20" F3.5 Telescope Mirror

The fabrication of this 20" F3.5 telescope mirror was a little unusual.  In almost all cases, I purchase mirror blanks that are "pre-generated" at the glass fab.  That means that they grind the rough curve into the mirror's face before I receive the glass.  This is economical, as they have large, fast diamond cutters that can quickly perform the work.

But in this case, I started work with a flat blank.  I have my own curve-generating equipment, but it doesn't produce as accurate and as clean a gen as is achieved by the glass fab.  In addition, after doing my own curve generation, I have to rough grind with loose abrasives, from #80 through #120, #220 and #320 silicon carbide grits.  Two of these images show the scale of the surface structure on the glass just after curve generation.

After rough grinding, I switch to Microgrit aluminum oxide abrasives for fine grinding.  The usual sequence I use is 25-micron, 12-micron and 9-micron.  Next comes rough polishing using the fixed-post polishing machine and a pitch polisher made from 64/55 mixed Gugolz pitch.  After the figuring process, a multi-pass, multi-diameter Foucault test is performed, and I use the FigureXP software package to analyze that data.

FigureXP is always optimistic in its analysis of premium mirrors, but this mirror has an exceptional final surface.  FigXP rates it at a little better than 1/25th-wave P-V on the wavefront.  The Strehl ratio is calculated as 0.996, and the Surface RMS error is 2.6 nm.  As usual, I'm not claiming these numbers are precise, as another test would certainly yield a slightly different result.  The report shows a slightly high center, about 10 nm above perfect.  There is also a comparable slight high area at roughly the 75% zone.

The results of the Millies-LaCroix analysis show the smooth quality of the figure.  All test points easily fall within the ML tolerance bands, and actually make quite a tight grouping around the center line.  In a perfect test, all points would actually lie on the center line, so the image of that test at right demonstrates the precision of this effort.  In general, the faster the mirror, the more difficult it is to score a great ML test.  At this fast F3.5 ratio, this surface quality should give nearly perfect star tests in the field, assuming proper collimation, a good secondary mirror, and excellent observing conditions.