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The future today: carved mirrors at 1/3 the weight

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#1 pbsastro

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Posted 12 January 2013 - 09:05 AM

http://www.kickstart...is-it-recycl...
http://www.opticwavelabs.com/index.php

This is the future. I hope 10 years from now, solid mirrors will be considered cave age technology, and most mirrors will be carved. There is simply no reason for us to carry all the weight we do today like mules, and take up with these long cool down times.
With 1/3 the weight and less than 1/10 of cool down time, I wonder why is this still not mainstream today?
Moreover, using less glass, once the process is established, it will result in cheaper and environment friendly products.

Carved material is not new in astro business. Astro-Physics was one of the big promoters of carved aluminum on their mounts and scope tubes, including their Maks.
I think carved glass is next, and it is here now. I plan/hope to get myself a 24" f/3.3 in a couple of years.

Pedro

#2 azure1961p

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Posted 12 January 2013 - 09:35 AM

Well it's a nice concept but being that glass will sag, bulge and everything se simply by sitting it wrong I don't see how super thin mirrors could ever keep their proper shape to say nothing over the wavefront value continually morphing as the scope is shifted.

Unless the glass is somehow stiffer I don't see how anything but the lousiest optics would come out of it.

Pete

#3 Jon Isaacs

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Posted 12 January 2013 - 09:47 AM

http://www.kickstart...is-it-recycl...
http://www.opticwavelabs.com/index.php

This is the future. I hope 10 years from now, solid mirrors will be considered cave age technology, and most mirrors will be carved. There is simply no reason for us to carry all the weight we do today like mules, and take up with these long cool down times.
With 1/3 the weight and less than 1/10 of cool down time, I wonder why is this still not mainstream today?
Moreover, using less glass, once the process is established, it will result in cheaper and environment friendly products.

Carved material is not new in astro business. Astro-Physics was one of the big promoters of carved aluminum on their mounts and scope tubes, including their Maks.
I think carved glass is next, and it is here now. I plan/hope to get myself a 24" f/3.3 in a couple of years.

Pedro


Cellular mirrors are not new. The Hale 200 inch mirror at Mt Palomar is block design. People are making cellular mirrors. There are issues, print-through is one.

Someone like Mark Cowan who posts here probably has made Cellular mirrors...

Jon

#4 pbsastro

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Posted 12 January 2013 - 11:14 AM

Pete, by carving a material the right way you can sacrifice very little of rigidity and save a lot of mass.
Some examples:
a) Astro-Physics mounts are all carved : http://www.astro-phy...1200gto/1200gto
b) Look at this Astro-Physics recommended "Flat Surface Adapter for the 3600GTO " near the bottom of this page: http://www.astro-phy...600gto/3600gto. Note the "Solid as a rock" on top of page.
c) And (attached) my own Rob Miller Tripod made of aluminium only 1.5mm thick !!!! If this 1.5mm pieces were single and not part of this structure I would be able to bend them with my hands, but this way they can handle more than 200 lbs rigidly.

Pete I understand your point that current solid mirrors have issues if not sited correctly. However that is because they are not thick enough. The required thickness for the required rigidity would be more than it would be acceptable weight wise.
However using carved mirrors we can actually get much more rigid mirrors than current mirrors and at same time with less weight. They will be more rigid because we can make them thicker, and they less weight because they are carved.

Pedro

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#5 pbsastro

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Posted 12 January 2013 - 11:25 AM

Cellular mirrors are not new. The Hale 200 inch mirror at Mt Palomar is block design. People are making cellular mirrors. There are issues, print-through is one.

Someone like Mark Cowan who posts here probably has made Cellular mirrors...

Jon

Jon I would be very interested to hear about the issues found. This print-through you mention is similar to what we get with too thin book pages?

Even if there are issues with cellular mirrors, there are also severe issues (to me) with current (large) mirrors: weight and cool down time. So even in that case it will be a trade-off and choosing what is more important to each one.

Pedro

#6 Jarad

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Posted 12 January 2013 - 11:27 AM

Print through is an issue, but one that can be dealt with with proper design and technique.

I think the bigger issue preventing cellular mirrors from becoming popular is economics. Paying for expensive glass, then breaking or cutting it up, then melting or annealing it, then figuring it costs significantly more than figuring a normal blank.

So if he can really get people to pay him to take their recycled pyrex, maybe he can change that. If a ready to figure cellular blank cost the same as a monolithic one, I think they would be much more feasible to market.

I wish him luck.

Jarad

#7 SeattleScott

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Posted 12 January 2013 - 12:52 PM

Another option is quartz mirrors. Due to their greater strength they can be thinner than glass mirrors. But again, they are pricey.

Most of the major observatories have these cellular mirrors, but then they also have platforms that can adjust many times per second in order to maintain the shape of the mirror as the weight shifts due to pointing in different directions. Also they cut it too close with the Discovery mirror (and some others) and made the mirror too thin, causing it to break. That being said, I have seen these mirrors made for amateur astronomy. I stopped looking though after I saw the price.

#8 ccaissie

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Posted 12 January 2013 - 01:10 PM

Conical mirror blanks.

#9 mark cowan

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Posted 12 January 2013 - 03:28 PM

Someone like Mark Cowan who posts here probably has made Cellular mirrors...


No, actually. I have gotten quotes on properly designed and fabricated cellular mirror blanks (can't say where, TOS) in large sizes but combining light weight and support IN the blank that way results in a price that'll make you go "ouch." This for borosilicate blanks.

And I've had various lower cost "solutions" in my shop for testing but none of them fly right.

Greg Rhode (no longer a vendor) and I discussed in some detail the practicality of carving out the back of monolithic quartz mirrors for light-weighting, but basically gave it up as (a) too expensive (b) not reliable (breakage) and © just generally problematic, given that you have to relieve stress in all the pockets AFTER you do the carving. Single crystal silicon can be carved without inducing stress - it's used for space bound mirrors - but the price is literally astronomical.

So I'm taking a different approach to this particular holy grail but due to patent secrecy requirements I'm not talking about it at all in public. :poke:

Best,
Mark

#10 Jon Isaacs

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Posted 12 January 2013 - 03:36 PM

So if he can really get people to pay him to take their recycled pyrex, maybe he can change that.



There was a thread in the ATM forum about using recycled Pyrex. One problem is that today, apparently most "Pyrex" is not Pyrex, i.e, Borosilicate..

Jon

#11 Jon Isaacs

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Posted 12 January 2013 - 03:53 PM

c) And (attached) my own Rob Miller Tripod made of aluminium only 1.5mm thick !!!! If this 1.5mm pieces were single and not part of this structure I would be able to bend them with my hands, but this way they can handle more than 200 lbs rigidly.



If you have ever ridden a top quality steel racing bicycle, the tubes are butted and the center sections are typically 0.5mm wall.

Mirrors are modeled as plates in bending. The rigidity of a plate increases with the cube of the thickness but the weight load increases linearly. This means that a mirror that is twice as thick is 8 times as stiff and but only has twice the load so the deflection is 1/4 that of the thinner mirror. Old style mirrors were thick enough that they did not require sophisticated support systems but modern thin mirrors that are designed to be transported and under go large temperature differences cool more quickly but require careful attention to the support.

Carving out a mirror essential redistributes the mass in a manner similar to an I-beam, with the same mass it is stiffer though for the same thickness it is not... But with a complex honeycomb design with support structures built into the backside of the mirror, the deflections become much more complicated because you no longer have a uniform structure that is being loaded.

When deflections are measured in nanometers, everything has to be just right.

Jon

#12 Jon Isaacs

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Posted 12 January 2013 - 03:56 PM

So I'm taking a different approach to this particular holy grail but due to patent secrecy requirements I'm not talking about it at all in public. :poke:

Best,
Mark



So that was you in ATM with the Mylar film mirror, I wondered who that was... :poke:

Sorry, I just couldn't help myself.. :(

Jon

#13 pbsastro

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Posted 12 January 2013 - 04:04 PM

So I'm taking a different approach to this particular holy grail but due to patent secrecy requirements I'm not talking about it at all in public. :poke:


Mark, thanks for chiming in.
That is great news that you are working on an alternative solution. No need to tell any info. All I would like to know are your estimations in relative (percentage) of comparison of your solution to classic mirror: weight reduction, cool time reduction, cost increase. Al this say for a 24" f/3 mirror.

Pedro

#14 pbsastro

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Posted 12 January 2013 - 04:21 PM

Old style mirrors were thick enough that they did not require sophisticated support systems but modern thin mirrors that are designed to be transported and under go large temperature differences cool more quickly but require careful attention to the support.

Carving out a mirror essential redistributes the mass in a manner similar to an I-beam, with the same mass it is stiffer though for the same thickness it is not... But with a complex honeycomb design with support structures built into the backside of the mirror, the deflections become much more complicated because you no longer have a uniform structure that is being loaded.
When deflections are measured in nanometers, everything has to be just right.


I guess with carved mirrors, the thickness will increase again. This way we can get the best of both worlds.
Regarding the complexity of the structure, today we have computers for that :). Look at how F1 cars changed in 30 years, due to computer modelation.

Pedro

#15 Jon Isaacs

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Posted 12 January 2013 - 04:40 PM


Old style mirrors were thick enough that they did not require sophisticated support systems but modern thin mirrors that are designed to be transported and under go large temperature differences cool more quickly but require careful attention to the support.

Carving out a mirror essential redistributes the mass in a manner similar to an I-beam, with the same mass it is stiffer though for the same thickness it is not... But with a complex honeycomb design with support structures built into the backside of the mirror, the deflections become much more complicated because you no longer have a uniform structure that is being loaded.
When deflections are measured in nanometers, everything has to be just right.


I guess with carved mirrors, the thickness will increase again. This way we can get the best of both worlds.
Regarding the complexity of the structure, today we have computers for that :). Look at how F1 cars changed in 30 years, due to computer modelation.

Pedro


Pedro:

I am a research engineer/scientist who works in the field of "advanced" materials. I am well aware of computer modeling for looking at the deflections in complex structures. I am sure that is the hope, that a properly designed structure can make for a lighter mirror while maintaining the necessary precision. Finite Element Modeling only takes you so far though..

Compared to the surface of a telescope mirror, a Formula 1 car is a crude approximation of perfection.

Jon

#16 careysub

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Posted 12 January 2013 - 09:47 PM

...
So if he can really get people to pay him to take their recycled pyrex, maybe he can change that. ...


I find Pyrex glass rod (the form the raw material comes in apparently) listed for glass-blowers at $8/lb.

Lets see, a 20" mirror blank - would it need to be full thickness? If so then it looks like it would weigh 108 lb and cost $850. If thinner, it would be cheaper. $850 is a lot for raw material, but premium 20" mirrors run $4000-6000 so it would looks like a premium for the ultra-light weight should be able to absorb it.

But would someone making these mirrors maybe be set up for melting their own blanks? Getting Pyrex mirror blanks seems much harder than just getting Pyrex. If so then scrap might could just be remelted.

#17 mark cowan

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Posted 12 January 2013 - 10:08 PM

So that was you in ATM with the Mylar film mirror, I wondered who that was...


In yer dreams, in yer dreams. :lol:

Best,
Mark

#18 bremms

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Posted 13 January 2013 - 12:36 AM

Thin mirrors make no sense when you can make an egg crate or a waffle mirror. They are a bit expensive to cast, but if annealed properly will hold a good figure.
F1 car?. Man there is more engineering in an F1 suspension arm than a telescopes mirror. Believe me, did work in both industries. An F1 car has more in common with Hubble space telescope than a family car.






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