carbon fibre ota vs metal for imaging?

[casstony]

I've only dipped my toe in the water as far as imaging goes, using an LPI with a LX90 and 80mm refractor. With the SCT ota the constant focus change due to cooling is a pain. My questions are:

(1). will a carbon fibre tube eliminate focus change in falling temperatures
(2). are there still other cooling based problems that will destroy the image when temperatures are falling.

Thanks,
Tony

[g__day]

1. Eliminate no, minimise yes.

You'll still need an autofocuser for long duration astro photgraphy (and an auto guider)

Carbon fibre has a much lower thermal expansion coefficient than aluminium or steel.

From: http://www.rcopticalsystems.com/overview.html

About 2/3 of the way down

Carbon Fiber = Low thermal mass, low expansion, and stable focus

Coefficient of Expansion... (x / unit length / Deg. C x 10 -6 power)
Aluminum = 18.35
Steel = 11.0
Titanium = 8.5
Invar 1.1
Carbon Fiber (High Modulus with Epoxy) = 0.9

see also

http://www.southern-astro.com.au/php/services/ccdprogram.php

2)

Falling temperature would be good for a CCD, eliminate more heat, but mechanical gears and Perodic Error (PEC) of your mount will change as temperature changes, unless your gears are made of welded dipolar exotic metals (one that shrinks matched to one that expands at exactly the same rate as the temperature falls or rises) like watch makers used to make the first chronograph (Harrisons H-1, 2, 3, 4 and Kendall's K-1 (a relative!) ) to pass the accuracy tests for determining Longitude at sea.

Refer http://www.astrosurf.com/demeautis/ep/pe.htm and go into each mounts specifics to see PEC mapped to temperature, e.g.

Losmandy G-11

http://www.astrosurf.com/demeautis/ep/leroy.gif

[Dennis]

In the PEC graph in the post above, I assumed that "Temps" = French for Time rather than temperature?

Cheers

Dennis

[JohnH]

:lol: I certainly hope so - 700C might be a problem for most mounts....

[Robert_T]

For long exposure imaging I can see the Carbon Fibre tubes being of benefit, but for short duration planetary imaging focus is regularly touched up and the most important thing is getting the scope cooled as quickly as possible... aluminimum probably has an edge for this.

cheers,

[astrojase]

CF is the go...

[Brad Moore]

Hi,

I have one of those fancy carbon fibre thingies!

From what I have measured I get the following:

2c shift in temp = .008 mm of movement between the primary and the secondary which = .093 mm at the focal plane.

My CFZ at F6.1 is .084mm at the focal plane. So if my temp moves more than 2c I will be out of focus.

If I image at F9 my CFZ is 0.181mm, which means I can withstand about 4c of temp change.

I also have temp compensation, but I've never used it as I rarely see such a big change over a hour during the night. I also re-focus between sub exposures.

Hope this helps.

Regard,
Brad Moore

[astrojase]

CF expansion? Carbon fibre does not expand...negative coefficient of thermal expansion. Or rather, has a very very low expansion coefficient - depending on the type of weave and direction employed. I am suprised that you were able to measure it Brad! Well done!

Still, compared to aluminium or that sad excuse for metal that most ota's (including some of my own) are made of these days, any improvement is a bonus. Excellent post brad, thanks for the info! ;-)

[Brad Moore]

I should correct myself. My OTA has two T6-6061 Aluminium Dovetails and two Rings as well. The truss is fully CF.

Simple to measure, the secondary focuser has 1/40,000 of a inch encoder res. I then do a focusmax run at the start of the night (before cool down) and one after the temp is stable. Do this over a few nights and you can calculate it.

Cheers,
Brad

[astrojase]

Love the elegant and simple methodology you employed - excellent, and thank you for sharing it too!!!

Just wondering then, where do you think the thermal expansion is coming from? Given that CF does have a very very small expansion coefficient and your measurements are in thousandths of millimetres, do you think it IS the CF trusses or the aluminium connectors?

Cheers mate,
jase.