<title>Status of the design and fabrication of the GTC mirrors</title>

Abstract

The Gran Telescopio Canarias (GTC) is a lOm-class telescope which is under construction and will be operational at the Observatorio del Roque de Los Muchachos at the end of 2003. The goal of this paper is to describe the current status of the design and construction of the primary, secondary and tertiary mirrors of the GTC and their opto-mechanical supports. It also summarizes the optical performances expected from the GTC and the error budget of the optical system.The primary mirror is segmented into 36 hexagonal segments. Each segment can be actively deformed to correct figure distortion. In addition, an active stabilization ioop permits the relative position between the segments to be maintained so that they will function as a single optical surface. The GTC will work with a single secondary mirror, which will be used for both visible and infrared observations. Only the system of baffles will be changed between observations in he visible and infrared. The secondary mirror will be a lightweight glass mirror moved by a driving unit with the capability of active alignment and fast tip-tilt correction; the latter function will be used for chopping and image motion correction. The tertiary mirror will permit the light beam to be directed to the Nasmyth and folded-Cassegrain focal stations. A trade-off of several alternatives that we have evaluated will be presented as well the current concept for the mirror and the support.

Optical design for an adaptive anastigmatic five-mirror extremely large telescope

“…As preliminary contacts with optical manufacturers have resulted in positive feedback on the feasibility of an aspheric M1 at an affordable cost (Castro et al 2000;Nelson & Mast 1990), it is not obvious that a telescope with a spherical primary mirror will be cheaper -it may turn out to be more expensive. In addition, most of the desirable features mentioned above would not be available.…”

Reduction of the diffraction pattern in segmented apertures

“…Although our analysis can be easily generalized to any geometry, we assume a primary mirror design consisting of a close-packed hexagonal array. We have chosen this geometry because it is in use in present 10-m telescopes, [10][11][12] and it is the preferred geometry in most extremely large telescope projects. 13 In this Keck-type hexagonal segmentation geometry, the number of segments is N =3M͑M +1͒, where M is the number of rings and the central segment has not been included.…”