HIGH-RESOLUTION IMAGING WITH A TIP-TILT CASSEGRAIN SECONDARY

A prototype adaptive secondary mirror has been developed on a 2.3-m telescope to stabilize rapid image motion caused by atmospheric turbulence. The instantaneous position of an image's centroid, or brightest speckle, is measured by a near-infrared camera system which enables the tip-tilt mirror to recenter the image. Correction is accomplished at closed-loop frequencies of less than or equal to 100 Hz on guide stars with K (2.2 mu m) magnitudes less than or equal to 8. Image motion is reduced to <0.1'' rms, removing over 90% of low-frequency (f less than or equal to 5 Hz) tilt power, leaving less than or equal to 1 rad(2) mean square wave front tilt phase error. In seeing conditions where D/r(0) similar to 4, long exposures in the H band (1.6 mu m) exhibit a nearly diffraction-limited resolution of 0.19'' FWHM (Strehl ratio similar to 0.14)-a factor of 4 improvement over uncorrected images. Reduction of the present static optical errors will improve the Strehl ratio another factor of 2, leading to similar to 70% of the maximum possible Strehl ratio.