Re: State of the art?

Bill Thomas <bthomas32000@...>

for polishing i use a lap slightly larger than 70% with a max overhang of 25%,? table at high speed (just short of spraying fine mist) and slow eccentric? -? which strange as it may seem does not produce a TDE - some times is an antidote for a TDE.?

then beginning figure (slow table 4 RPM, faster eccentric) Zambuto's smoothing (Zambuto's log - figuring a 12.5",5_Eng.pdf? - NOTE: M-o-M are not all the same) with that 70% instead of a sphere the outer zones are nearly a parabola with a hump in the center.? then the problem is dealing with the center hill with a 45% lap without having the next to outer zone going below to hyperbola hell - the antidote for hyperbola is high table speed, 50% lap working the 70% zone for a short period - if too long and then nearly a sphere the result is a big TDE.

my mental picture of the figuring is a glide slope as with landing a plane and not crashing.? but actually figuring is what i enjoy the rest of the process grinding polishing is just what i have to do to get to the fun part - figuring.

but with the M-o-M type the blank should have little run out i.e., it should be edged.? i try for having the optical center over the table shaft center less than 0.005" (M-o-M manual states 0.032" - seems large to me)? to avoid astigmatism.? i mark the edge at three 120 degrees points.? then take the average dial readings at those points and then nudge the mirror to get each point to the average.? then set the 4 cleats against the edge of the mirror with a 0.002" feeler gauge in between - with outdoor carpet under, a circular blank will auto rotate - but if not circular, the mirror will hangup on a high spot - and that is a BIG PROBLEM.

my understanding from a well respected mirror maker, his Zeiss does not need an edged blank - as is by hand over a barrel.
the SIT can't detect astigmatism - let alone fix it - do get a hint all is not well when getting different results at different mirror orientations - but its just a hint!? then its back to fine grinding and a lot of hope.

thankfully i have not had to deal with astigmatism - others using the SIT and doing what was suppose to be just refiguring (of large mirrors) turn out to be astigmatic and then its back to 80 grit!?

so being able to fix an astigmatic mirror without having to going back to grinding is a BIG DEAL!

the book, "A Manual for Amateur Telescope Makers",? by Karine and Jean-Marc Lecleire describes a "wire" apparatus for detecting and correcting astigmatism - the key to the apparatus is the wire can be rotated without ANY lateral displacement to some small portion of a wavelength of light - in that an astigmatic mirror has multiple focal points which are nearly identical to a few wavelengths of light.

that is why i think Shark Hartmann is significant.

and for the professionals there is the $10K Shack Hartmann? fits into the focuser - same concept as with medically measuring eyes

On 11/16/2019 4:33 AM, Rien wrote:
What kind of lap diameter do you use for polishing? Same as mirror?, or undersized but fixed, or variable sized depending on the correction you need to make?

Thanks for mentioning the pivot point position, it makes sense to choose that as low as possible. (This will also place a minimum on the lap size, i.e. of the kind: the minimum lap diameter should be N times the height of the pivot point above the mirror))

On 16 Nov 2019, at 07:19, Bill Thomas <bthomas32000@...> wrote:

from what i have experienced, polishing requires the greatest amount force - my 24" machine
uses 1/2 hp and 1/4 hp - two stock M-o-M systems - using ball bearings - more than enough power.? i should replace the M-o-M system with 2 monster variable speed control motors which a friend gave me - would eliminate the belt hassle changing.? the other change i need to make is moving the arm pivot back further from the table axis so as to increase the radius of? the "quill" arc -? also on the website pictures the "arm" is counter balanced like the old high end LP record player - so as to control the downward force on the lap - which i have found necessary to stop chattering of smaller laps (noticed this in a picture of a Zambuto machine).

(need to send along a picture - worth a thousand of the following words) also to that end found i needed to get the pivot (toggle pad) attachment as low as possible to the back of the tool/lap - use a 5/8" short threaded stud into the back of the aluminum plate of the tool/lap.? the toggle pad 5/8 NC female is then threaded on that stud.? the 1" top of the toggle pad then fits into a machined adapter which on the other end is then threaded on to the 5/8" NC arms quill.

attached is a "strokeSetup.pdf" which i use to setup my modified M-o-M and record the settings.? also attached is a spreadsheet i use with my homemade sphereometer - the radius of the 3 legs from the center of the dial indicator was done with a bar of approximate 3" radius - then the actual radius was determined by measuring a mirror of known focal length (using excel solver).? it is vary accurate.

but i don't think a M-o-M type machine will be what you need for a closed loop system - would be more like a Zeiss machine?? also think you will need to know the exact RPM of the table and arm to avoid repatriation of strokes? - one crude method is to mark on the edge of the mirror where the tool/lap crosses the edge - those dots should be uniformly spread out.? stroboscope would be better?

On 11/15/2019 3:11 AM, Rien wrote:
Again, I have to thank you Bill. To me this is like opening another door of which I did not even know it existed.
I will study both the site (yubagold) and the pdf. I am however not yet in a knowledge state that would allow me to make sensible comments on it.

Btw, I have pretty much decided to make the CNC machine -that I was planning anythow- sturdy enough to also handle grinding/polishing forces. That does not impact my schedule too much and at least gives me the option to start experimenting with CNC grinding/polishing whenever I feel ready.


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