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24" noob dob

beginner DIY dob moon mirror making
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#26 Jon Isaacs

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Posted 10 November 2019 - 08:34 PM

Guys:

 

I hate to be a Debbie Downer but the corrosion is far thicker than the tolerances of an optical mirror. Polishing with any grit will destroy and optical precision it might of had.

 

Figuring a 24 inch F/3 mirror to optical tolerance requires a highly skilled optician whose made numerous mirrors and has the skills to test the mirror..

 

Steve Dodds has made numerous mirrors. 

 

Jon


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#27 brave_ulysses

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Posted 10 November 2019 - 08:46 PM

hi bob,

 

here is another metal mirror thread:

https://www.cloudyni...-mirror-making/

 

jon isaacs post #11 will lead you to the work of cn member marcos baun

https://www.cloudyni...363-marcosbaun/

http://atmalm.blogspot.com/

 

good luck


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#28 PETER DREW

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Posted 11 November 2019 - 09:31 AM

The way I see it is that the level of corrosion is likely that the pitting would require a re-grind rather than just a re-polish. If so, then it would be a good point to grind the curve to a more appropriate focal ratio for a visual telescope. I've no practical experience with metal mirrors but I would expect that an AL mirror could be coated with a non corroding surface like chromium to preserve the surface.

Edited by PETER DREW, 11 November 2019 - 09:32 AM.


#29 bobsorenson

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Posted 11 November 2019 - 09:39 AM

Thanks Peter, this is just what I thought.

I don't know about the coating just yet...I have been reading 3-4 hours a day trying to learn as much as possible before proceeding.



#30 bobsorenson

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Posted 11 November 2019 - 09:45 AM

Jon Isaacs, I do not understand the logic of your comment.

 

It's obvious that the finish is corrupted hence it must be restored.

If in the process the mirror needs refiguring, then isn't that just what one does with any type of mirror?

 

And finally, is the intent of your comment to discourage me or other amateurs from trying, learning and doing? Should I just give up and go out and buy something new that someone else made?



#31 bobsorenson

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Posted 11 November 2019 - 09:47 AM

Thank you brave ulysses, this looks very helpful.



#32 JOEinCO

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Posted 11 November 2019 - 11:15 AM

Jon Isaacs, I do not understand the logic of your comment.....It's obvious that the finish is corrupted hence it must be restored. If in the process the mirror needs refiguring, then isn't that just what one does with any type of mirror?

 

And finally, is the intent of your comment to discourage me or other amateurs from trying, learning and doing? Should I just give up and go out and buy something new that someone else made?

I think Jon's comments are more to stress what is involved. Because the corrosion (3 dimensionally) is HUGE compared to the surface one needs prior to recoating. So you are at the point where you are going to have to regrind the surface to an accuracy of 1/10th the wavelength of light just as if you were starting from scratch. And there's not an ATMer out there who is going to suggest your first mirror project be a 24"er. 

 

There is a big difference between "discouraging" and "hard truth". Sometimes the hard truth is far from what we want to hear.


Edited by JOEinCO, 11 November 2019 - 11:17 AM.

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#33 bobsorenson

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Posted 11 November 2019 - 11:31 AM

Very well put Joe, thanks.

 

In the end however, consider that

1. I have these two mirrors that are worth say, $X as scrap.

2. If one or both of the mirrors ends up a disaster and completely unusable in a telescope because I am "not a trained optician with years of experience" then

3. the mirrors are still worth $X as scrap and

4. I most likely have learned a lot.


Edited by bobsorenson, 11 November 2019 - 12:34 PM.

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#34 davidc135

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Posted 11 November 2019 - 12:29 PM

What process would be used to remove Al? Grinding or planing? Recently I've used an aluminium ring to deepen the sag of an 8'' glass mirror. The ratio of material lost to Al tool and glass mirror was between 1: 50 and 1:100. The plastic metal seemed to soak up the force of the splintering 180 grits whilst remaining mostly intact.   David



#35 bobsorenson

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Posted 11 November 2019 - 12:45 PM

David, I don't know. Pretty much this question in how to deal with the material is the reason I joined this group.

Clues I've gleaned so far are;

that sometimes the much harder abrasive particles used in grinding can get trapped in or under the Al surface and cause problems later, and

the motherload of information about working solid Al mirrors is compiled on a Yahoo group based in Brazil and is in Portuguese. I'm considering joining up with those people just to see if I can get something useful. I'm hoping one or more of them speaks english and will be interesting in helping out.

What I do know about the Brazilians is that they use a tool covered with polyester and they use colloidal Al as a polishing agent.


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#36 brave_ulysses

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Posted 11 November 2019 - 12:59 PM

hi bob,

 

maybe a sympathetic local machinist/atm could help you get the party started. any nearby makerspaces?

 

clay

 

edit:

 

well, you have 2 mirrors

https://www.bbastrod... binoscope.html


Edited by brave_ulysses, 11 November 2019 - 01:33 PM.


#37 JOEinCO

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Posted 11 November 2019 - 02:44 PM

In the end however, consider that

1. I have these two mirrors that are worth say, $X as scrap.

2. If one or both of the mirrors ends up a disaster and completely unusable in a telescope because I am "not a trained optician with years of experience" then

3. the mirrors are still worth $X as scrap and

4. I most likely have learned a lot.

waytogo.gif waytogo.gif waytogo.gif waytogo.gif



#38 BGRE

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Posted 11 November 2019 - 04:55 PM

The way I see it is that the level of corrosion is likely that the pitting would require a re-grind rather than just a re-polish. If so, then it would be a good point to grind the curve to a more appropriate focal ratio for a visual telescope. I've no practical experience with metal mirrors but I would expect that an AL mirror could be coated with a non corroding surface like chromium to preserve the surface.

The classical coating was kanigen (electroless nickel) but bimetal effects during temperature changes can be an issue.



#39 Jon Isaacs

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Posted 11 November 2019 - 06:07 PM

Jon Isaacs, I do not understand the logic of your comment.

 

It's obvious that the finish is corrupted hence it must be restored.

If in the process the mirror needs refiguring, then isn't that just what one does with any type of mirror?

 

And finally, is the intent of your comment to discourage me or other amateurs from trying, learning and doing? Should I just give up and go out and buy something new that someone else made?

 

A glass mirror does not corrode or lose it's figure. If the mirror needs refiguring,  it is because it wasn't made properly in the first place. The surface of a poorly figured glass mirror will probably still be with 1/4 wave of light. 

 

The surface precision needed is about 0.000025 inches. A hair is 0.004 inches.

 

My hope is that you understand the enormity of the task you are facing.

 

Very well put Joe, thanks.

 

In the end however, consider that

1. I have these two mirrors that are worth say, $X as scrap.

2. If one or both of the mirrors ends up a disaster and completely unusable in a telescope because I am "not a trained optician with years of experience" then

3. the mirrors are still worth $X as scrap and

4. I most likely have learned a lot.

 

As Joe said, I am trying to provide a dose of reality. What you have are two pieces of aluminum. they are not mirrors and it's most likely they never were, at least mirrors of sufficient quality for optical wave lengths.

 

Assuming the focal ratio is F/3, making a 24 inch F/3 mirror of sufficient optical quality for a telescope is a project for a very experienced optician, its far from a beginners project.  There are many ATMs and commercial opticians in this forum.  I think every last one would consider a F/3 24 inch mirror a very difficult challenge.

 

If you really want to make a serious attempt are turning one of these pieces of aluminum into a mirror, the first step would be to put them aside and learn to make a small, relatively slow mirror so you have some understanding of the techniques and skills required. You will learn how to test the mirror and to interpret the tests so you can figure the mirror to the extreme precision required. 

 

Again, I'm not trying to be a Debbie Downer, I'm trying to add a dose of reality. 

 

Jon


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#40 555aaa

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Posted 11 November 2019 - 07:11 PM

Everybody on this thread needs to go and watch "Real Genius," especially the part where the bad guy puts fingerprints on an optical window in (pre-Batman) Val Kilmer's laser experiment to literally blow it up. Say you have a megawatt laser, and it hits a 99% reflective mirror. How much power goes into the mirror? 10,000 watts. Hence the metal mirror.

 

ABL (Airborne Laser) and MIRACL were very high power NIR lasers as examples, not saying these mirrors had anything to do with that. But very star-wars.

 

Man, those were the days to be in optics, even though most people thought it wasn't gonna work.



#41 bobsorenson

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Posted 11 November 2019 - 08:24 PM

Thanks for your concern Jon.

 

BTW, I believe your use of the term "corrosion" is incorrect here. What is seen on the surface of the mirror is Oxidation, is it not?

 

I will be working on these mirrors and am here looking for constructive advice.

 

Your cautionary opinions have been duly noted.


Edited by bobsorenson, 12 November 2019 - 09:59 AM.

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#42 WarrenK

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Posted 11 November 2019 - 10:37 PM

This is a long shot but it might be worth your while to contact the Cosmic Ray Group, Physics Dept, Univ of Utah. They have numerous detectors in Central Utah and these mirrors might be surplus from one of their installations. If nothing else they might be able to give you some guidance

#43 bobsorenson

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Posted 11 November 2019 - 11:08 PM

Thanks Warren,

I know for a fact that the mirrors came from the Strategic Defense Initiative installation at the Toole Army Depot's west desert testing range.

I've also been to the Cosmic Ray Detection Array about 100 miles west of where I am. It's kooky out there!



#44 Oberon

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Posted 12 November 2019 - 03:41 PM

Thanks for your concern Jon.

 

BTW, I believe your use of the term "corrosion" is incorrect here. What is seen on the surface of the mirror is Oxidation, is it not?

 

I will be working on these mirrors and am here looking for constructive advice.

 

Your cautionary opinions have been duly noted.

Oxidisation is corrosion.

 

“Corrosion is a natural process that converts a refined metal into a more chemically-stable form such as oxide, hydroxide, or sulfide. It is the gradual destruction of materials (usually metals) by chemical and/or electrochemical reaction with their environment.”

 

Wikipedia


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#45 Oberon

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Posted 12 November 2019 - 03:51 PM

Thanks Warren,

I know for a fact that the mirrors came from the Strategic Defense Initiative installation at the Toole Army Depot's west desert testing range.

I've also been to the Cosmic Ray Detection Array about 100 miles west of where I am. It's kooky out there!

In that case it seems likely that you have a pair of reflectors designed for pointing high power lasers. I guess it is also possible (but less likely) that they were coupled with an IR detector for detecting/tracking rocket exhausts, and in either case you could have some fun making a very effective (if dangerous) solar cooker. 


Edited by Oberon, 12 November 2019 - 03:52 PM.


#46 bobsorenson

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Posted 12 November 2019 - 03:54 PM

Oberon, thank you.
I had read several definitions of both and was confused I guess. l took the distinction between corrosion and oxidation to be a matter of the degree of degradation.
IMHO, the degree of surface degradation is key to any success I may have in restoring the mirror. That is, I fully realize that I will likely be unable to properly figure the surface so it is my hope that it is oxidized only to the extent that if i am very careful with the surface then the figure may be retained somewhat intact and polishable and useable by an amateur.
Thanks again

Edited by bobsorenson, 12 November 2019 - 03:58 PM.


#47 bobsorenson

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Posted 12 November 2019 - 04:04 PM

Oberon, yes.
An obsession I have is solar concentration and I've built many projects. Your idea is why I purchased them in the first place for the price of scrap. And in the end they may be used as secondary concentrators in a 50,000 sun's smelter I've designed.

#48 wells_c

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Posted 12 November 2019 - 10:54 PM

Hehe I’ve dabbled in solar concentration, the pinnacle of which involved tearing down a free (thanks, street!!) rear projection large screen tv for its fresnel lens. Somewhere around 1.5 meters squared of sunlight gets directed to a spot less than 25 cm across. Stacks of coins slump over and run off immediately. My local beach has magnetite black sand, and I melt that. Can’t remember melting temp but couple thousand Celsius at least :)

In any case, your failure outcome has been promoted from metal scrap to precision solar concentrator!! Way less reason to not go for it :)
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#49 bobsorenson

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Posted 12 November 2019 - 11:39 PM

Ha! Fresnel lenses are the bomb for sure.

Once I made a F lens with 10 linear mirrors focused on an evacuated tube. The idea was to put liquid in but the heat was so intense, the liquid would boil within seconds and fly out of the end of the tube with much enthusiasm!



#50 bobsorenson

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Posted 13 November 2019 - 12:01 AM

I have been furiously reading about Foucault and Ronchi testing, watching many Gordon Waite videos and studying various threads on the CN forums.

Perhaps most importantly I have absorbed valuable information on CN member Marcos Baun's blog detailing his methods of grinding and polishing solid Al mirrors. The methods and materials detailed there are such that I hope to put together a good effort at trying to make one of the mirrors work. Too bad I don't speak Portuguese.

 

Of course, concurrent with the work on the mirror, testing is required and I will be making a Foucault tester. All I've read on this subject shows testing on uncoated glass...

 

Can anyone tell me if there are special considerations for testing a reflective mirror?

 

TIA


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