<title>First tests of the Cassegrain adaptive optics system of the Mount Wilson 100-in telescope</title>

Abstract

In January 1994, we began construction of a modern adaptive optics system for the newly refurbished 100-inch telescope. The design philosophy of the adaptive optics system is to achieve a working system in the visible in a short time at relatively low cost. This means wavefront sensing with natural guide stars and implementation at the bent Cassegrain focus of the telescope. The system has an integrated wavefront sensor and finder camera, and is automated for one-person 1 It uses off-the-shelf components where possible. The deformable mirror, which has 241 actuators, is on loan from the Air Force. The use of an existing mirror imposes constraints that have driven some of the design considerations. The system is operating at the telescope, with early results described below. DESIGNThe primary shortcoming of the Atmospheric Compensation Experiment (ACE) system used on the 60-inch telescope at Mount Wilson Observatory was the limitation to bright (B<6 mag) guide objects.2 As a result of experience with ACE, we sought to design an AO system with good sensitivity for the 100-inch telescope. The improvements were implemented in the following four areas:1. Operation at the Cassegrainfocus avoids the long paths and extra reflections arising from operation at Coude. 2. A beanz splitter optimized to the specific research objective feeds light to the wavefront sensor. The two modes of beamsplitting each use a field stop with selectable diameter for the wavefront sensor. Such a field stop is necessary for astronomical adaptive optics with faint natural guide stars, and should allow compensation on a bright star during daytime.3 The two beam-splitting modes are:Object-plane splitting (coronagraphic mode), wherein the guide star is not of interest or is to be attenuated. The splitter is a plane mirror with a calibrated hole in the mirror coating. Placed at the object focal plane, this splitter acts as a field stop for the wavefront sensor and as a means of sending light to a science camera. Little light is lost for either science or wavefront sensing in this mode.Pupil-plane splitting (normal mode), wherein the guide object is of scientific interest, so a neutral or dichroic beamsplitter is used in a collimated beam. When pupil-plane splitting is employed, the objectplane splitter is not removed, but acts as a field stop for the wavefront sensor, and sends light to a finder or wide-field camera.3. Modal correction offers flexible control of the degree of correction by matching to the seeing conditions and the brightness of the guide object. Bright objects are corrected to relatively high order, and faint objects only to low order. The adaptive optics system remains useful in the case of still fainter or extended objects because it can correct static aberrations. In the initial system configuration, a relatively simple zonal wavefront reconstruction is used, with precise 72 ISPIE Vol. 2534 O-8194-1893-5/95/$6.OO Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05/14/2015 Terms of Use: http://spiedl.org/terms

Astrometric and photometric measurements of binary stars with adaptive optics: observations from 2001 to 2006

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