<title>Optomechanical design of the Gemini multiobject spectrograph on-instrument wavefront sensor</title>

Abstract

The Gemini' Multi-Object Spectrographs (GMOS)' will exploit the excellent delivered image quality of the Gemini8-metre telescopes. An integral part of the GMOS optical design is the On-Instrument Wavefront Sensor (O1WFS). TheOIWFS measures image tip-tilt and focus (1TF) information and feeds this back to the telescope's active and adaptiveoptics systems. Since GMOS is Cassegrain mounted, the OIWFS was designed to be insensitive to the varying gravity vectoras the telescope tracks. The OIWFS utilizes a two-rotational-axis robotic platform to position the pickoff optics within thetelescope field. The mechanical design, optics, kinematics and an analysis of the optical measurement error due tomechanical flexure of the OIWFS are presented.Keywords: wavefront sensor, spectrograph, optics, kinematics, flexure