22 replies to this topic

[H.I.]

Hello all! I'm kind of stuck and could really use some advice. I have a 10" mirror that I've fine ground to what appears to be a focal point (based on wetting and focusing) and I need to start polishing/figuring. My biggest obstacle is understanding what to do next, and I would really appreciate your advice or reading material suggestions. I've watched videos and will pour the pitch this weekend, but outside of just polishing COC I'm not sure what to do. For example, do I polish once for half an our and test with Foucault, or polish 3 times for half an hour each and then test with Foucault. And which comes first, Foucalult testing or Ronchi tesing? I can build a Foucault tester, and have some Ronchi film on the way (thank you @sputnik), and I'll grind (pun intended) my way through this but if you have good links, articles, jokes, or advice (besides quit ), for beginners I would really appreciate it. Thank you all very much for the help!

Edited by H.I., 24 March 2025 - 10:54 PM.

[Jkrtush]

Hi, welcome to the grind. I have made 1 mirror. Back around 83. I was surprised how much deeper it got as I kept going thru the grits. And how still the air needs to be for testing. And how long the wait was to get it coated lol. I did the grind and polish outside and got a scratch during polish, but it did not hurt too much. Buy a good luck charm to figure it, that will be easier than skill .

[Dale Eason]

You now polish out the pits and do not need to do any testing except to check that the pits are gone.  There are several good ways but first is just look to see if there is any haze left.  Then use a strong magnifying glass of 10x or more (I use a 60x) to look for pits.  Keep polishing until they are gone.  This is going to take awhile.  Then after they are gone start testing with either ronchi or foucault or both.  There are many good sites and books that explain this. 

 

 

Once polished out you can then once again using flashlight or sun test to check the mirrors ROC without having to wet the surface.

 

Here is a good online resource.    https://stellafane.o.../atm/index.html

Edited by Dale Eason, 25 March 2025 - 01:01 AM.

[davidc135]

Also, two basic but good suggestions would be:

 

Standard handbook for telescope making.  Neale E. Howard

 

All about telescopes.  Sam Brown

 

Sam Brown's work is part of a large on-line archive. Maybe someone can give details?

 

More comprehensive:

 

How to make a telescope  Jean Texereau

 

I made my first Newtonian with the help of Howard's book. The first two book suggestions keep things simple which can be an advantage for a beginner. But Texereau was the master!

 

David

Edited by davidc135, 25 March 2025 - 01:01 PM.

[Pierre Lemay]

Hello all! I'm kind of stuck and could really use some advice. I have a 10" mirror that I've fine ground to what appears to be a focal point (based on wetting and focusing) and I need to start polishing/figuring. My biggest obstacle is understanding what to do next,

Please provide more details of your progression so far. In particular:

• What grit sequence did you use during fine grinding and how long did you grind with each grit size?
• Did you do a pencil or sharpie test to make sure you have perfect contact between tool and mirror?
• What is the current radius of curvature, or focal length? 
  ​

[ccaissie]

Doing the fine grind properly will make sure other issues don't creep in and make the next stages difficult or impossible.  We hope for your success, so glad that you have asked.

 

Dale and Pierre's questions above are crucial.  

 

The mirror must be spherical.  By now you should have perfect contact between the mirror and tool.  A few extra wets with the final abrasive should be done to ensure it really is completed.

 

If you are asking your questions, I suspect you are not following a standard procedure, and that can be trouble.  Methodically following the process as developed by thousands of mirror makers is absolutely crucial.  

 

Your questions:

Do I polish for 1/2 hour and then test?

NO.  Testing frequently while polishing is deceptive, reveals little, and wastes a lot of time.

 

By this time you should have NO pits other than the last grit size.  Larger pits will probably never polish out unless you polish for many tedious hours, wasting time, so check this before making a lap. It's heartbreaking to polish and find big pits...then you have to go back to grinding...a bummer.

 

Also, spherical.  If you made a tile tool, it gives a better final grind by distributing the abrasive better.  Your final grinding should have been with short strokes, slowing down, and taking it real easy.  If you have a solid glass tool like in the old days, watching the grit distribution and air bubbles through the back of the Mirror  will confirm that the mirror matches the tool and both can only be spherical to fit prefectly.

 

If that's the case, use the pencil and sharpie test.  Mark up the face of the mirror and then do a short fine grind to confirm that the marks are evenly wearing.  Any areas that do not wear off evenly tells you that you need to do more fine grinding.   If you don't get a perfect match and fit now, you will be fighting a non spherical surface for many hours.

 

Also I assume you have been supporting the mirror evenly if it's on the bottom, rotating the tool and mirror regularly to get a symmetrical figure. Did you try Mirror on the bottom occasionally?

 

You're asking questions that need a lot of information before anyone can give you a firm answer, but making sure you have done the fine grind correctly is MOST important.  If you can confirm this, then make a lap and POLISH using methods recommended by the experts until you have the pits gone.  Like 6-10 hours.  Then test.

 

Tool type

Thickness of mirror 

Radius/ f number

final grit size

method to check for pits

stroke pattern

MOT/TOT

Edited by ccaissie, 25 March 2025 - 07:29 AM.

[Ranger Tim]

The second book to have, after Texereau: “All About Telescopes” by Sam Brown. Scroll to the chapters on grinding/polishing. The Foucault diagrams are awesome. Click on it and save it!

 

Thanks to Edmund Optics for making this available.

[ccaissie]

The second book to have, after Texereau: “All About Telescopes” by Sam Brown. Scroll to the chapters on grinding/polishing. The Foucault diagrams are awesome. Click on it and save it!

 

Thanks to Edmund Optics for making this available.

Yes.  Each step is clear...it just takes dedication to make sure you understand each step.  Be thorough and patient.

[H.I.]

Thank you all very much for the insight and your contributions. It's both helpful and inspiring. I'm going to check for pits and do a pencil test this weekend. If all goes well, sounds like starting polishing (after a bit more reading) will come next. This is exactly what I needed.

 

You now polish out the pits and do not need to do any testing except to check that the pits are gone.  There are several good ways but first is just look to see if there is any haze left.  Then use a strong magnifying glass of 10x or more (I use a 60x) to look for pits.  Keep polishing until they are gone.  This is going to take awhile.  Then after they are gone start testing with either ronchi or foucault or both.  There are many good sites and books that explain this.

This is great advice, and coupled with the other advice in this thread it's really helped clarify things. Knowing to polish before testing was essential to moving forward. Thank you.

 

Also, two basic but good suggestions would be:

 

Standard handbook for telescope making.  Neale E. Howard

 

All about telescopes.  Sam Brown

 

Sam Brown's work is part of a large on-line archive.

I'll see if I can grab a used copy of Howard and search online for Brown. Thank you.

 

 

Please provide more details of your progression so far. In particular:

• What grit sequence did you use during fine grinding and how long did you grind with each grit size?
• Did you do a pencil or sharpie test to make sure you have perfect contact between tool and mirror?
• What is the current radius of curvature, or focal length? 

Mostly following Texereau's grit to wet ratios. So far I have done the following with silicon carbide (estimates):

80 grit - 20 wets

120 grit - 20 wets

220 grit - 15 wets

600 grit - 15 wets

9u Aluminum - 12 wets

5u Aluminum - 10 wets

 

I did MOT mostly, rotating to TOT occasionally. I rotated my position around the stand, and the mirror itself every 8ish strokes, and occasionally rotated the tool. I have not done a pencil test, but now I have my weekend marching orders Current focal length is roughly 60 inches.

 

Doing the fine grind properly will make sure other issues don't creep in and make the next stages difficult or impossible.  We hope for your success, so glad that you have asked <--- Thank you!

 

If you are asking your questions, I suspect you are not following a standard procedure, and that can be trouble.  Methodically following the process as developed by thousands of mirror makers is absolutely crucial. 

...

 

Also I assume you have been supporting the mirror evenly if it's on the bottom, rotating the tool and mirror regularly to get a symmetrical figure. Did you try Mirror on the bottom occasionally?

 

making sure you have done the fine grind correctly is MOST important <-- [good advice].  If you can confirm this, then make a lap and POLISH using methods recommended by the experts until you have the pits gone.  Like 6-10 hours.  Then test.

...

Thanks for the thorough response. I was basically following Texereau but when I got to polishing I got a little overwhelmed and turned to more videos, searching, etc.. which really just piled on. This thread has helped me simplify my next steps and provided great reading material.

 

Tool Type: Ceramic Tile

Thickness: 2 inch

Radius/f number: 5 inch radius f5 (ish)

Final grit size: 5u Aluminum Oxide

Method to check for pits: None. Will do this weekend. Ty!

Stroke pattern. Basic w stroke outline by Texereau.

MOT/TOT: Mostly MOT but occasionally TOT, especially in the later stages of grinding.

 

 

The second book to have, after Texereau: “All About Telescopes” by Sam Brown. Scroll to the chapters on grinding/polishing. The Foucault diagrams are awesome. Click on it and save it!

 

Thanks to Edmund Optics for making this available.

This sounds like a great way to further my knowledge. Ty.

 

Yes.  Each step is clear...it just takes dedication to make sure you understand each step.  Be thorough and patient.

I'm learning this for sure. Thx.
 

[hamishbarker]

Great responses so far. Just one thing to add, from my experience: how does the fine grinding feel? Does the tool feel as if it sucks onto the mirror at the middle of the stroke? If so, it is likely that the shape isn't nicely spherical. Fine grinding needs little pressure. Its important to clean up any non spherical shape well before progressing up to the finer grits.

This is a subtle point which I only truly understood before having the problem on a few mirrors ( at worst, really badly seized mirror/tool can result) and then seeing Texerau perfectly describe the problem in the text of his book.

[H.I.]

Great responses so far. Just one thing to add, from my experience: how does the fine grinding feel? Does the tool feel as if it sucks onto the mirror at the middle of the stroke? If so, it is likely that the shape isn't nicely spherical. Fine grinding needs little pressure. Its important to clean up any non spherical shape well before progressing up to the finer grits.

This is a subtle point which I only truly understood before having the problem on a few mirrors ( at worst, really badly seized mirror/tool can result) and then seeing Texerau perfectly describe the problem in the text of his book.

Good question. Fine grinding feels good at first, smooth and suction free but at some point (not immediately) it does get harder to push. That happened throughout as the grit got smaller and I assumed it was because of the fit between the mirror and the tool. I then wet and added new abrasive and everything started smooth again. I assumed that's kind of normal. I'm going to do a pencil test this weekend so I'll keep an eye on it. However that does make me think. Would a pencil test scratch the mirror since I'm already into fine grinding? Please let me know if you have thoughts. Thank you!

Edited by H.I., 26 March 2025 - 12:10 AM.

[hamishbarker]

pencil will not hurt the mirror.

 

if you have access to a copy of texerau, a careful review of section II-12 "Fine grinding and smoothing", particularly top of page 42 (Willmann Bell second edition 1984) ""the procedure in other respects is that described in Section II-9, except that now the sole pressure applied to the work is the weight of the upper disk plus that of the hands resting naturally on the work and acting only to control the motion. [my italics] Fine grinding takes place uniformly and without appreciably changing the curvature provided that in alternate wets we work with the mirror above and the mirror below." [ Texerau's italics].

 

Perhaps a point to add is that when the curvature is being shortened, I think the shape tends to oblateness (not that anyone should be testing any mirror shape before it is fully polished out). I suspect that this non spherical figure might be making suction against a spherical or hyperbolic tending tool at the centre of the stroke, leading to seizure in the worst case.

 

The remedy, as texerau writes, is alternating MoT and ToT (mirror, and tool -on-top respectively) at each wet. And no excess pressure when cleaning up or fine grinding.

[davidc135]

It sounds like everything is going to plan.

 

At each stage of going through the grits, the surface can be examined with a back to front Plossl or similar with the mirror held up to light. The position of the largest pits can be marked on the mirror back surface. When those pits have gone, it's time to move on to the next grade. Hopefully, all is well.

 

I would do the last few, fine grinding wets MOT to guard against astigmatism.

 

After say 1/2 hour of polishing, I would check to see that wear was reasonably even across the mirror surface, although it's to be expected that the outer zone will lag a bit. If the outer part is more or less untouched, there is a problem, but that's not likely.

 

David

Edited by davidc135, 26 March 2025 - 03:29 AM.

[Pinbout]

After you start polishing , you start perform reflection tests to make sure you totally polished out to the edge

Also use a laser to see if the surface is totally transparent

Do this before even thinking about a ke or ronchi

https://youtu.be/mWO...6XllXh5CGTmgKUR

[Pinbout]

The test of fine grinding another reflection test from a 12 micron grind

do this on 5um too 

https://youtu.be/kjp...JUlP_hdu7hV</p>

Edited by Pinbout, 28 March 2025 - 07:19 AM.

[H.I.]

After you start polishing , you start perform reflection tests to make sure you totally polished out to the edge

Also use a laser to see if the surface is totally transparent

Do this before even thinking about a ke or ronchi

https://youtu.be/mWO...6XllXh5CGTmgKUR

Very helpful, thank you.

[H.I.]

Quick update. The pencil test evenly erased the center marks, but left about a 1 inch ring around the outside edge. According to Stellafane (https://stellafane.o...tml#Pencil-Test) I should grind MOT to correct so I will follow that. My only concern is reducing the focal length since my understanding is that figuring a short focal length is more difficult. But the chase is the adventure Thanks all for the help.

Edited by H.I., 29 March 2025 - 10:24 PM.

[H.I.]

Any suggestions on which grit to go back to? I feel like the aluminum oxide won't move the needle but would like to avoid going all the way back to 80 if possible.

 

So far I've used:

 

80 grit

120 grit

220 grit

600 grit

9u Aluminum

5u Aluminum

 

Thanks.

[PrestonE]

Any suggestions on which grit to go back to? I feel like the aluminum oxide won't move the needle but would like to avoid going all the way back to 80 if possible.

 

So far I've used:

 

80 grit

120 grit

220 grit

600 grit

9u Aluminum

5u Aluminum

 

Thanks.

If you lay a Straight Edge carefully across the Depression, can you visualize a change in the curve in the outer 1 inch 

that did not clean up with the Pencil Test?

 

If not, likely try 600 Grit working MOT and that will not change your ROC very much at all.  If that cleans up the 

problem then you can continue with the finer grits...if not, you can move back to 220 grit.

 

Try 10 minutes of 600 and see if the 1 inch Radius changes to 3/4 in or at all.  That will give you an idea of how

much Slope Difference that there is and how much time to correct the problem.

 

Regards,

 

Preston

Edited by PrestonE, 30 March 2025 - 03:10 PM.

[H.I.]

If you lay a Straight Edge carefully across the Depression, can you visualize a change in the curve in the outer 1 inch 

that did not clean up with the Pencil Test?

-- Yes, that's exactly what I saw. About 1 inch that remained all around the edge.

 

If not, likely try 600 Grit working MOT and that will not change your ROC very much at all.  If that cleans up the 

problem then you can continue with the finer grits...if not, you can move back to 220 grit.

-- Ty, it was pretty extreme so I went with 220 and did 10 wets (8 MOT, 2 TOT) and a new test. Looked MUCH better after that. Nice even wear from edge to edge I think it's time to move to the 600 now.

 

Edited by H.I., 30 March 2025 - 07:40 PM.

[luxo II]

And don't worry about figuring - the comes after polishing - not before !

[Don H]

I think 220 to 600 is a pretty big jump. If that's all you have, maybe spend a lot of time on 600, checking for pit reduction often.

[H.I.]

I think 220 to 600 is a pretty big jump. If that's all you have, maybe spend a lot of time on 600, checking for pit reduction often.

Currently that's all I have. I'll give it a go and check pits often, thank you for the advice. If it's not making enough progress I'll find something in the middle...fingers crossed