Jump to content


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Carbon fiber primary mirror

This topic has been archived. This means that you cannot reply to this topic.
26 replies to this topic

#1 StarGazer235

StarGazer235

    Lift Off

  • -----
  • Posts: 16
  • Joined: 11 Sep 2009

Posted 09 May 2010 - 11:51 PM

So there I was daydreaming about building a scope and thinking, "hmmm, a carbon fiber tube would be nice and it would make it more portable...."

Then I thought, "hmmm, that heavy mirror kind of kills the weigth advantage of a carbon fiber tube..."

Then I thought, "Hey, why not make the primary mirror out of carbon fiber? Wow, that would be genius..."

Carbon fiber has exceptional strenght and thermal stability - not sure about flex, but I would think it very rigid.

Here's how I imagined it:
- mold some carbon fiber into the general shape of a primary mirror, as precise as possible
- make it thick enough or honeycombed/sandwich structured to ensure proper stiffness
- as the carbon fiber's surface is most likely not optical grade, pour glass (or another substrate?) into it while spinning to get a fairly thin, even coat
- figure that new coat as you would figure a normal glass/pyrex blank

Of course, this is mostly theoretical, but nonetheless, would it be feasible?
Could anything else than glass be used for the optical coat?
Could epoxy be used? Vitrified ceramic?

(As for existing carbon fiber stuff, these guys make awesome scopes: Dreamscopes. )

#2 polaraligned

polaraligned

    Viking 1

  • -----
  • Posts: 694
  • Joined: 26 Dec 2008

Posted 10 May 2010 - 05:23 AM


(As for existing carbon fiber stuff, these guys make awesome scopes: Dreamscopes. )


Did you see Dream's 24" mirror that weighs 28#?
http://www.dreamcell...lc.com/60cm.htm
How much lighter can you ask for. And the mirror is not
haphazardly lightened. It has a ton of PhD level engineering
behind it. It is made to sit in a custom carbon fiber cell
that has a CTE closely matched to the glass. I think Dreams
total weight for a 24" f/3.5 imaging OTA is going to be about 100#. Compare that to an RCOS 24" that is 350#.

#3 m. allan noah

m. allan noah

    Viking 1

  • -----
  • Posts: 769
  • Joined: 14 Aug 2009

Posted 10 May 2010 - 07:57 AM

pour glass (or another substrate?) into it (carbon fiber shell)


That would be a really neat way to turn carbon fiber into smoke :)

allan

#4 dane lambson

dane lambson

    Ranger 4

  • -----
  • Posts: 324
  • Joined: 18 Sep 2007

Posted 10 May 2010 - 08:43 AM

I remember seeing an 8" cellular carbon mirror on display in a museum of aeronautics at Tuscon Az. I don't remember which firm made it but it was for a satellite. It looked to weight less than a pound, so it has been done.
later
Dane

#5 StarGazer235

StarGazer235

    Lift Off

  • -----
  • Posts: 16
  • Joined: 11 Sep 2009

Posted 10 May 2010 - 09:37 AM



(As for existing carbon fiber stuff, these guys make awesome scopes: Dreamscopes. )


Did you see Dream's 24" mirror that weighs 28#?
http://www.dreamcell...lc.com/60cm.htm
How much lighter can you ask for. And the mirror is not
haphazardly lightened. It has a ton of PhD level engineering
behind it. It is made to sit in a custom carbon fiber cell
that has a CTE closely matched to the glass. I think Dreams
total weight for a 24" f/3.5 imaging OTA is going to be about 100#. Compare that to an RCOS 24" that is 350#.


Yeah, their mirrors are amazing, and probably the lightest available.
How much lighter could I want it? Hmmm, i dunno, how about: LIGHTER!
As in near carbon fiber light.
As in I won't need a 20lbs counterweight.
As in my granny could haul it out.

But you miss the point of the discussion: regardless of what is on the market now, is the idea plausible/feasible or not?

#6 StarGazer235

StarGazer235

    Lift Off

  • -----
  • Posts: 16
  • Joined: 11 Sep 2009

Posted 10 May 2010 - 09:46 AM

pour glass (or another substrate?) into it (carbon fiber shell)


That would be a really neat way to turn carbon fiber into smoke :)

allan


I thought carbon fiber was very resistant to high temperatures?
Some carbon fiber is heat treated at over 2200C: would it resist to molten glass?

(I have about zero engineering knowledge of carbon fiber - or any material for that matter...)

#7 TxStars

TxStars

    Apollo

  • *****
  • Posts: 1410
  • Joined: 01 Oct 2005

Posted 10 May 2010 - 09:48 AM

Using Ceramic for the mirror base with glass over that would be easier.
It would not be as light as carbonfiber but it would take the heat of melting the glass with no problem.
Think about something along the lines of Shuttle tiles covered with glass.

Might be able to use fused silica even. :question:

And it would be a lot cheaper than the Carbon-Carbon space miror. $$$$

#8 thrawn

thrawn

    Viking 1

  • -----
  • Posts: 983
  • Joined: 14 Mar 2010

Posted 10 May 2010 - 09:59 AM

Speculating for you, there's no need to pour hot glass on the thing: use a very thin generated blank, and stick it on a very stiff CF or else ceramic substrate. Not sure, perhaps you could use a special glue so the mirror part feels like it's floating.

#9 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 46024
  • Joined: 16 Jun 2004

Posted 10 May 2010 - 10:14 AM

I thought carbon fiber was very resistant to high temperatures?
Some carbon fiber is heat treated at over 2200C: would it resist to molten glass?



The reality is that when people talk about Carbon Fiber, there are actually referring to Carbon Fiber Reinforced Composites, i.e. composites made from carbon fibers and and a matrix material which bonds the whole mess together. Typically the matrix is an epoxy.

The limiting temperatures depend on the less of the two, whether it is the matrix or the carbon fibers. Carbon will burn... heat treatments would be done in an inert atmosphere.

I don't think that composites are a good choice for a mirror. They just have the important properties that make for a stable structure at 50 nanometers scale.

Jon

#10 MitchAlsup

MitchAlsup

    Surveyor 1

  • -----
  • Posts: 1694
  • Joined: 31 Aug 2009

Posted 10 May 2010 - 11:33 AM

The problem with carbon fiber is the epoxy (resin). Thus if you had a perfet blank that had been constructed for zero thermal expansion and wanted to grind and polish you are faced with an inhomogenous surface (35% carbon fibers (hard) and 65% expoy (not so hard).) Finding a lap that would polish both at the same time is "nadda gonna be easy"

#11 dave b

dave b

    Aurora

  • -----
  • Posts: 4501
  • Joined: 09 May 2005

Posted 10 May 2010 - 12:01 PM

those carbon mirrors you see are Silicon Carbide, not pure carbon:


http://www.esa.int/e...QD_index_0.html

#12 StarGazer235

StarGazer235

    Lift Off

  • -----
  • Posts: 16
  • Joined: 11 Sep 2009

Posted 10 May 2010 - 12:20 PM

Interesting!

I like the idea of sticking/cementing a very thin blank to a CF cell and then grinding and figuring out a mirror out of that assembly...

How thin do you think you could safely go for the blank ( % vs sagitta)?

Do you think a CF cemented very thin blank would be stiff enough to be optically stable?

#13 dave b

dave b

    Aurora

  • -----
  • Posts: 4501
  • Joined: 09 May 2005

Posted 10 May 2010 - 12:51 PM

also, dont forget that as the epoxy/polyester resins age, the figure of the mirror will be lost

#14 StarGazer235

StarGazer235

    Lift Off

  • -----
  • Posts: 16
  • Joined: 11 Sep 2009

Posted 10 May 2010 - 10:51 PM

Ah ha! found it!
I knew there had to exist some no-shrinkage epoxy:

"Epoxy resin copolymer with zero shrinkage can be obtained by copolymerizing epoxy resin E51 with the expanding monomer [..lots of incomprehensible chemistry stuff...]. This epoxy resin copolymer with zero shrinkage has been used as an adhesive to join the optical parts of a large optical telescope."


It does sound very exotic, but at least it's one for the 'feasible' column! ;)

More info here: Exotic epoxy white paper

#15 Ken Kobayashi

Ken Kobayashi

    Ranger 4

  • *****
  • Posts: 347
  • Joined: 09 Apr 2007

Posted 10 May 2010 - 11:54 PM

Here's one paper by former colleagues of mine. Though I don't think any further work was done by that group.

http://adsabs.harvar...SPIE.5487.1084T

#16 nytecam

nytecam

    Voyager 1

  • *****
  • Posts: 11635
  • Joined: 20 Aug 2005

Posted 11 May 2010 - 02:53 AM

This subject seems to be going around-and-around :shocked: so a carbon fibre mirror can be shaped and stable to a fraction of the wavelength of light :roflmao: More like a crude flux collector maybe :lol:

#17 JohnH

JohnH

    Vanguard

  • ****-
  • Posts: 2213
  • Joined: 04 Oct 2005

Posted 11 May 2010 - 09:31 AM

The lightest mirror I have ever seen was a framework of beryllium with a polished surface. The weight was less than 800 grams for around 11 inches in diameter.

Considering the cost was "on the order of $65,000" it seems unlikely amateurs would be lining up for these.

#18 Ken Kobayashi

Ken Kobayashi

    Ranger 4

  • *****
  • Posts: 347
  • Joined: 09 Apr 2007

Posted 11 May 2010 - 10:33 AM

The lightest mirror I have ever seen was a framework of beryllium with a polished surface. The weight was less than 800 grams for around 11 inches in diameter.


And the JWST uses beryllium mirrors. Each segment is about 51 inches in diameter, and weighs 46 lb.

#19 TxStars

TxStars

    Apollo

  • *****
  • Posts: 1410
  • Joined: 01 Oct 2005

Posted 11 May 2010 - 01:40 PM

If you need an epoxy for some crazy project look at what these guys offer:
http://masterbond.com/overview.html

#20 refractory

refractory

    Apollo

  • *****
  • Posts: 1337
  • Joined: 05 Feb 2005

Posted 11 May 2010 - 04:36 PM

You've likely seen graphite, which is made of microscopic plates stacked together. Graphene is a newly synthesized 2D crystal based on the same chemistry. Already there is work on something called graphene paper, where relatively large sheets of this material are bonded together.

Graphene, done right, is absolutely smooth, though individual sheets in solution exhibit waves propagating through the material. But bonded multilayer structures should be pretty stable.

Graphene can also stretch to a good degree without splitting, so it should be possible to figure out a way to form the stacked structure in or on a mandrel without surface distortions, if such stretches can be spaced right and locked in.

Then you'd add other materials to reflectivize (perhaps under the first layer, thus protecting this surface indefinitely, since graphene is impervious to oxygen or other larger molecules passing through. Graphene transmits about 97 or 98 percent of visible light.

In the end you would have an ultralightweight, stable mirror with ultrasmooth surface. I've already posted elsewhere that a single sheet would be ideal for space telescopes- manufactured in space itself one could make sheets millions of miles across, with only a tiny total mass.

Jess Tauber

#21 mconnelley

mconnelley

    Viking 1

  • -----
  • Posts: 500
  • Joined: 14 Mar 2006

Posted 12 May 2010 - 01:08 AM

Hello:

One epoxy I've had my eye on is Masterbond EP30LTE-LO. This is a low CTE epoxy with low shrinkage and low outgassing. The CTE is advertised to be 12e-6 /C and the shrinkage is supposed to be 0.02%. I'm interested in it to bond two zerodur pieces together for our secondary mirror.

Cheers
Mike Connelley

#22 TeaDwarf

TeaDwarf

    Lift Off

  • -----
  • Posts: 11
  • Joined: 20 Feb 2010

Posted 12 May 2010 - 11:40 AM

There is on-going work in this field -- stiffness is the main concern, rather than outright weight (obviously the two are related).

e.g.;

http://spiedl.aip.or...gifs=yes&ref=no

#23 salies

salies

    Explorer 1

  • -----
  • Posts: 70
  • Joined: 10 Aug 2009

Posted 15 May 2010 - 05:31 PM

hello! I am in my 3e test on a hybrid mirror glass and Carbon.
the main challenges is to find a bubble-free bonding
for now, I pray for a lot of tinkering, but it allows me to power controller that the bonding does not move in time, it is as well!
http://forums.futura...nsportable.html

#24 jpcannavo

jpcannavo

    Viking 1

  • *****
  • Posts: 924
  • Joined: 21 Feb 2005

Posted 15 May 2010 - 08:29 PM

In bonding glass to graphite you now introduce the issue of differential coefficients of expansion (same principle that allows a bimetalic strip to measure temperature). If these coefficients are not identical - or very close - you would get optically significant distortion of the surface with temperature changes.
As for Dream blanks - these do seem to cut to the chase! I am toying with the idea of a cellular mirror for a fast 22"-24". What I am not clear on, however, is who figures their blanks? I wonder if the issue of print-through comes up for opticians who are used to monoliths?
One construction point: Cellular blanks, being lighter, also favor large dobs, where a shorter truss assembly yields a shorter lever to oppose the moment arm at the mirror box.
Joe

#25 Owen

Owen

    Viking 1

  • -----
  • Posts: 668
  • Joined: 21 Jun 2007

Posted 16 May 2010 - 03:32 AM

Hi Joseph,

I suspect print through is common, when the tool grinding is of significant weight - the thin unsupported glass deforms to 'avoid' the grinding, so slow and gentle, should avoid some of this...

Owen


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.



Recent Reviews


Imaging/Sketching Contest






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics