High-resolution imaging with a tip-tilt Cassegrain secondary.
CLOSE L.M. and McCARTHY D.W.
Abstract (from CDS):
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 </= 100 Hz on guide stars with K (2.2 microns) magnitudes </= 8. Image motion is reduced to < 0.1" rms, removing over 90% of low frequency (f</= 5 Hz) tilt powr, leaving </= 1 rad^2 mean square wavefront tilt phase error. In seeing conditions where D/r_0∼4, long exposures in the H band (1.6 microns) exhibit a nearly diffraction-limited resolution of 0.19" FWHM (Strehl ratio ∼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 ∼70% of the maximum possible Strehl ratio.