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The Future Today Part II: CF Mirrors

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#26 JM La Galette

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Posted 09 February 2013 - 03:59 AM

Why then not use a demoulding agent commnly used in composite industry? No need for a chamber. Just need to apply this as uniform as possible. I need to check the thickness once applied but this is pretty thin.

For example this:

http://www.henkelna....d=8797571973262

JMarc

#27 careysub

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Posted 09 February 2013 - 04:10 PM

Why then not use a demoulding agent commnly used in composite industry? No need for a chamber. Just need to apply this as uniform as possible. I need to check the thickness once applied but this is pretty thin.

For example this:

http://www.henkelna....d=8797571973262

JMarc


Because you are transferring optical surfaces that must be preserved. It has to optically perfect, the same tolerances that apply to mirror coating with aluminum.

Dust may not be a serious problem (creating a little roughness) but the layer has to be optically even and I expect vapor deposition in the absence of air currents will be necessary to achieve this.

It may be possible to deposit the even release film using a liquid process analogous to silvering - which coats evenly because the precipitation rate is even and chemically controlled. But silicone can't be created in situ through aqueous chemical reaction, and would have to be deposited from solution or suspension somehow.

And with mirror->mold->mirror scheme you are doing the optical surface transfer twice, imperfections will be cumulative.

#28 FlorinAndrei

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Posted 09 February 2013 - 04:51 PM

There's also the question whether what works well for a professional telescope, installed at a fixed location, in a controlled environment, would also work well for a portable amateur instrument that will be used in a wide variety of locations, environments and temperatures.

Playing devil's advocate:

1. Thermal expansion issues, due to the composite nature of the substrate. Also, composites are typically good insulators, therefore it may actually be harder to achieve thermal equilibrium.

2. Stability issues of the whole telescope, due to the lack of a large stabilizing mass at the bottom.

3. Durability issues, given that the composite material may or may not "flow" in time, and lose its initial perfect shape. Also, glass is chemically stable over very long periods of time, the composite may or may not be quite so stable (e.g. outgassing under the aluminum layer).

4. What happens to the composite if you leave the scope outside, in the summer, under the desert sun, and everything in the main box gets cooked? Will it continue to hold the precise shape at lambda/4 or better?

5. Mechanical shocks. Glass either breaks or doesn't; as long as it doesn't break, it holds its shape with great precision, no matter how much it's knocked and rattled. Would the same remain true for composites, all the way down to 100 nanometers or less?

#29 careysub

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Posted 09 February 2013 - 07:54 PM

I can make some comments about this from the available information.

There's also the question whether what works well for a professional telescope, installed at a fixed location, in a controlled environment, would also work well for a portable amateur instrument that will be used in a wide variety of locations, environments and temperatures.

Playing devil's advocate:

1. Thermal expansion issues, due to the composite nature of the substrate. Also, composites are typically good insulators, therefore it may actually be harder to achieve thermal equilibrium.


The production process used by Composite Mirrors involves prepreg CF/epoxy that is cured at 120C with the optical mold and then cools to room temperature. This is a 100C temperature change where the optical surface is apparently preserved. This suggest some robustness with regard to temperature.

CF/epoxy has about the same thermal expansion as Pyrex (very little) - one of its attractive properties for mirrors.

CF is an excellent thermal conductor, less so when combined with resin, but it is transverse conductivity is still nearly as good as stainless steel. Across the layers the conductivity is about the same as soda-lime glass, but composite mirror itself is quite thin.


2. Stability issues of the whole telescope, due to the lack of a large stabilizing mass at the bottom.


An appropriate design - that treats the very light mirror as an asset and not a problem is needed, yes. Simply using designs developed for bottom heavy scopes won't work very well.

I have read that one mid-century telescope building manual (early edition of Texereau maybe?) thought a telescope that we now see in 100 lb Dob form must necessarily weigh in at 2000 lb or so.

Very light mirrors would look like nothing but a blessing on GEM mounts I would think.

3. Durability issues, given that the composite material may or may not "flow" in time, and lose its initial perfect shape. Also, glass is chemically stable over very long periods of time, the composite may or may not be quite so stable (e.g. outgassing under the aluminum layer).


It has to have backing to give it stiffness (and making a light-weight one that is economical is an issue to be addressed).

Flowing even at the 100 nm level seems very, very unlikely - this isn't a bulk polymer but very high modulus carbon fiber cloth laminated with multiple orientations which never bears any load other than its own very low weight, and does so with the support of its backing material.

Chemical stability is a real issue. It seems much less likely that a CF mirror would remain ready for use a century after fabrication like glass mirrors can.

One major problem is that CF and aluminum are incompatible materials if in physical contact. A very thin layer of epoxy, if unbroken, would protect it, but if there is electrical contact (not even physical contact) then electrochemical erosion of the aluminum would occur. Composite Mirrors found a thin epoxy surface film was necessary anyway to prevent print-through (a problem with other earlier CF mirror attempts). But it is imperative that this layer be preserved.

Also whether conventional coating processes would work is an open question. CM apparently uses a modified process.

4. What happens to the composite if you leave the scope outside, in the summer, under the desert sun, and everything in the main box gets cooked? Will it continue to hold the precise shape at lambda/4 or better?


Maybe not. It would partly depend on the backing.

As I mentioned the CM production process has the material initially at high temperature, but that was under controlled conditions and before coating. This is an extreme case of the thermal issues mentioned earlier.

5. Mechanical shocks. Glass either breaks or doesn't; as long as it doesn't break, it holds its shape with great precision, no matter how much it's knocked and rattled. Would the same remain true for composites, all the way down to 100 nanometers or less?


Mechanical shocks do not generally strike the mirror surface but would be absorbed by the sides or back - i.e. by the mirror support backing. Resistance will depend on how that is implemented - light weight materials though contain a lot of air, transmit shocks poorly, and don't acquire a lot of kinetic energy if you drop it.

#30 JM La Galette

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Posted 10 February 2013 - 05:22 AM

Quote:
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Why then not use a demoulding agent commnly used in composite industry? No need for a chamber. Just need to apply this as uniform as possible. I need to check the thickness once applied but this is pretty thin.

For example this:

http://www.henkelna....d=8797571973262

JMarc


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Because you are transferring optical surfaces that must be preserved. It has to optically perfect, the same tolerances that apply to mirror coating with aluminum.


Careysub, I'm pretty sure coating of demolding agent at that level exists, I need to check. I've in the past also looked at what exist in the patent world and this is also interetsing, a lot to learn.

I personnally think building a composite mirror is an attempt I'll try in the future. But this needs to be carefully prepared, every step is important as you mentionned.

JMarc

#31 FlorinAndrei

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Posted 10 February 2013 - 06:16 PM

An appropriate design - that treats the very light mirror as an asset and not a problem is needed, yes. Simply using designs developed for bottom heavy scopes won't work very well.

I have read that one mid-century telescope building manual (early edition of Texereau maybe?) thought a telescope that we now see in 100 lb Dob form must necessarily weigh in at 2000 lb or so.

Very light mirrors would look like nothing but a blessing on GEM mounts I would think.


I think this is the major point of interest for this design. An RC or DK astrograph, made entirely from composites, including the mirrors, that would stay on a GEM all the time, protected under a dome (no temperature extremes to warp the optics), with an understanding that the relatively cheap optics (much cheaper than conventional glass optics) would have to be replaced every decade or two, and having huge amounts of aperture due to the lightweight design - well, that sounds pretty interesting.

#32 careysub

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Posted 10 February 2013 - 06:35 PM

Thanks, I haven't tried doing a patent search on this yet. Let me know what you find - personal mail if you don't want to post.

#33 JM La Galette

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Posted 11 February 2013 - 03:17 PM

OK, let me a few days and I'll send you an e-mail.
JMarc

#34 Datapanic

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Posted 11 February 2013 - 08:05 PM

I will be stopping by Composite Mirrors Associates this week to bring a mirror to them for recoating - they do amateur glass mirror recoating as a service and it's local so no worries about shipping.

Going to ask them if I can see some of their scopes they've made - will report back later!

#35 UmaDog

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Posted 11 February 2013 - 09:55 PM

Cool, let us know!

#36 Datapanic

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Posted 12 February 2013 - 11:24 PM

Today, I brought in my '72 Cave 8" f/4.4 mirror to Composite Mirror Associates (CMA) for the recoating process. I selected CMA because it's local to me and consequently, no risk of damage by shipping. If they do good by this mirror, then I'll bring in some more mirrors that need it - my '75 Cave 8" f/7, the Horsetrail Cave 12.5" f/7.5 and an RV-6 mirror.

I only got to enter the receptionist area. The first thing I saw were a bunch of composite mirrors hanging on the opposite wall - all over 12" in diameter. The second thing I noticed was the 16" Cassegrain prototype on a modern computerized mount in front of the receptionist desk. The receptionist came out in about a minute and I introduced myself and shortly afterwards, Bob Romeo, the CEO of CMA came out! He inspected the Cave mirror and secondary and told me it would be about a week for them to be recoated, I replied that is fine because I have the entire scope in pieces and need to order new stainless fasteners and paint parts and that'll take much longer than a week. Bob is a really bright guy. He's been in the optical profession for quite a while and I think went to the University of Chicago as well as the University of Arizona.

I asked if composite mirrors was his idea, he told me that it was originally developed at JPL and he took the idea and ran with it many years ago. CMA has been around for 20 years. I asked him about thermal equilibrium and he said that these mirrors don't experience thermal adjustment problems like traditional mirrors. He even mentioned that tests using propane torches on the surface of the mirrors did not adversely affect surface integrity for more than a minute - temperature accumulations are not an issue.

I asked him about going commercial, and he said there are plans for that eventually.

When I go back in a week or two to pick up my mirror and bring in some others for recoating, I hope to take a look at the 1-meter Cassegrain prototype. It was on loan to the University of Arizona and to be returned later this week.

CMA seems to be on top of the technology, building the next generation of optics and utilizing Adaptive Optics as well.

This technology is definitely leading edge and hopefully will fall in to the commercial scope world soon.

#37 JM La Galette

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Posted 13 February 2013 - 04:07 PM

Interesting, thanks.
JMarc

#38 careysub

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Posted 14 February 2013 - 01:28 PM

That is exciting.

Maybe some time soon they will open up pre-orders. :grin:

#39 csrlice12

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Posted 14 February 2013 - 01:52 PM


Keep the traditional design and use sandbags/water bladders inside the rocker box?


I'm not sure I would want the balance to shift as sand shifted.

On the other hand, perhaps wood slabs with lead 'sticks' inserted into holes drilled into the wood, and then sealed with putty would work well. If the slabs slid into brackets attached to the inside of the mirror box, there should be essentially no shift as the altitude of the scope changed.


Or just a couple of bricks velcroed to the bottom of the box for that matter.

But my thinking is to have a large aperture hand-transportable scope; lugging lead weights around would defeat the purpose. Whereas water or sand could be obtainable at the observing site, and disposed of immediately after the observing session. Bladders with "bulkheads"(separated compartments) would prevent the sand or water shifting. In oil tanker ships the oil is separated into compartments so that when the ship rolls on the swell, the liquid inside doesn't slosh around and compound the roll. Same idea.

Come to think of it, why not make all the structural elements (rockerbox, UTA, trusses) hollow sealed CF units? - light, transportable, snaps together. Fill it with water at the observing site for added mass and stability, empty it out when done. If you wanted to get really clever you could design it in such way that the water leaks out at a precise rate that alters the balance and pointing of the scope...at sidereal rate... :grin:

I think Careysub could be right, perhaps it's time to look to innovative designs that take into account a much less massive bottom end...


only problem is if the hole gets blocked, and the water inside freezes......water expands when it freezes.....goodbye mount....

#40 PEterW

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Posted 19 February 2013 - 02:36 PM

Sounds like just a case of scaling up the production to get prices down.... Recoating would be interesting, but a decent protective coating would negate this. Cheap, BIG mirrors would really be a game changer and turn the market upside down! For low power views we don't need super fine accuracy either. It would be a shame if this technology stays too costly.

PeterW






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