The importance of thermal design and thermal analysis of primary mirror is stated in this paper. The deformation of the primary mirror in temperature field and its effect no the image quality of the optical system is analized. The deformation of surface figure of a primary mirror in radial static temperature field

K9 and fused silica responsively

is calculated and analized in detail. It is found that as though the structure is not different

the deflection of the surface figure of the mirror is not only connected with alpha and other thermal and structural paremeter

but also directly connected with the magnitude and direction of the temperature gradient.