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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.

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The upper part of the European Extremely Large Telescope (E-ELT) altitude structure is one of the most critical areas of the telescope's structure. This part hosts sensitive optical elements of the telescope. Its structural performance has a major impact on the whole system. The most critical requirements are low optical path obscuration, high static and dynamic performance (high specific modulus), high mechanical safety (high specific strength), low wind cross section and low weight.Composite materials are ideally suited to meet these requirements. This study is carried out in order to quantify the relative advantage of composite material over mild steel, in terms of performance and costs. The mechanical behavior of the steel structure can be easily improved with a structure manufactured with composite materials. This structure is significantly lighter than the steel one and reduces relative displacements between primary and secondary mirror. Consequently, optical performance is improved, assembly process is simplified and transport cost is reduced.The support structure studied takes part of the E-ELT Altitude Structure, and works as the interface between the substructure supporting the primary mirror and the sub-structure supporting the secondary one. It is around 30m height, and a high effective stiffness is desired to minimize the relative movements between the mirrors. To achieve that, trusses legs internally braced with structural ropes are used. Figure 1 shows the configuration selected for this structure. This configuration has been selected after a deep optimization study based on stiffness criterion: