15 replies to this topic

[chantepierre]

Hi,

 

I am starting to polish my larger and last (except for small 6" pieces and a doublet I'm working on the side) mirror for a while, a 16.5" porthole supposedly from JD's stock that Augustus sent me a few years ago.

 

This size and thickness (1:16.5) are quite new to me so I'll write what I am doing and how it is going here.

 

I took the 12" tool from a 12" f4 I did last year, and it pressed on the mirror under its own weight for a week, separated by baking paper. Today I dug channels and did a first 30 minutes session TOT. Polish is quite even and the surface looks mostly spherical-ish, but with a long edge. I'll try to bring it up to speed in the next 5 hours.

 

I planned a playlist of 17.5 hours to get to a good optical polish before figuring.

 

Clear skies !

[chantepierre]

I finished building the test stand. This mirror is quite thin so I might as well get repeatable results with the Bath, otherwise I'd soon be running in circles.

This is not its real mirror cell, just a test jig.

 

[CrossoverManiac]

I have some questions about using porthole glass for a mirror blank.

1. Is the porthole glass pane glass or borosilicate?
2. Where did you purchase it?
3. What is its thickness?

I've heard that John Dobson started making his own primary mirrors from porthole glass that were surplus from the military (probably WWII) and I've been interested in it ever since seeing the price for mirror blanks.

[Gerhard_S]

Thin and flexible, and longing for repeatable results on testing. Better use just one support at the bottom. Eliminate Astig by rotating mirror by 90 degree an average the wavefront. More simple, less stress in the glass more repeatable. V Support is a good choice for the telescope. Gives treefoil and so nicer stars. Testing is different.Avoid zhe triangles. Thats what I think. Thumbs Up for your project.

Edited by Gerhard_S, 14 November 2024 - 04:21 PM.

[MeridianStarGazer]

I have some questions about using porthole glass for a mirror blank.

• Is the porthole glass pane glass or borosilicate?
• Where did you purchase it?
• What is its thickness?

I've heard that John Dobson started making his own primary mirrors from porthole glass that were surplus from the military (probably WWII) and I've been interested in it ever since seeing the price for mirror blanks.

Most port holes are pyrex, since it can bend more before breaking. Very important when a wave crashes.

1" thick.

Obtained from Augustus, from a treasure trove.

If you want new borofloat, stathus firstlight seems the best source.

...

...

I took the 12" tool from a 12" f4 I did last year, ...

I'm glad the tool is getting re-used.

Edited by MeridianStarGazer, 14 November 2024 - 04:21 PM.

[chantepierre]

You all might be right on the stand, but now it exists and seems to work well.

 

I'm now at 1/7 of the initial polishing time. I work 30 minutes to 1h at a time, monitoring the shape, rotating the blank every 3 to 4 minutes on the mat. Visible astigmatism is now gone but I'll learn more when I switch to the Bath.

 

It went from oblate spheroid with a down edge to a weak paraboloid with a clean edge in the last sessions, but I have a raised center now.

 

I'm learning to control the initial sphere with the 70% tool and think the next two sessions will help me blend the center and the rest of the surface.

 

Anyway, there are still pits and I promised myself to push this for 17,5 hours before figuring. This leaves me a lot of time to control this initial shape.

Edited by chantepierre, 17 November 2024 - 07:49 AM.

[Augustus]

Most port holes are pyrex, since it can bend more before breaking. Very important when a wave crashes.

1" thick.

Obtained from Augustus, from a treasure trove.

If you want new borofloat, stathus firstlight seems the best source.

...


I'm glad the tool is getting re-used.

Most portholes are plate glass

[Augustus]

I have some questions about using porthole glass for a mirror blank.

1. Is the porthole glass pane glass or borosilicate?
2. Where did you purchase it?
3. What is its thickness?

I've heard that John Dobson started making his own primary mirrors from porthole glass that were surplus from the military (probably WWII) and I've been interested in it ever since seeing the price for mirror blanks.

There are usually a couple floating around any given time on eBay - eg this one that was used as a tool but would work as a blank https://www.ebay.com...c101949.m162918

Someone gave me this one, I got another 16.5" porthole that's near identical recently and am grinding to f/5.5 or so

1" thick plate glass

[chantepierre]

I modified my turntable to finish polishing out with spin polishing. Will remove the attachment to parabolize.

 

[chantepierre]

After approximately 10 hours of spin polishing, I do not discern pits anymore. I will resume work with a bit of smoothing sessions (fixed post spin made a few zones) at spring when pitch is happier.

[Pierre Lemay]

Chantepierre,

Like you, I'm currently making a 16 inch f/3.3 mirror (actually, two). The substrates are not plate glass but Schott Borofloat (borosilicate) so the behavior when working the blanks (grinding and polishing) will differ. Among other things I believe plate glass is softer and perhaps easier (faster) to polish?

 

Nevertheless, I encourage you to take precautions and double check for any presence of remaining pits during polishing. Maybe 17.5 hours of polishing is enough in your situation, maybe not. There Is no harm in polishing too much. On the other hand, polishing too little will have a detrimental effect on contrast should any pits remain. In addition, it's very embarrassing to see that hazy ring around the periphery after the mirror comes back from the coater. Ask me how I know that...

 

So to be extra careful and critical, inspect the glass surface for any signs of a lack of polishing and keep polishing if you have any doubts. I don't only rely on the laser test to check surface polish. I also examine the surface with a small magnifier like this one available for a very reasonable price on Amazon:

 

 

 

 

I've currently polished both mirror blanks about 18 hours. Don't know how well the pictures will turn out but, here is what the edge, just inside the bevel, looks like through the small microscope at a magnification of 60X. Many pits still visible, although much fewer than just after fine grinding ended:

 

 

 

And here is what the glass looks like about 1.25 inches (32 mm) away from the bevel, showing very few pits:

 

 

 

As for the remaining portion of the central 12 inch surface, the mirrors are fully polished.

 

A few additional details about the polishing technique I'm using for these two mirrors: 

• Mirror blanks are meniscus in shape and only 1/2 inch (12 mm) thick 
• To prevent scratching the thin, fast mirror blanks, I ended fine grinding with 9 micron aluminum oxide
• All "rough" polishing is being undertaken mirror on top over the glass tiled grinding tool, covered with polishing pads
• The mirror blanks are stroke polished on a machine at a rate of 28 strokes per minute over the full size tool

I expect close to 30 hours of machine polishing per mirror will be needed to get rid of all the pits. If more is required, I will keep going until all pits have been polished out. I will then switch to a pitch polisher and polish an additional 5 hours per mirror to make the mirror shape perfectly smooth and spherical before undertaking parabolizing.

 

 

[MeridianStarGazer]

I get the feeling that polishing the back side makes it easier to see if the front has haze. But it is easy to do a laser test, although only a super polish would fully pass that. You can compare spot brightness over time and vs radius, and compare to pictures.

Or do the haze test with the back side in water.

Edited by MeridianStarGazer, 19 December 2024 - 04:00 PM.

[Dale Eason]

Back side does not bother the test for laser or magnifier.  So nothing special needs to be done for either of those tests.

[chantepierre]

Hello Pierre,

Thank you for your advice, I hope we will see updates of those two mirrors !

 

Doubt creeped in with your post. I used my microscope on the mirror (same kind of microscope that you are using) and did not see remaining pits.

 

I fine ground with 120, 180 and 320 carbo, then switched to alox, did 25µ, 15µ for a long time, 9µ, got scratches, went back to 25µ, 15µ, 9µ, and did an additional hour of 5µ.

 

I do not use a laser anymore to check for polish, it seems very error-prone. I use a very, very strong LED bicycle headlight and look for haziness around the crisp reflection of the LED, after correctly cleaning the outer part of the mirror.

 

But before I start figuring, I want to get to a better starting sphere, so I'm only out of rough polishing.

 

From my limited experience plate glass polishes a lot faster than borosilicate.

[Pierre Lemay]

I do not use a laser anymore to check for polish, it seems very error-prone. I use a very, very strong LED bicycle headlight and look for haziness around the crisp reflection of the LED, after correctly cleaning the outer part of the mirror.

 

But before I start figuring, I want to get to a better starting sphere, so I'm only out of rough polishing.

 

From my limited experience plate glass polishes a lot faster than borosilicate.

I'm glad to read you are in control of your polishing process and are leaving nothing to chance. I also use a bright LED flashlight to examine the state of polish, in addition to using the laser test (which I agree should not be a final arbiter) and the small microscope. I've also noticed that one can even tell polishing is still needed just by the change in friction felt when wiping the glass with a paper towel. 

 

 

And I concur: borosilicate seems to take much longer to polish than plate glass. I've read of people polishing out 16 inch plate glass mirror blanks in 10 hours. One thing that doesn't help me is having to always polish mirror on top. This is due to the very thin 16 inch blanks I'm working on, which only weigh 8.5 and 9.5 pounds (one came out of grinding a little thinner). Polishing tool-on-top normally accelerates polishing of the glass located closer to the periphery of the blank.

 

Bonne chance avec ton projet. Je suivrai les progrès avec intérêt.

Edited by Pierre Lemay, 20 December 2024 - 12:10 AM.

[hamishbarker]

My 12 inch likely porthole ( rough edge, rather green glass) polished out exceedingly fast after finishing grinding with 15 and I think 9 micron alumina microgrit). My notes say surface was very hard to see any pits at only 3 hrs, and that was my last mention of pits in polishing notes. although I assume there MUST have been some perhaps at twice that time.

End of figuring was at 23.5 hrs , mainly because I spent a lot of time getting rid of a deep hole I dug just after polishing out to oblate. I should have pressed a special lap ( depressed central star with plastic to bring the hole up by not polishing the centre) much earlier, as that eliminated the last vestiges of the hole in only half an hour once implemented it. Lesson: special star or ring star lap worked great! ( Risk of zone though).