tokens: This is a great advantage of such a Nasmyth system. Any optical designer of telescopes can observe that in this Nasmyth system the exit pupil is near the plane fold mirror, and that if some small adjustments are made to this system, an AO deformable mirror can be added. After this AO deformable mirror, a plane fold mirror can still be added to reflect light to the Nasmyth focus. In such an arrangement the length of path from the exit pupil to the pupil formed by the secondary mirror and that from the exit pupil to the Nasmyth focus should be equal. These two plane mirrors can be used, respectively, as AO deformable mirror and tip-tilt mirror for all instruments at Nasmyth platform and coudé focus. This is the second great advantage of this Nasmyth system. Such a system has been applied in the Nasmyth system of 39 m European Extremely Large Telescope (Delabre 2008 ; ESO 2011 ). However, if an AO deformable mirror is added in such a Nasmyth system, there will be some shortcomings as well. The size of the AO deformable mirror is bigger than those AO deformable mirrors set up separately in Nasmyth instruments and coudé instruments. It is difficult for a big AO deformation mirror to perform the high temporal frequency movement. Besides, an AO mirror is unnecessary for some Nasmyth instruments and coudé instruments. As it is located in the tube of the telescope, this AO mirror will produce some harmful thermal effects, especially since this mirror needs more power due to its large size. Besides, servicing also possess some difficulty. Given these circumstances, at present we only intend to use one plane fold mirror, which will serve as a tip-tilt mirror in our Nasmyth system. Nevertheless we insist that the addition of an AO deformable mirror in this Nasmyth system is an important advantage and we will continue to consider whether or not to make such an addition during future research and design of Nasmyth and coudé instruments. Excellent image quality has been obtained in our Nasmyth system. In the next section one can witness the excellent image quality also in our coudé system design. We are convinced that when we make some small readjustments for adding an AO deformable mirror, the excellent image quality can still be obtained in Nasmyth and coudé systems. In Nasmyth platforms many instruments will be installed. In order to meet this challenge, we propose (Su, Wang Cui 2004b ) the use of four Nasmyth platforms in this telescope, i.e. two Nasmyth platforms I on the upper level and two Nasmyth platforms II on the lower level. In such an option, the two light bundles to Nasmyth platform II come perpendicularly up (or down) towards this platform; for some instruments that is an advantage. Since the lengths of paths from secondary mirror to Nasmyth I foci and to Nasmyth II foci may be different, M2 and M3 will be moved to make the image arrive at the required foci. In these situations fairly large off-axis aberration may occur and FOV will be
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