Why don't really high-end refractors use carbon fibre instead of aluminium tubes?

[g__day (Matthew)]

Qu - Why don't really high-end refractors use carbon fibre instead of aluminium tubes to minimise thermal change induced re-focusing?

Reading up about imaging with APOs - I finally go it that finding and maintain critical focus is a big part of the challenge - and probably why mega expensive, thermal correction, micro calibrated computerised, motorised focuser exist. Phew what a mouthful.

I've re-focused my C9.25 carbon fibre - twice in a year. The Carbon Fibre simply doesn't seem to expand or contract at all - so focus always seems spot one - even when you look really closely. I am sure computer based focusing software would do it a bit better that my eye - but there isn't wild variation.

Why don't high end apos come more commonly in carbon fibre - with its 1/20 the thermal expansion of aluminium? Is it their critical focus zone is so small compared to a SCT that a sub-atomic focuser is always going to be needed ?

[MortonH]

Some do. Check out

http://www.tetontelescope.com/index.php?cPath=53_12_28

If you look at the prices, I think your question is answered!!!

Morton

[Phil]

Depend where you are imaging the focus will change no mater what the tube is made of. I find at bigging of imaging i may be closer to the east
and when i get closer and closer to the zenith the focus will change a little.
Phil

[Kal (Andrew)]

A quote from Roland Christen from Astromart forums (you need to be a member to view)

By the way, Carbon fiber is not an advantage in a refractor, in fact it is a disadvantage if you will be imaging with it. As the temperature drops, the focal length of the lens shortens quite noticeably, so a thermally stable tube that does not shorten will exacerbate the defocus problem. Aluminum at least contracts enough to counteract some of the defocus.

The other problem that I found was in increase in tube currents when using carbon fiber. This was in a 5" refractor, so it might not be as much of a problem in a smaller 90mm size.

Rolando