Optical Design Of An All-Reflecting, High Resolution Camera For Active-Optics On Ground-Based Telescopes

"Seeing" effects of the Earth's atmosphere cause motion and blur of stellar images produced by ground-based telescopes. In order to sharpen the images over long exposures it is necessary to add a camera which produces an image of the pupil, i.e. of the primary mirror, followed by the reimaged field of stars. A vital feature of such a camera is the provision for analyzing the imperfections of the image of a guide star at the center of the field. Any movement of the guide star in excess of a small threshold is sensed and corrected by feedback to an active mirror located at the pupil. Small, rapid motions of the active mirror sharpen both the guide star and the nearby images that lie within the "aplanatic patch" where seeing effects are correlated with those of the guide star. The aplanatic patch covers a field of about 2.5 arcmin at most. Usually, some magnification of the images is required to match the pixel size of the detector. Optical designs are given of cameras optimized for use at the secondary foci of two different telescopes: the 3.6 meter Canada-France-Hawaii Telescope (CFHT), which has classical optics (paraboloidal primary); and the 2.2 metre University of Hawaii telescope, which has Ritchey-Chretien optics (hyperboloidal primary). Absorption in the infrared is avoided by using only mirrors in the cameras.