This shows the main mirror mounted on the mirror cell. The mirror itself is conical and was ground and figured by Mark Suchting and is of very high quality (strehl ratio ~ 99%). The mirror is mounted via a threaded rod that is screwed into a nut epoxied in a hole in the back of the mirror. This rod goes through a hole in the mirror cell and is then fastened by some wave spring washers and nut, which applies the necessary tension while allowing for thermal expansion / contraction of the mirror & cell. Adjustment for the mirror cell is provided by push/pull screws near the outer edge which provide fine control as well as excellent rigidity.

Note also the 5 stainless threaded inserts in the side of the mirror cell backplate; this allows flexibility for positioning the mirror in the tube to cater for any variations in the focus position. Two of these inserts can be seen on the table to the right of the mirror.