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"
http://www.astrosurf.com/gap47/T400/Machine/Utilisation_machines/Methode_Zambuto_12,5_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 http://ecee.colorado.edu/ecen5606/2014/Wavefront_SensingAOL2014.pdf is significant.
and for the professionals there is the $10K Shack Hartmann? http://www.alcor-system.com/new/SH/Shackscope40x40.html fits into the focuser - same concept as with medically measuring eyes http://www.cvs.rochester.edu/yoonlab/research/ows.html
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?