Aluminium or Carbon Fibre OTA

matt · #1 03-05-2007, 07:58 PM

I'm casting my eye around at the moment for a potential new OTA.

Just need a quick bit of advice.

I'm wondering what the benefits or drawbacks are of a carbon fibre OTA are over aluminium?

Are there some climates where carbon fibre is better or worse than aluminium?

jase (Jason) · #2 03-05-2007, 09:00 PM

The low thermal expansion/contraction of a carbon fibre tube or trusses can assist with astrophotography, but I personally think much of this is hype. Manufactures claim that carbon fibre tubes/trusses can reduce the amount of times you need to refocus during an astrophotography session. This statement depends on the size of the critical focus zone.

Generally, the faster the optical design (f-ratio), the smaller the critical focus zone (in microns). So if your OTA shrinks or expands due to temperature changes through the night by 100 microns or so and you're using a fast optical system you'd need to check focus regularly. But in all seriousness I've never come across this. More importantly, who doesn’t recheck focus after a few exposures or change of a colour filter? I know I do.

If anything I think the carbon fibre benefits are strength and weight, compared to maintaining critical focus.

iceman (Mike) · #3 03-05-2007, 10:07 PM

Great advice Jase.

What are you looking for matt? What size/type?

matt · #4 03-05-2007, 11:23 PM

Thanks, Jase.

Mike - I'm looking at a number of scopes at the moment.

Among them is a carbon fibre C11, which I've been offered at a very attractive price. Been considering this offer for weeks now.

I can also get the aluminium C11 at an even better price, albeit not a great deal less. However, both prices are very good.

Was really wondering whether it was worth bothering with the CF, while I'm even giving thought to a C11?

Like I said, it's just one scope in a fairly broad range of options. Maybe a Mewlon. Maybe an Intes Micro Mak-Newt...blah blah blah.

Whatever it is, it'll be my new planetary imaging scope.

matt · #5 04-05-2007, 10:49 AM

Of course, it's most likely I'll be selling a very nice C9.25 to help fund this project.

This is the scope which has produced all of my images this Jupiter season and is a proven performer

It's pretty much in 'as new' condition. It was the replacement unit sent by Celestron, after my initial troubles with my original 9.25.

bojan · #6 04-05-2007, 11:28 AM

Another thing to consider is vibration, or ability of the tube to absorb vibration.
My recent experience with resonances inside tube (and spider assembly), triggered by running stepper motors is telling me that Al tube might be the worse choice here, if appropriate steps are not taken (like, a lack of layer of absorbing material inside the tube).

matt · #7 04-05-2007, 11:32 AM

That's probably a fair point, bojan, and one I hadn't factored into my deliberations.

Fewer vibrations would be a good thing for imaging

iceman (Mike) · #8 04-05-2007, 11:37 AM

The stepper motors will not be running while you are imaging, it's just during focus.

matt · #9 04-05-2007, 11:38 AM

OK. Thought he was refering to the tracking motors in his mount

bojan · #10 04-05-2007, 12:15 PM

Quote:

Originally Posted by matt

OK. Thought he was refering to the tracking motors in his mount

Yep, tracking motors.

iceman (Mike) · #11 04-05-2007, 12:20 PM

oh ok. I wouldn't have thought they would introduce vibrations?! That would be very annoying.

matt · #12 04-05-2007, 12:31 PM

Yes. That's what we're talking about

RB (Andrew) · #13 04-05-2007, 12:36 PM

To be honest, I've never heard of vibrations from the drive motors causing such probs.

matt · #14 04-05-2007, 12:38 PM

Nor I, RB.

bojan · #15 04-05-2007, 01:17 PM

It was very annoying, and quite visible at high magnifications.
I think I solved this by attaching the vibration absorbing material at the spider centre, and changing the PWM frequency of the motors.

acropolite (Phil) · #16 04-05-2007, 04:05 PM

Nobody has mentioned cooling but I'm sure I've seen it mentioned elsewhere that Aluminium OTA's reach equalibrium quicker that composite models.

matt · #17 04-05-2007, 05:54 PM

Now, that too is important to keep in mind.

Given I live in Canberra, where the temperature differential between day and night can be quite large, perhaps aluminium is best?

gbeal · #18 04-05-2007, 06:01 PM

You may want to research that temp thing a bit more, as I thought I read it differently with the closed tube. Mind you I could have been wrong.
Gary

matt · #19 04-05-2007, 06:52 PM

Gary.

I have read it on a number of sites that carbon fibre, while very stable thermally, also contracts and expands slower than aluminium.

As a result, it takes longer to come to equilibrium.

Whether that's more of a problem than the primary mirror doing the same thing, I have no idea?

Dennis · #20 04-05-2007, 07:42 PM

Hi Matt

I have a CF C9.25 and the main reason I went for it over the Al variant is that for long exposure, auto guided DSO astro-photography, I wanted the minimum dimensional change in OTA with falling temps, so I didn't have to re-focus the 'scope.

The scenario I am describing is say, taking 20 x 3 min Luminance of M83, followed by 10 x 3 mins each RGB, giving an elapsed time of almost 3 hours (including image download times).

I don’t have a temperature compensating focuser nor do I want the additional cost/complexity of getting one, so I figured a CF tube would minimize the need to re-focus over a 3 hour period.

For Lunar & Planetary imaging, I would be prepared to re-check focus more often, especially with large changes in temperature.

Cheers

Dennis