The UH wavefront curvature sensor.

Abstract

Wavefront sensors currently used in adaptive optics operate only on relatively bright sources. One of the most sensitive sensor ever built is the Hartmann sensor of the European COME-ON adaptive optics system developed for infra-red astronomy [1]. It is equipped with an electron-bombarded CCD and can stabilize images on a 3.6 m telescope with a mag 13 star. With some improvement in the optical transmission, the sensor should be able to operate up to a mag. 15 star. Because only a few astronomical objects are bright enough or are close enough to a bright star to enable the sensor to work, it is generally thought that artificial laser guide stars are necessary to benefit from the full potential of adaptive optics in astronomy. However the aforementioned sensors are far from the theoretical sensitivity limit An image stabilizer developed for the CFH telescope (HR Cam [2]) was recently equipped with four photon-counting avalanche photodiodes in a quadrant detector, and image stabilization was demonstrated on a mag 19.5 star. Such a sensitivity allows images to be sharpened over the whole sky. The UH wavefront sensor was developed to reach HR Cam performance on tip/tilt compensation together with higher order compensation using brighter natural reference sources. It is estimated that under average Mauna Kea seeing conditions a Strehl ratio improvement by a factor 10 can be obtained over 30% of the sky at 1.6μm with a 3.6 m telescope.