It's about time to get serious with this silicon telescope mirror project. Having ground this out to f/8, I cut up a urethane pad used for high speed high pressure silicon polishing and stuck the pieces on my tool in a pie shaped pattern. Now, I know that both pads and machine polishing do not lead to a precision polished parabola, but for polishing out the grey, its the way to go. At 60 psi on my dc drive Strausbaugh polisher, it polished out in approx. 1 hr. As we know, pads do not flow, so ended up with heavy zoning (rings). Re covered tool with pad pieces in small squares (3/4") approximating a pitch lap pattern. Much better, still slight zoning. Was using an over center 1/4 stroke. Pulled pin out approx. .5" and increased stroke to 1/3. Even better. At this point all pitting is gone, with a slightly zoned, but overall fairly spherical surface, TIME FOR PITCH!!
So we're first trying Tropical Pitch (much thanks to Bratislav) made with rosin, woodflour, and castor oil. Well, first attempt at pitch lap too soft, will cut back on castor oil and try again tomorrow. Another possibility is Cycad Pitch (Cycad Products, Las Vegas, New Mexico) which is supposed to be more resistant to the alkalinity (high pH) of silicon polishing slurries./
Which brings me to the question for the nite, for all of you out there still hanging on.....
Ratio of Diameters: Mirror/Tool
In the world of plano optics(flat work), it has been established that when controlling test (flatness) that the ratio of tool to work sizes is very important,basically at a 1/1.25 ratio.
In cap polishing (tool on top, work on bottom) this means say a 8" tool on top of a 10"block (work). Or, a 10" tool on a 12.5" block.
In block polishing (tool on bottom, work on top) this means a 8" block(work) running on a 10" tool, or a 10" block on a 12.5: tool.
With these arrangements, small adjustments in stroke, pin location, speed of eccentric, etc, will produce slight changes in the never ending progression between concave and convex.. Nothing is ever static...always changing... the trick is to catch it just right.
With these techniques, we can produce 1/10 wave optics on parts .200 to 4" dia. On a 10" or 12.5" block however, using machines, it is very difficult to produce test better than 1/4 wave. The ideas of people out there pushing glass in their garage to 1/8th or even 1/10 wave over 10" or larger surfaces is amazing, as that is the trick of the master optician.
I have not seen much info on sub diameter tools, except that with larger mirrors the full sized tools are hard to push around. Does anyone out there have any comments, or references on the use of sub dia tools in controlling spherical test.(Obviously it impractical to consider sub dia mirrors as it would be impossible to keep a larger pitch tool pressed out, which is too bad as block polishing controls test better than cap polishing, at least with plano surfaces)
Anyway, hopefully this time tomorrow I'll know how well this tropical pitch stuff works (or doesn't)
Thanks for your patience.
Bill Marriott Forest Knolls Ca. btk@ix.netcom.com