22 replies to this topic

[Astro Daddy]

Hey guys just purchased a piece of glass to practice on before I attempt a refiguring on the mirror I took out of my Zhumell. Just wondering where to get supplies for make a mirror? Also would say a 10" grinding kit be a good way to go? This is my first time doing this so it is all a learning experience at this point. CCassie told me the blank is stressed so I will also be able to see what that looks like as well. The blank has been polished on before and has a bad figure at present so I may see if I can make the figure correct before taking it to a sphere then parabolizing it again.

John

[photoracer18]

No. I would not recommend refiguring a Chinese mirror until I had it tested. They tend to be machine made which makes the figures pretty consistent (usually means acceptable but not bad and not great either). Just rougher and not as smooth as good stateside mirrors (at least in the modern era).

[John Rose]

In the past it was always recommended to start with a 6 or 8 inch mirror. Check the Stellafane web site. Also check for clubs in your area. Perhaps you will find some experienced mirror makers with in reasonable distance. Nothing like some hands on help.

Good luck, John

[dan_h]

CCassie told me the blank is stressed so I will also be able to see what that looks like as well. The blank has been polished on before and has a bad figure at present so I may see if I can make the figure correct before taking it to a sphere then parabolizing it again.

John

It is going to more than difficult to know what your strokes are doing for you if the stressed glass is influencing the result. When you are trying to learn something, you need to have all known factors under control. Otherwise you will be chasing the figure endlessly.

 

dan

[TOMDEY]

Handcrafting mirrors is supreme fun, but is time consuming, slowly-nurtured skill. Something you gota love for its own reward. If that sounds exciting, making a 6-inch first would get that And your 10-inch finished faster than jumping right on the ten-inch. A friend (highly-skilled optician) made two 16-inchers for me. #1 finished immediately, wonderfully. #2 took forever because the glass was (apparently) stressed. After huge effort, he replaced the blank and succeeded. Pushing glass can be frustrating. I still recommend it, but you have to inherently love that aspect of optical fab. Every mirror is different, its own personality. Similar to wood-working.  Tom

[Astro Daddy]

No. I would not recommend refiguring a Chinese mirror until I had it tested. They tend to be machine made which makes the figures pretty consistent (usually means acceptable but not bad and not great either). Just rougher and not as smooth as good stateside mirrors (at least in the modern era).

Yes I can testify to the roughness of my Chinese mirror. The images were not the best and after getting another one to install in my scope I could really see the difference. The Foucault test shows a lot of roughness but the Ronchi test shows good figure at least to more experienced CNers. As I said this piece of glass is just for going through the motions. Making the plate and testing and such. Also getting familiar with tools and strokes.

[Astro Daddy]

Handcrafting mirrors is supreme fun, but is time consuming, slowly-nurtured skill. Something you gota love for its own reward. If that sounds exciting, making a 6-inch first would get that And your 10-inch finished faster than jumping right on the ten-inch. A friend (highly-skilled optician) made two 16-inchers for me. #1 finished immediately, wonderfully. #2 took forever because the glass was (apparently) stressed. After huge effort, he replaced the blank and succeeded. Pushing glass can be frustrating. I still recommend it, but you have to inherently love that aspect of optical fab. Every mirror is different, its own personality. Similar to wood-working.  Tom

Yes more than likely the blank after I am finished practicing on it will be flipped over and become a tool. I am wanting to test and see stress first hand. I am going to need to check the original mirror for stress. Someone on CN had a Zhumell mirror and sent it to Ed for a refiguring and Ed found it badly stressed. So if I strip my Zhumell mirror and find the same thing I guess I will have two tools. The mirror I have in my scope now was used but I was able to resolve six stars in the Trapizuim which I had never done

John

[mark cowan]

There's no good reason to do this just to "go thru the motions." 

 

Figure to actually make a mirror  from the start and you'll be a lot more motivated - even if you do it several times over.

 

Never work with stressed glass.  Either have it properly annealed (Newport Glass Works) or give it the fire hydrant treatment.

 

John Dobson said a few things in his class about mistakes you could make, in no particular order:

 

"The sins of the tool are not inherited by the mirror."

 

"There's no error you can commit making a mirror that will consign you to hell."

 

"Anything that can be polished in can be polished out."

 

And most importantly, about chips and such:

 

"Any chip you make on the mirror is just another part of the universe that won't be your mirror."

Edited by mark cowan, 16 April 2018 - 06:25 PM.

[Astro Daddy]

There's no good reason to do this just to "go thru the motions." 

 

Figure to actually make a mirror  from the start and you'll be a lot more motivated - even if you do it several times over.

 

Never work with stressed glass.  Either have it properly annealed (Newport Glass Works) or give it the fire hydrant treatment.

 

John Dobson said a few things in his class about mistakes you could make, in no particular order:

 

"The sins of the tool are not inherited by the mirror."

 

"There's no error you can commit making a mirror that will consign you to hell."

 

"Anything that can be polished in can be polished out."

 

And most importantly, about chips and such:

 

"Any chip you make on the mirror is just another part of the universe that won't be your mirror."

Yes I would have liked to have studied mirror making under him. I have viewed many videos of the man. Thanks for your advice. But are there better grits than others or do they all originate from the same source?

John

[mark cowan]

grits?  Use silicon carbide up to about 500 and aluminum oxide down to 5-9 micron, basically. 

 

https://www.hisglass...loose-grit.html is a good source, gotgrit.com and firsthanddiscovery.com as well.

[Astro Daddy]

Thanks for the link Mark. In my searches I have never came across  His Glass.

John

[mark cowan]

It caters to fused glass (art) hobbyists but they have good supplies, clean packaging, and excellent shipping.  I found them when searching cerium oxide suppliers. 

[Astro Daddy]

Mark I didn't realize there were so many types of polishing compound. Is this the type you use? I see something about optic in the name. Cer-Optik Cerium Oxide. When I have been researching making a mirror I found that  Gotgrit has a package kit for different size mirrors as well as First Hand Discovery but First Hand wants one to specify the pitch type and hardness. Is there a middle of the road pitch type and hardness or one that for most common to use? I have seen on some mirror making articles that mirror making outcomes and technic and almost like a signature a little bit different with everyone's style. I am guessing the pitch that one uses may be the same way. That being said is there one for  whom does not have that experience or expertise, could you suggest something?  

Thanks John

[ccaissie]

The 10" thick American-made piece is very slightly stressed, so that's why it will probably never make a premium mirror. Someone polished it out, but then gave up...maybe the chip on the back discouraged him?  But who knows?  If you figure it to a smooth sphere and test it under changing temps on the bench, you might see the impact of strain..or maybe it will be too slight to make a difference.

 

Once you can see that strain, by polarization methods, you'll have a good measure for the Zhumell.  Also the figure on that practice piece is so horrible that you will learn a lot by making a 6-8" lap and smoothing and working interior zones, etc.

 

Like you say, the figure on the Zhumell is not too bad.  I know of a 12" that has a good figure, but wicked rough.  So by practicing on the interior zones on the practice disk, you'll get a good handle on how to control and smooth a surface.  Then the Zhumell will respond well to your work.

 

Right, maybe the Zhumell is strained?  You'll know how to determine that once you see what strain looks like.  If the Zhumell is not worse than the practice piece, you're good.

 

Too bad I didn't think to send you some Barnesite.

 

I attach a stolen pic from Lockwood's site showing the type of strain pattern that I saw on the practice blank, though your practice blank is much LESS severe than that shown in Lockwoods example. 

Attached Thumbnails

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Edited by ccaissie, 17 April 2018 - 08:46 AM.

[ccaissie]

You will only need to polish, not grind, so pouring a plaster lap and making a pitch lap is all you need.  And if you're just smoothing/figuring rather than going for a full polishing out...(like hours of polishing out the grinding pits when making a mirror from a raw blank)....use a medium to soft pitch...gugolz 64 or equivalent, is soft in my estimation...

 

 

[Astro Daddy]

Weird looking. The pictures I have seen have a x shape strain.

I have been studying Ed's video with his tool making method and it looks like he does a very good job of it. I plan on doing it his way. He melts the pitch in a foil form and mates the mirror with it then cuts the grooves. I think I would have a more difficult time making a pitch lap like I seen John Dobson do on a video.

Thanks for the pointers

John

[ccaissie]

Lockwood explains strain very well, and provides examples of some really bad patterns,  He said the irregular strain pattern like in the pic is the least detrimental.  Your practice mirror should be ok, according to his discussion.

 

I have rubber molds for lap making. Got them from Stan Brower/Laboratory Optical.   I also liked Mackintosh's method of making a mold out of thin wood strips, and casting pitch in long rectangular strips, then laying the strips on the tool...channelled one-way, it works very well.  Cutting channels is a pretty messy way to do it.  Pressing them in isn't too bad.  Gouging channels with a soldering gun works, but your lady friend might not like the fumes...

Attached Thumbnails

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[mark cowan]

Mark I didn't realize there were so many types of polishing compound. Is this the type you use? I see something about optic in the name. Cer-Optik Cerium Oxide. When I have been researching making a mirror I found that  Gotgrit has a package kit for different size mirrors as well as First Hand Discovery but First Hand wants one to specify the pitch type and hardness. Is there a middle of the road pitch type and hardness or one that for most common to use? I have seen on some mirror making articles that mirror making outcomes and technic and almost like a signature a little bit different with everyone's style. I am guessing the pitch that one uses may be the same way. That being said is there one for  whom does not have that experience or expertise, could you suggest something?  

Thanks John

You can use what Discovery or GotGrit sell without any problems.   Standard Gugolz 64 will get the job done for most people - the warmer the climate the harder the pitch and v.v. - 55 is softer, 73 is harder.  There is no pitch grade to recommend for no experience however, as the mirror doesn't know that. 

[mark cowan]

Weird looking. The pictures I have seen have a x shape strain.

I have been studying Ed's video with his tool making method and it looks like he does a very good job of it. I plan on doing it his way. He melts the pitch in a foil form and mates the mirror with it then cuts the grooves. I think I would have a more difficult time making a pitch lap like I seen John Dobson do on a video.

Thanks for the pointers

John

Strain can be irregular or the classic regular Maltese cross like this (not too severe):

 

\
 

This kind is more or less workable, the irregular kind isn't.

[ccaissie]

Lockwood states on his site that the cross-type strain tends to produce a potato chip type distortion, whereas the irregular strain can more likely statistically cancel out.  Re: the 'practice blank', the visible strain is way less than the Lockwood photos, so the piece in question would probably do ok.   Once the OP sees how some careful polishing works miracles on the horrible figure, he'll be another hopelessly-addicted glass pusher....

 

I'd include a pic of the strain, but the practice mirror is in the mail.  Maybe Astro Daddy can send a pic to round out this thread when he receives it, as he's going to check it.

 

Hope OP likes the bag of Cerium I added.....Universal Photonics #363.  Heirloom polish, goes well with the vintage glass.

 

Lockwood and Cowan can thrash out the discussion on which strain is worse...I think badly strained blanks should be specially reserved for Yolo/Scheif secondaries....8^) 

Edited by ccaissie, 18 April 2018 - 01:01 PM.

[mark cowan]

Speaking from experience, anywhere you see a sharp strain contour in the glass you will see effects on the figure nearby on the surface. 

 

The regular cross produces a stable strain on the surface which is, marginally at best, workable. At least whatever you get is mostly stable.  Been there, done that once, and only as a customer request.  Best to just have it annealed properly or replaced.

 

Irregular strain will tend to shift with temperature and cause weird effects on the figure.  At a constant temp it could maybe be a decent learning device. 

 

No thrashing needed  - all strain basically sucks if you're trying to make an excellent mirror.

Edited by mark cowan, 18 April 2018 - 03:45 PM.

[Astro Daddy]

Speaking from experience, anywhere you see a sharp strain contour in the glass you will see effects on the figure nearby on the surface. 

 

The regular cross produces a stable strain on the surface which is, marginally at best, workable. At least whatever you get is mostly stable.  Been there, done that once, and only as a customer request.  Best to just have it annealed properly or replaced.

 

Irregular strain will tend to shift with temperature and cause weird effects on the figure.  At a constant temp it could maybe be a decent learning device. 

 

No thrashing needed  - all strain basically sucks if you're trying to make an excellent mirror.

Yep! I work for my county's school system, they have several kilns. I would love to

see if I could reduce or eliminate the strain but the energy used would be the issue. My boss is on point on that subject

John

[ccaissie]

Send a pic of the strain.  We'll be underwhelmed, I think.