Progress in diffraction-limited imaging at the Multiple Mirror Telescope with adaptive optics

Low spatial frequencies of atmospheric turbulence are especially troublesome to astronomers because the phase distortions that these frequencies cause have a large amplitude. We have begun experiments at the Multiple Mirror Telescope to remove these errors with tip, tilt, and piston control of pieces of the wave front that are defined by the telescope’s six 1.8-m primary mirrors. We present long-exposure images that were recorded at the telescope with a resolution of as high as 0.08 arcsec under piston control, and 0.32 arcsec under tilt control, by use of an adaptive instrument designed to restore diffraction-limited imaging in the near infrared. Of particular importance for high-resolution imaging is the control of the piston or the mean phase errors between the segments. These errors can be calculated 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 a theoretical analysis of the effects of photon and detector read noise on the derived piston errors and computer simulations of wave-front reconstructor algorithms. We find that a Wiener filter combined with nonlinear weighting of the piston errors minimizes the impact of noise. Finally, we summarize expected improvements to our system and discuss the application of these techniques to forthcoming large telescopes.