20+inch+F3.8+Figuring

Figuring progress for the 20 inch F3.8 mirror.

The relative sucsess for figuring this mirror so far is attributed to the good 'near' spherical surface at the end of fine grinding. The mirror was polished in only 5 hours!! The fine grinding was done with a glass tile tool similar to as used on the remake of the 30 inch TRO. The final grade was BM304.

Soon after polishing started, the edge had some minor correction with 70% of the middle near spherical. This figure did not change during the rest of the polishing phase wich concentrated most of its time around the 50 to70% zones.

Figuring has been done with a 6 inch lap initially that was turned into a 6 point star lap an hour into the figuring process. Only one stroke has been used so far for all of the polishing (except for 30 mins concentrating on the middle 50% of the mirror. The stroke used has been edge to edge with 1.5 inch overhang firm pressure and 1 to 2 inches off centre.

02/07/2011 shows the mirror to be 2.4 waves PV on the wavefront

03/07/2011 shows the mirror to be 1.94 waves PV on the wavefront

05/07/2011 first session test shows the mirror to be 1.42 waves PV on the wavefront

05/07/2011 second session test shows the mirror to be 1.1 waves PV on the wavefront

The mirror performed OK at low power but I was not happy with it's performance at higher power. Our developments in optical testing using the Bath Interferometer allowed us to complete the mirror to a higher specification at 18nm RMS on the surface. The image below is early in the later stages of figuring. Projecting the error map down onto the glass makes life easy for error correction.

The image below shows that the Wavefront error was 1/15.6 RMS. This for OpenFringe is (0.064 x 550nm/2) = 17.6nm RMS on the surface. Note that OpenFringe uses 550nm as the reference, no matter what frequency you laser is. Individual Wavefront files for a number of rotational positions of the mirror in the tube were required to remove support system astigmatism and flexure as the mirror was only tilted back 20 degrees. These files where then re-rotated and averaged. The image below has the central obstruction removed. This flexure of the optic in the mirror support system is not really noticeable visually at the eyepiece on sky at low altitudes. Thicker glass is of course not as flexible! This is 25mm plate glass. Never again will I make something out of plate this thin again. The profile shows a wavefront PV of less than 0.25 waves, nominally 0.2 PV. The surface rms is 17.6nm.

Configuration of OpenFringe for the mirror.