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Is it possible to combine a bunch of thin glass to make a mirror?

Beginner Equipment Mirror Making
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#1 henruth

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Posted 22 July 2021 - 11:33 PM

Hello. I'm trying to find the glass to make an 8 inch mirror and the book I'm reading has said that it's best to get a piece of glass that is 1.5 inches thick for an 8 in mirror. The local glass shops said that would be crazy expensive and one of the things I read online was the idea of stacking a bunch of thinner cuts of glass to make the desired width. I don't really understand how to do that though which lead to a whole bunch of questions. Are they just gluing it together? Do they cast it in a kiln? How feasible is it to go down that route? 

 

I'd really like to go down this route rather than buying a mirror kit because I want to spend as little money as possible. If it's just cheaper to buy a kit or even a finished mirror, I'd go down that route, but this is currently looking the most affordable. One of the things I read online was that they just went thrifting for glass and, as long as it wasn't tempered, they could create a thicker piece of glass by stacking and combining smaller pieces. Any help is much appreciated!



#2 Astrojensen

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Posted 23 July 2021 - 01:45 AM

1.5" thickness for an 8" mirror is the old standard, before it was figured out that it was better to have a thinner mirror and a more complex mirror cell. Today, I'd say that it's better to go for a mirror that's 0.8" to 1" thick. This is what I see everyone doing. 

 

A thin mirror means faster cooldown and a lighter telescope, which are some pretty major advantages.

 

 

Clear skies!

Thomas, Denmark


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#3 luxo II

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Posted 23 July 2021 - 01:55 AM

I'd really like to go down this route rather than buying a mirror kit because I want to spend as little money as possible.

If you're struggling to buy 2 blanks I'd have to say you had better stick to a pair of binoculars.

 

The blanks are just the start; assuming you do manage to finish the mirror the next step is coating it and that is not DIY. Bottom line though is very few make mirrors anymore because its cheaper to buy a finished one from GSO or Edmund for less than the cost of coating.

 

Then there's the rest of the scope...

 

Or buy something secondhand here on CN or AM.

 

Or better yet - find a paying job, work hard at that for a while till you can afford to buy a scope because in making one, you're going to expend an insane number of hours to make something that isnt worth $1,000 - less than $1 per hour.

 

It's a lot more sensible to work a paying job and earn rather more at a better hourly rate, and buy one.


Edited by luxo II, 23 July 2021 - 02:00 AM.

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#4 barbarosa

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Posted 23 July 2021 - 02:50 AM

Hi, I don't know anything about you or your situation, your skills, aspirations or finances. I only want to say that I admire anyone who makes his own mirror. Years ago when I wanted to do that it was always just out of reach for one reason or another. 

 

I see this price on the web for coating a mirror --

 

Mirror Size (in.) Protected Aluminum (PAL) Enhanced Aluminum (EAL) MaxR (Enhance EAL)

         8"                             $75                                        $110                                 $130

 

Before Covid a large local astronomy club in my area conducted a regular mirror making workshop at a public observatory. 

 

From the course description--"The workshop is free; participants pay only for the mirror blanks and grinding tools, which generally cost between $100-$300, depending on the size of the mirror. All the instruction, grinding grit, testing equipment, and camaraderie is free of charge!" The cost will have gone up some to reflect the higher prices for some supplies. One big advantage of a class is that the you work with people who know what they are doing in a fully set up workshop/class situation. 

 

One of the local 2 year colleges ha a 1 unit astronomy lab course in which you grind a mirror an build the whole scope. The enrollment fees are about $70 and "Note: For this course, students will pay a minimum cost of $375 for a completed telescope."

 

This company sells a $44 8" blank.

 

Here is an article about using thin glass layers.

 

Good luck. There are a lot of options and a range of prices. People still grind mirrors though how much money you can save is something for you to research.


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#5 Augustus

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Posted 23 July 2021 - 09:51 AM

Yes and no.

 

A 3/4" thick 8" blank is more than thick enough. The 1:6 standard is outdated and I wouldn't follow it for anything larger than 6 inches. Generally anything around or thicker than 1:10 to 1:12 in ratio doesn't need a super complicated support either while grinding or in use in the scope, and is desirable due to the lower weight, cost, and cooldown time. I would think you could get a 3/4" thick 8" disk from many glass suppliers and if not ordering one is relatively cheap, probably less than $50. Even if that's not so, blanks show up on CN/Astromart and sometimes eBay often enough that I wouldn't contemplate anything like fusing glass together for an 8-incher.

 

That being said:

 

Mirrors in the 1:14 to 1:18 or 1:20 thickness ratio are generally the thinnest that can be made conventionally, and the greater the proportion of the thickness is eaten up by the depth of the curve of the mirror the more difficult it is to figure one or support it adequately. The old 16" x 1" plate glass portholes used by John Dobson are at the limit of what most mirror makers are comfortable with. I have a 0.8" thick 14.7" but it's quartz which is a little stiffer than regular glass.

 

Mirrors thinner than 1:20 (such as Mel Bartels' ~1:50 25" f/2.6 mirror) have been accomplished by amateurs by making them meniscus-shaped so that the curve is convex on the back to the same degree it is concave on the front side. This makes the mirror behave somewhat as though it's of whatever thickness the curve depth + glass thickness is, and helps ease the support requirements, though they are still pretty "floppy" and certainly not advisable for a beginner. The jury is kind of still out on whether or not these can become a more "mainstream" and practical approach for large mirrors as only a handful of (expert) ATMs have ever bothered with one and even Mel had a fair amount of difficulty - though now that techniques for working with them have been established, it might be easier.

 

It's possible to "sandwich" together glass slabs into a thicker mirror but this process requires a lot of knowledge of glass work. If you're going through the trouble of a kiln, etc. in the first place you probably would want to try a cellular design which is hollow, allowing air to flow through and reducing weight and glass use. These have been achieved to various degrees of success by companies such as Stabilite, Hextek, Fullum, Hubble Optics, and BVCTek the latter three of which are currently offering finished mirrors and blanks which work really great. The key to a good blank with long-term stability and no print-through problems from my understanding is actually fusing the glass on a molecular level as opposed to "gluing" it with some kind of adhesive, which is quite difficult to accomplish. Stabilite had a lot of QC issues, Fullum's TechnoFusion process is extremely expensive, Hubble's process seems to use adhesive at some level of the process and BVCTek's results in a "wavy" mirror back which may not be the most desirable.

 

Fusing together solid sheets of plate glass to make a thick blank can be accomplished somewhat more easily but isn't the most practical approach. One of the issues is that it's hard to fuse them absolutely perfectly and bubbles and other issues always result between layers, so the sagitta of the mirror cannot go below the top layer which can be a bit of a nuisance. That being said, I've seen a few successful examples. Chris Fuld's 41" Dob uses something like a dozen sheets of plate glass fused together, but I recall hearing that he actually had issues with the glass devitrifying on one side of the blank due to the amount of heating/cooling it had to go through. Another guy has been making a lot of (small) fused blanks as well as unsuccessfully attempting cellular blanks with a really basic kiln setup.


Edited by Augustus, 23 July 2021 - 09:52 AM.

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#6 MeridianStarGazer

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Posted 24 July 2021 - 11:47 AM

Nothing wrong with wanting to save money. Maybe this person is retired or unemployed and has lots of time but not much money. Getting a job is still an easier way to get a scope. But ATM often is for the joy of building and learning, or to gain skills to go professional.

 

 

1.5" is definitely overkill. UnitedLens has 8" that is 1.33", which I think is thicker than I would want. But some people still like the old school thickness.

Many many ATM have made 0.75" thickness, and some as thin as 1/2", though that takes more care.



#7 LU1AR

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Posted 28 July 2021 - 12:02 PM

There is currently a trend towards the slumped mirror; which is made by placing a 1/2" window glass plate in an oven, over a mold that has the arrow at the desired F / D ratio.
These mirrors -over 18 "- are lightweight and very fast temperature equilibrium. They have almost no warping.
Remember that a telescope is about 25 milligrams of aluminum, attached to a heavy structure.
Regards.
Edgardo



#8 MeridianStarGazer

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Posted 30 July 2021 - 12:29 PM

Hello. I'm trying to find the glass to make an 8 inch mirror and the book I'm reading has said that it's best to get a piece of glass that is 1.5 inches thick for an 8 in mirror. The local glass shops said that would be crazy expensive and one of the things I read online was the idea of stacking a bunch of thinner cuts of glass to make the desired width. I don't really understand how to do that though which lead to a whole bunch of questions. Are they just gluing it together? Do they cast it in a kiln? How feasible is it to go down that route? 

 

I'd really like to go down this route rather than buying a mirror kit because I want to spend as little money as possible. If it's just cheaper to buy a kit or even a finished mirror, I'd go down that route, but this is currently looking the most affordable. One of the things I read online was that they just went thrifting for glass and, as long as it wasn't tempered, they could create a thicker piece of glass by stacking and combining smaller pieces. Any help is much appreciated!

Best to use a single solid piece. Zambuto has cut thick squares and heated them to spread out into blanks, possibly already having curvature. No bubbles. Also for those who treepan lots of blanks, the scrap is at least 25% the original mass and begs to be melted in a kiln in a mold with a curved bottom. Maybe there would be bubbles, but not nearly as much as with melting powder or even smaller pieces.

 

I've seen thin stuff fuzed together in kilns. That is an option if all you have is thin stuff. But a lot of thin stuff is tempered, so you have to stress test it. Also there will likely be some air trapped between. There then is a limit on how deep you can hog.

 

 

 

Thick plate glass has thermal issues. Thin is often used for that reason, as it can compete with thick supremax. You can buy thick supramax for less than the cost of a kiln. But if you already have a kiln and want to have some fun, go for it. Just read about devitrification and bubbles.


Edited by MeridianStarGazer, 31 July 2021 - 06:06 AM.


#9 greenglass

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Posted 31 July 2021 - 12:47 PM

try and find a big broken aquarium with half inch glass and then find a company that has a waterjet cutter, go directly to the person at the cutter and he might do it for free.  I did this and saved alot.



#10 MeridianStarGazer

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Posted 31 July 2021 - 01:33 PM

try and find a big broken aquarium with half inch glass and then find a company that has a waterjet cutter, go directly to the person at the cutter and he might do it for free.  I did this and saved alot.

They will do samples for free in hopes of getting bigger business. I've been quoted 50 cents per square inch to about tripple that price, $18 per 8" circle.

 

As for diamond blades, they can wear out after 15 cuts, though I hope a lot of that is from insufficient irrigation. Makes the first quote more reasonable.
 




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