The C9.25 XLT OTA has landed. - Page 2

ving (David) · #21 05-04-2006, 03:34 PM

that scope looks wicked!! :drool:
congrats

Robert_T · #22 05-04-2006, 04:47 PM

Wot Asi, you got a c9.25?

On the alum versus carbon fibre, if it's planetary imaging cooling is more important than expansion and the ALum one will probably cool faster. Focus is constantly rechecked and only has to remain fixed for a minute or two at a time so not such an issue with planetary.

cheers,

g__day (Matthew) · #23 05-04-2006, 07:40 PM

I don't understand the relevance of that statement when one compound has almost zero expansion coefficient. Hot or cold it will be the same size over a very wide temperature range.

Or are you refering to the optics at the ends of the tube? Their thermal expansion must be minimal.

I'd imagine with carbon fibre equipment cool down is completely irrelevant.

Starkler (Geoff) · #24 05-04-2006, 09:57 PM

I believe Rob was referring to the CF tube retaining heat inside the ota, whilst the aluminium may better transfer heat to the outside environment, allowing the scope to approach ambient temp quicker.

djacko · #25 05-04-2006, 10:35 PM

Hello Lads

Just new to this forum but thought I'd put in my two bob on the C9.25. It is a popular telescope option in the U.K and we have a couple of friends who swear these scopes are better than Meade, but I can't say with direct experince this is the case but I do know that they produce lovely views. Perhaps because the waether is colder in the north the difference between a carbon fibre and aluminium is very minor. To be honest, I don't really understand what the difference would be considering you're talking about the tube that houses the optics. The only difference would be overall weight and perhaps a little price difference. I've been thinking about a Celestron but am still shopping around at present.

g__day (Matthew) · #26 06-04-2006, 12:29 AM

Starkler,

I understand that, but I fail to see why one needs care about the scope reaching ambient temperature if the optics holder itslef is unreactive to temperature, unless you're saying the lens or mirrors distort with temperature changes, not only any aluminium tubing used?

http://www.atscope.com.au/rcos.html#

Q: Why use a Carbon Fibre tube assembly?
A: Carbon fibre has excellent thermal and strength for weight properties that make it an ideal material for telescope tube assemblies. The carbon tube has significantly less thermal expansion and contraction than an aluminium tube, yet weighs a fraction of a similarly rigid plastic or sonotube (i.e. cardboard) assembly. As a result the telescope will not sag or lose alignment under different orientations or shift focus as the temperature changes during a night's observing

djacko - welcome aboard!

djacko · #27 06-04-2006, 02:22 PM

Thanks for the welcome Tech Guru and the Celestron carbon tube promotional spliff. I can easily visualise, and have experienced flexure in a long tube newtonian, but find it very difficult to visualise a noticeable amount of flexure in a short tube SCT. I'm sure the thermal coefficient properties between them are obvious, in a practical sense it comes down to material thickness and rigidity. It would be nice to see a properly measured comparison between to two models and whether or not there genuinely is a difference that a user should concern themselves with. I think it would be very minor.

g__day (Matthew) · #28 06-04-2006, 04:20 PM

Techie refers to PCs BTW. Personally I’d like to hear from those with practical experience versus me with a physics book and a few years of engineering experience under my belt.

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http://en.wikipedia.org/wiki/Coefficient_of_thermal_expansion#Li near_thermal_expansion_coefficient
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Aluminium’s linear thermal coefficient of expansion is 23 to steel 10.8 to carbon fibre 0.2. So do not expect minimal expansion, and especially if you are at a high magnification factor then your tolerance to thermal expansion will be very low (increasing magnification significantly decreases your window / tolerance for being in-focus).
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So I don’t know, but I expect temperature variances of between 10 – 20 degrees from sunset to sunrise (with the most dramatic falls after sun set and early morning) will cause discernible changes to your focal points with a metal telescope housing. Mind you this is probably less than 1 – 2mm in an aluminium scope (and maybe only ¼ that) but that is sufficient to throw your focus out, especially long exposure and/or high magnification imaging.

PS

I was thinking linear expansion or contraction (not tube flexing, sagging or bending) would be the worry here.

GrampianStars (Rob) · #29 06-04-2006, 04:39 PM

Nice "Asi"

might have to give big "M's" the heave ho
& go to the dark side and buy a "C"

g__day (Matthew) · #30 06-04-2006, 04:46 PM

Nice article about this here:

http://www.tenagraobservatories.com/article2.html

excerpt:

The bad boy on the block when it comes to focusing is the ubiquitous Schmidt-Cassegrain (SCT). How lovely and compact they are! How lovely the first 50 images look after you have focused. How disappointing are those donuts you see in the images after it has gotten colder by 6 degrees C. How aluminum they are. Therein lies the rub. Aluminum loves to contract when the temperature drops. Its coefficient of expansion (contraction) is large compared to steel, invar and carbon. So as the temperature drops, the distance between the primary and secondary in your SCT is smaller therefore the focal point moves farther out. The effects of the pyrex primary mirror shrinking has the opposite effective; this would move the focal point inward. So, given the evidence it is clear that the culprit is aluminum when the telescope focus changes due to temperature. Then why do the scope manufacturers use it? It is light (look at those airplanes flying around) and it is relatively cheap.

casstony · #31 06-04-2006, 11:20 PM

In addition to the advantages of CF, here is some interesting reading I pulled from the MAPUG archives regarding C9.25 design advantages for astrophotography:

"The interesting thing about the C9.25 is that it is *not* the same optical
formula as is used on all other Celestron and Meade SCTs. Instead of an f/2
primary and 5x secondary it has an f/3 primary and 3.3x secondary. That's a
significant difference because it makes for a flatter field at f/10 and
significantly less sensitivity to temperature shifts.

These are fairly important differences. A temperature shift of just half a
degree Celsius (not uncommon during a typical film exposure) will move the
focal plane of a conventional 10" or 11" SCT by 175 - 200 microns, throwing
it out of focus and resulting in the blobby-looking stars commonly seen on
many long-exposure SCT images. On a C9.25, however, that same temperature
shift would only shift the focal point about 70 microns, still within focus
at f/10. It's hard to overstate how important an advantage that is for
long-exposure astrophotos"

I take notice of such discussion as I'm thinking of purchasing an ota larger than my 8" SCT and might want it to be suitable for ccd work. However if someone's getting rid of one of those clunky aluminium C11's for a good price I wouldn't turn my nose up at it :-).

-Tony