Technical description:
To accommodate the changes in focal length of the telescope with temperature, and the focus position changes induced by the various filters, the camera must be able to move along the optical axis of the telescope. The focus stage assembly (FSA) accommodates this motion, supporting the camera and its shutter on a motorized stage bolted on top of the upper end platform. In order to follow the apparent motion of the sky due to the Earth's rotation, two small cameras fix on stars outside of the field of view, providing automatic guidance of the telescope and measurements of the focal changes. 
The FSA is seen here being lifted to the top of the base of the MegaPrime upper end at the time of the MegaPrime integration. The top of the wide field corrector will go through its base, and the camera with its shutter will be fixed on top of the structure. 

Credit line: "Canada-France-Hawaii Telescope / 2003"

Copyright © 2003 Canada-France-Hawaii Telescope Corporation