We have begun experiments at the Multiple Mirror Telescope (MMT) to remove these errors with tip, tilt, and piston control of pieces of the wave front defined by the telescope’s six 1.8 m primary mirrors. We show long exposure images taken at the telescope with resolution as high as 0.08 arcsec under piston control, and 0.32 arcsec under tilt control, using an adaptive instrument designed to restore diffraction-limited imaging in the near infrared. Of particular importance for high resolution imaging is the control of piston, or mean phase errors between the segments. These errors can be found from the Fourier transform of the short exposure combined-focus image, but the accuracy of the reconstructed wave front depends critically on the signal-to-noise ratio of the data. We present theoretical analysis of the effects of photon and detector read noise on the derived piston errors, and computer simulations of wave front reconstructor algorithms. A Wiener filter combined with non-linear weighting of the piston errors is found to minimize the impact of noise. Finally, we summarize expected improvements to our system, and discuss the application of these techniques to forthcoming large telescopes.