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Understanding polishing stroke selection when polishing a mirror and how pitch works?

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#1 Fly2High


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Posted 15 April 2024 - 01:28 PM

I would like a better understanding of how material is removed when polishing with a pitch lap.


To abbreviate this:

As I understand it, the tool is covered ith pitch and is set on top of the ground a figured mirror with water and CeO, etc.  


Once set, a pattern of channels is cut offset making squares so that none line up the same with the center. 



As I understand it, this tool has the shape of the mirror at this point.


This is where I expect I am wrong.:

I expect that the pitch is relatively firm but might deform slightly over time.  During a single grinding session, I think it would mostly remain fixed in shape.

Since the tool is convex, most of the action will take place at the center as the tool is moved about.


1.  If the tool is kept evently pressed, Is there only action occurring up to the point where the center contact?


2. How does a turned down edge occur if the tool only slightly overhangs a blank?  I understand you can press offcenter but I do not expect most are when doing polishing initially unless the attempt is to focus in a particular zone.  How is the edge getting ground?


3.  Is it better to do uneven pressure when working a zone or to move the tool?


Ultimately, I am looking to better understand polishing with pitch and how to determine what strokes to use based on testing results.  I undeerstand you can 'only level a hill and cannot fill a hole'.


I have not yet begun my mirror work but would like to understand how to make choices in stroke or tool size before I begin.


I will be using Foucault and Ronchi testing.  I would prefer to focus on after determining what the tests indicate but feel free to provide your own pictures if it helps to discuss stroke and tool selection.


To me, this is the black art of telescope making.


With much appreciation,



Edited by Fly2High, 15 April 2024 - 02:28 PM.

#2 JohnH



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Posted 15 April 2024 - 02:00 PM

Process of polishing is somewhat misunderstood. It was thought that the Polish removes incredibly microscopic amounts of glass. But this isn't quite the case. Analysis is shown that Polishing Compound iron oxide ceramoxide, gets embedded into the surface of the glass so it is more like a weird kind of deformation that happens.

#3 walt r

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Posted 15 April 2024 - 02:00 PM

All good questions. I have very little experience - only re-figured one 4.25" mirror.


What is the diameter and f/ of the mirror you have?

Have you done the grinding yet?


Have you studied the info from Stellafan:



Once you have digested the Stellafane info then search this forum (ATM) for mirror polishing and figuring threads that 'talks' someone through the process and methods of correcting various issues like TDE.


This is how I learned to figure the mirror I did (used this mirror un-coated with EP projection for group observing the Partial Solar eclipse last week.

Edited by walt r, 15 April 2024 - 02:05 PM.



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Posted 15 April 2024 - 02:59 PM

Frank, if your goal is to produce a good figure on your mirror, read Texereau a couple times through. To this day, the actual mechanism of shaping the surface is still somewhat mysterious and subject to debate... but the successful application of the process is well described by Texereau.     Tom

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#5 Mike Spooner

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Posted 15 April 2024 - 04:06 PM

Pitch is fickle. 

Temperature is important.

Humidity is critical.


Understanding what is really happening to the mirror surface is challenging. I hear of folks who think they have TDE when they still have edge pits! 


Lots of excellent info here compared to years ago but takes some effort to ferret out what works for you. 

Mike Spooner (still trying to figure it out) frown.gif

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#6 Pinbout



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Posted 15 April 2024 - 04:07 PM

polishing with a pitch lap is like using a melting piece of ice to polsih the mirror.


you have to repress to get the shape same-same.


you alternate tot / mot while polishing...tot does the edge more, mot does the center more


you watch the channels as your polishing and see if they start to close up.... then you have to rechannel to open them and repress.


all 1/3 overhang center of center strokes for polishing... no zones to worry about... that's for figuring. and the edge of the tool cuts more so you use the edge of the tool to work zones that need to be lowered. but agian that's for figuring not polishing.

Edited by Pinbout, 16 April 2024 - 02:21 PM.

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#7 starspangled


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Posted 15 April 2024 - 04:41 PM

Get hold of Sam Browns All About Telescopes . The mirror making section is not too long but it goes in to some good depth with figuring strategies and strokes with good diagrams . It is out of print but you can download the PDF from Edmund scientific site .  That will give you a good idea of how polishing works .


I think  A  Manual for Amateur Telescope Makers ( translated from the French ) is a worthy successor to Texereau being in the french intellectual tradition , available from Sky Publishing .

#8 Dale Eason

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Posted 15 April 2024 - 05:02 PM

Watch this video to learn more.  https://www.youtube....h?v=snz7JJlSZvw   for polishing start about at time 25 minutes.

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