stupid mirror idea?

[rowan46]

I m at home home at the moment recovering from an operation. Consequently have a lot of time for idle speculation. would it be possible to make large carbon fibre mirrors for a dobsonion. Although expensive I was thinking that with a mould they could be fairly mass produced lighter and stronger than glass without the thermal problems. is this possible or is it the product of too much time at home?

[themos]

Can you polish carbon fibre to the extent that you can do with glass? I doubt it.

[rowan46]

was thinking of some kind of ceramic scrim over the top was thinking of their main use being in the 800mm plus apparture some kind of rib at the back to stop it flexing and voila a lightweight very large mirror.

[LukeSkywatcher]

Can you polish carbon fibre to the extent that you can do with glass? I doubt it.

I was thinking the very same. While it is possible to get a high finish on carbon fibre, i doubt you could polish it to the same degree/level as glass.

Maybe worth a try but i think its gonna be uuber expensive. Mind you...............IF it works you could revolutionize the telescope world.

Try it on a small scale first................say a 1-2" disc.

[FraserClarke]

Maybe worth a try but i think its gonna be uuber expensive. Mind you...............IF it works you could revolutionize the telescope world.

Quite.

It is possible, but is still a research level project. A few protoypes up to ~1m have been made recently, though I'm not sure of the current status. They are the result of several years work, so probably budget around £100k/mirror at the minute...

The tricky bits are I believe the polishing (you can't polish the CF directly, as far as I'm aware), and avoiding print-through of the fibres into the optical surface.

If you can make it work you'll get a lot of interest from professional astronomers and telescope makers...

[LukeSkywatcher]

The tricky bits are I believe the polishing (you can't polish the CF directly, as far as I'm aware), and avoiding print-through of the fibres into the optical surface.

If i understand that comment correctly, i was also thinking the same thing.

CF is a composite which includes FIBRES in its construction. Just how much can you polish a composite containing fibres until those fibres start to be eroded and exposed.

Interesting that so far experiments have been able to make mirrors of 1m diameter. Long way to go before they get to the size of glass mirrors.

But these scientists are a clever bunch of people.

Q: you say that the cost of a 1M CF mirror is about 100K. What is the price to make a similar sized mirror with glass?

[Revs]

I've godda admit I smiled when I read the OP. Polishing carbon fibre?!

But, I Googled it and to my surprise there are people experimenting with carbon fibre composites for mirrors. They're trying to make a huge telescope to take to the moon! Obviously weight is a serious issue for getting it there, so this is something they're working on. Very interesting.

[rowan46]

it was just a thought, i can see some solutions to the weave problem but they may have problems. a scrim of some sort seems the best approach but if this has already been thought of and disregarded then I'm out of ideas oh well back to the daytime tv

[FraserClarke]

CF is a compound which includes FIBRES in its construction. Just how much can you polish a compound containing fibres until those fibres start to be eroded and exposed.

You don't polish the fibres themselves, you coat the CF substrate in a thin layer of metal and polish that. The problem comes that because the substrate (CF) is not homogenous, you get slight changes in the pressure on the polishing surface as the tool moves over it, and that gives you ripples/print-through into the polishing surface.

Interesting that so far experiments have been able to make mirrors of 1m diameter. Long way to go before they get to the size of glass mirrors.

V true -- but you don't need to get to the size of glass mirrors. The current generation of 8m glass mirrors are the largest that will ever be built. Beyond that size a single piece of glass becomes too unweildy to transport (even for 8m mirrors you need to take down street lights and widen tunnels to get them up to telescopes!).

The next generation of extremely large telescopes will be built from a mosaic of 1.4m mirrors...

Q: you say that the cost of a 1M CF mirror is about 100K. What is the price to make a similar sized mirror with glass?

Not sure, but I'd probably be happy budgeting 50-60k£ for one; and not have much risk in getting an optician to deliver it within a six-months to a year.

CF mirrors will become feasible, and they will become cheaper than glass based mirrors -- but not for a little while.

[LukeSkywatcher]

it was just a thought, i can see some solutions to the weave problem but they may have problems. a scrim of some sort seems the best approach but if this has already been thought of and disregarded then I'm out of ideas oh well back to the daytime tv

Rowan.................if you can solve the issue of polishing the weave..................you my friend need to patent it and this time next year you could be a millionaire.

My analogy of the CF mirror is similar to a car tyre.

How long can you drive (polish) before the threads become exposed?.

Only testing will answer that and from that we will find out if CF mirrors for scopes is a viable option.

I cant help but think that if it fails with large mirrors.............it MAY still be a viable option for small mirrors but the price of a scope of maybe 4-6" with a CF mirror would still cost 10 times more then a standard glass mirror.

If nothing else i ADMIRE your thinking. Necessity is the mother of all invention.

My brain is doing double overtime trying to come up with "The next best thing".

Never stop dreaming and thinking.

One of these days you could have a eureka moment and make MILLIONS from it.

Can you tell that i am a HUGE Dragons Den fan?

[FraserClarke]

it was just a thought, i can see some solutions to the weave problem but they may have problems. a scrim of some sort seems the best approach but if this has already been thought of and disregarded then I'm out of ideas oh well back to the daytime tv

If you've got experience with CF, and you're interested in trying to make mirrors, I would hate to discourage you. You may well be able to apply knowledge others don't have, and come up with a good solution. I understand the variety in fibre and resin is huge, so there could be a good combination out there waiting to be used in a mirror...

It is a research area; but not a very big research area at the minute to be honest. Still perfectly possible for someone to make a breakthrough I would have thought. The devil will be in the detail though; making the CF substrate is the easy bit -- turning it into a lambda/10 optical surface is the hard bit...

[Starblazer]

I should imagine that another problem if using cf as the backing to support a thinner glass mirror would be the difference in shape through time and temperature changes.

[LukeSkywatcher]

You don't polish the fibres themselves, you coat the CF substrate in a thin layer of metal and polish that. The problem comes that because the substrate (CF) is not homogenous, you get slight changes in the pressure on the polishing surface as the tool moves over it, and that gives you ripples/print-through into the polishing surface.

V true -- but you don't need to get to the size of glass mirrors. The current generation of 8m glass mirrors are the largest that will ever be built. Beyond that size a single piece of glass becomes too unweildy to transport (even for 8m mirrors you need to take down street lights and widen tunnels to get them up to telescopes!).

The next generation of extremely large telescopes will be built from a mosaic of 1.4m mirrors...

Not sure, but I'd probably be happy budgeting 50-60k£ for one; and not have much risk in getting an optician to deliver it within a six-months to a year.

CF mirrors will become feasible, and they will become cheaper than glass based mirrors -- but not for a little while.

Ah ok. Thanks for that. You have made things make more sense then my brain is doing.

You mention a mosaic of smaller mirrors making up the size of one larger mirror. Isnt that already been used?. Its called "Adaptive Optics". Each smaller mirror can move independently and "adapt" to create the perfect image.

[LukeSkywatcher]

I should imagine that another problem if using cf as the backing to support a thinner glass mirror would be the difference in shape through time and temperature changes.

Rowan pointed out in his opening post that thermal problems would not be an issue.

I can only take his word for that seen as the most i know about CF is that F1 cars and many boats are make of the stuff.

[FraserClarke]

You mention a mosaic of smaller mirrors making up the size of one larger mirror. Isnt that already been used?. Its called "Adaptive Optics". Each smaller mirror can move independently and "adapt" to create the perfect image.

Not quite. Adaptive optics makes fast (100's Hz) changes to a small (~10-20 cm) mirror to correct for atmospheric seeing. These are typically a single sheet of thin (~mm) glass with piezo-electric actuators glued onto the back. You push/pull the actuators to bend the glass very slightly.

Active optics (annoyingly subtle difference of name!) is changing the shape of the telescope primary mirror on the <0.1Hz timescale to optomise it's shape. Yes, in a segmented mirror you would do that by adjusting tip/tilt/piston on each segment. Here is a link to an image of the SALT (10m) primary mirror, which is made up of ~90 hexagonal segments -- to make a bigger mirror, you just add more segments I think the image is with dummy segments in the mirror, which actually makes them easier to see.

File:Salt Mirror.jpg - Wikipedia, the free encyclopedia

Interestingly though, one of the key applications for CF mirrors is to do adaptive optics for big (>20m) telescopes -- because CF is so stiff, you can make a large (few meter) mirror, and throw it about at 100s of Hz. If you tried that with glass, it would vibrate itself to death...

[rowan46]

I have to admit to not having any more than a passing aquaintance of either CF or optics it was just an idle idea to take my mind off the jeremy kyle show it seemed a good idea but I don't have the skill or the knowledge to make anyhing of it so as I said idle speculation on my part

[LukeSkywatcher]

Active optics (annoyingly subtle difference of name!) is changing the shape of the telescope primary mirror on the <0.1Hz timescale to optomise it's shape.

Yes thats the one i meant.

[LukeSkywatcher]

Living on Earth may be expensive, but it includes an annual free trip around the Sun.

Haha i just saw this and it is BRILLIANT.

[thesatanbug]

If it's just a light weight structure you need it could be bonded to ultra thin metal film that could be polished.

[Peter Drew]

It seems that the professional experimenters have use in space or lunar optics in mind where the conditions should be stable and gravity either zero or much less than earthbound. As far as cost is concerned re a glass 1 metre blank, we have a 42.5" diameter by 3.5" thick BVC blank that is generated to F4, edged, back ground flat and a 3" diameter core hole, shipped from Canada at a total cost of under £5K.

[yesyes]

how about making a CF base and pour liquid/molten glass into it making a layer of glass on the CF base? The glass should have just the right thickness to be polished into shape. The CF will give stability.