A possible design and fabrication approach for the optics of the 25-m telescope

The optical prescription of the primary mirror of an astronomical telescope is an essential design driver, with major implications on cost and complexity. The sheer size of an alt-az 25-m class telescope emphasizes the need for a short focal ratio of the primary mirror. The optical solution for the 25-m telescope is that of a 4-mirror axial design. Cost and fabrication constraints require the segmented primary mirror to be spherical; on similar grounds, we conclude that the same statement most likely applies to the secondary mirror as well. This makes the correction of spherical aberration and field aberrations comparatively more complex than with a 2-mirror Ritchey-Chretien design, and the very high asphericity of the fourth mirror will constrain the minimal achievable focal ratio of the primary mirror. A parametric study will be presented, showing the available field of view and the difficulty of fabrication of the optics of the telescope as a function of the f/ratio of the primary mirror. The peculiar properties of the design will be reviewed, and it will be shown that the f/ratio of the primary mirror shall most likely fall in the f/1.20 to 1.40 range, as a result of fabrication, testing and alignment constraints. Novel optical testing methods will be proposed as well.