GSO 10" RC white or carbon fibre?????

[batema]

I am possibly looking at recking the run of fine weather on the sunshine coast and wonder if there is a benefit for the carbon fibre tube over the aluminium white. I have looked at the weight and found a site that tells me the astro tech 10"Rc which I think is the same scope has the carbon fibre at 36 pounds and the white aluminium at 33 pounds. Luke from Andrews weighed the CF today OTA and it came in at 16Kg where I have seen it listed at 13.6Kg. I have no idea which way to go as I am looking at stretching the limits of my EQ6Pro with a longer counterweight bar and different lasmandy style head. Any advice on which tube. Allan I see you have the white. Was there a reason for this preference??

Thanks

Mark

[Sarge]

Mark,

If you can afford it, go for the CF tube. It will reduce cooling time, it is considerably lighter and also stronger.

Clear skies

Rod
:D:D

[LightningNZ]

And the focus won't change nearly as much with the change in temperature as it cools.

To be honest I don't know why they offer the metal tubes when the difference is only $200 on a $1000+ item.

-Cam

[g__day]

My CF C9.25 hold focus as temperatures change amazingly well...

[Paul Haese]

Carbon fibre is what you want. I am a bit puzzled why GSO have not brought out the 12 with a carbon tube. I have heard some interesting buzz though about future developments.

[casstony]

I recall a post from someone at Astronomics to the effect that they weren't keen to introduce a carbon fibre 12" because the CF 10" had not sold well; I imagine GSO takes cues from them since Astro-Tech is probably a big part of their market.

[Marke]

Price difference between the 2 at moment is $500 it was $700 at one point.
The CF does hold focus better but the steel tube will cool much quicker and for the price difference you can buy a robo focus or similar with temp compensation - just my 2c worth.

[allan gould]

CF vs Aluminium tube. Agreed less weight but the cf tube has two large Losmandy style plates top and bottom. Would these not try to expand and contract re temperature variation?
I find with my 10" al tube that running the fans quickly brings the temp down for cooling whereas I suspect the cf tube would be at a higher temp for longer. Practically I find that if I refocus after 30-45 min of imaging then the focus stays put, it sometimes has a very small focus shift.

[clive milne]

Really... are you sure about that?

One of the advantages of CF is that it has a very low thermal mass (meaning it doesn't have to transfer a whole lot of heat energy to the air to reach thermal equilibrium)

That is not to say that all CF tubes are created equal.
The percentage of epoxy and filler content obviously has an impact on performance and varies from manufacturer to manufacturer...

But yes, tough call as to whether a good ascom compliant focuser with temperature compensation might be a better way to spend the extra cash.

~c

[Marke]

I am positive - cf is an insulator so it takes a lot longer to equalise temp
as it will keep the inside of the tube at what ever temp it was left at for longer. Steel tube will transfer the heat much quicker reaching equalibrium
faster which is also why it changes focus more quickly . You can have one or the other.
Its why you dont see big refractors in CF either they would never cool down the inside air would be insulated even more .

[clive milne]

Mark, you are getting a couple of things confused here.

[brian nordstrom]

:question: I think its more to do with expansion / contraction issues as the temperature changes rather than insulation . ;)
An alloy tube will move up to 1mm in extreme temperature changes where the CF will only expand/contract 1/10th of that .and 1mm a lot at critical CCD focus .
Thats why CF is better , oh yes its also a little lighter and looks so sexy .
Brian.

[Marke]

I am pretty sure , I am not talking about expansion and effects on focus I know thats a given . My statement regards cooling down an ota
will equalise temps a lot slower if its made of CF than it will if its metal .

[clive milne]

OK, in the interests of trying to illicit a more complete understanding of the issue, I'll present the response in the form of questions... (thereby taking my opinion out of it)

Disregarding focus issues, what effect does the temperature of an OTA have on the wavefront?

More succinctly... why does the temperature of the OTA impact the wavefront?

[brian nordstrom]

:) True , but I would rather have a longer , in time 1/10th mm contraction , rather than a faster 1mm contraction .I could live with that .
Thats my own opinion .
Brian.

[bert]

The reason why most to top end refractor manufacturers use aluminium, is that when the objective cools and moves the focus point inwards. Thus, a shrinking aluminium tube compensates somewhat for this effect as it also cools and shrinks.

Roland christens thought on the matter:

http://tech.groups.yahoo.com/group/ap-ug/message/55064

Also note that mr Christen uses carbon fibre on his cassegrain designs.

Brett

[clive milne]

Brian, when you take in to account the surface area of the OTA, the emissivity and R values of the respective materials, the difference in cool down times of CF versus steel is measured in minutes.

[brian nordstrom]

:thanx: I hear you but if you are worried about those values , you would not have a mirror/lense made out of glass? would you not ;) .
Sorry its not a perfect world we live in , in more ways than one , CF is still sexy , and a good tube material .
Brian.

[clive milne]

Hi Brian, That is exactly my point... the mirror has 40x the heat capacity of the CF tube and 1/10th the surface area, so it takes several orders of magnitude more time to equilibrate... It also follows that the contribution to OTA tube currents due to heat shedding is proportional to the ratio of thermal masses... ie) the disturbance to the wavefront from the OTA cooling would not be perceptible over the contribution from the main mirror.

Also, the tube will quickly pass through ambient temperature as a result of radiative cooling to the sky and will generally sit a degree or so below ambient. At this stage, if the OTA is made with something that has some sort of insulation properties, there is probably some benefit.

fwiw) The specific heat of CF is 0.16 kJ/kg K
The specific heat of steel is 0.5 kJ/kg K ... so if the steel tube is 3 times heavier, it will store 9x the heat energy of CF..

By any measure other than price, it is the absolute best material to build a telescope tube from.... period.

regards,
~c

[Adsyadsman]

This is a very interesting thread as I've been asking myself the same question - steel vs cf?
So the consensus is cf is better for less thermal expansion, but in the real world environment, say if I was to leave the scope outside for a couple of hours before use to equalise with the ambient temperature, and considering the temperature during the night hours will only drop say 10 degrees max, what effect would this have on the focus of a steel vs cf scope? Would it really throw out focus that much? And if so, even with a cf tube, would you still need to tweak the focus slightly anyway?
Reason Im asking is, if you still need to tweak the focus of a scope with cf tube, even though its less, wouldn't getting a cheaper steel tube kit with motor focus be a better option as then you could almost guarantee (electronic/software glitches aside) your imaging session would be more hassle free?

Just to add, I was thinking of a fast scope like a f4 newt that I understand would exacerbate the issue. I'll start another thread on this, but wanted to ask anyones real life experiences with the GSO f4 8" cf newt vs the SW twin.

Just my boggle?

Thanks also to all you experienced and knowledgeable bunch out there for sharing your wealth of wisdom with us, really helps to enjoy astronomy further. :)

[clive milne]

If it was my money, the priority would be for an ascom compliant focuser.

[brian nordstrom]

That may be true in Adelaide where you live (10 degree temp change ) but what about the people living in Tassie or the bottom of the south island of NZ ? 10 degrees is nothing to them , more like 20 ?
CF is the material of choice for most , f they can afford the extra$$$ .
Brian.

[Paul Haese]

Just going off my own experience with the GSO RC8, cooling is fast and focus is well held. I found that I only had to refocus every 4 hours of so when imaging with that scope. With the RC12 I have had to refocus every 40 minutes or less to maintain good focus.

[clive milne]

Whilst it is true that fast telescopes are more sensitive to focus position, it is also true that fast instruments have shorter tubes with less focus drift due to thermal expansion.

[Marke]

With the $700 difference i paid i got a feathertouch with temp compensation focus control

[Adsyadsman]

Thanks for you input guys.
Paul, your RC's, were they the alum tubes or CF's?

[Capricorn1(Tom)]

Irespective of the expansion rates of the different materials, you will be fixed by the losmandy plates top and bottom. Would agree if the materials were similar that is the plates were CF, top and bottom.
I think its a marketing ploy----the only advantage is the weight component.

[MrB]

... and it looks mint ;)

[Adsyadsman]

Good points about the mounting plates.
I guess if you wanted a steel tube to look as schmick you could wrap it with cf vinyl wrap :lol: or even a custom paint job :rofl: some flames or a GT stripe?

But seriously, if you guys with cf tubes ar still needing to tweak focus slightly thoughout the night albeit only once or twice, maybe saving coin on a steel tube being spent on a robofocuser might be a viable alternative :question:

But, from what I've read, looks like most steel tubes, especially for DSO imaging still needs some reinforcing around the focuser due to image train weights affecting collimation...

[allan gould]

this point was made in post #8 and re-inforcing has never been done on GSO RC. I think you are confusing this with those that have Newtonian scopes and since their focusers are off axis they need reinforcing due to lateral stresses.

Depending on the temperature drops during the night my Metal tube 10" GSO RC only needs one tweak generally an hour after starting imaging and I bet thAts about par for the CF tubes as well. I spent my money on a better focuser.

[Adsyadsman]

Sorry Allan, I was referring to newt scopes requiring reinforcing due to the focuser hanging off the side of the tube. My bad :sadeyes: