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Carbon vs Steel

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#1 nyx

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Posted 30 June 2020 - 10:32 AM

I am considering buying a 6" F/4 Newton astrograph which comes with a steel or carbon tube. The carbon option is not available right now (COVID-19) while the steel one is. Should I wait for the carbon tube or go for the steel one?

 

Clear skies! :)



#2 pyrasanth

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Posted 30 June 2020 - 10:53 AM

Carbon fibre is lighter and more thermally stable & since F4 optics have quite fine focusing requirements you don't want to be refocusing more frequently. I would wait for the carbon as a reduction in weight is always useful to be had as an additional bonus.



#3 nyx

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Posted 30 June 2020 - 11:07 AM

Carbon fibre is lighter and more thermally stable & since F4 optics have quite fine focusing requirements you don't want to be refocusing more frequently. I would wait for the carbon as a reduction in weight is always useful to be had as an additional bonus.

Re-focusing isn't my biggest concern to be honest. I have the ZWO EAF and in conjunction with NINA, I can trigger the auto-focus routine when the temperature or HFR changes. I am mostly worried about collimation. My sensor (ZWO ASI533MC) is fairly small though so....I'm not sure how big of a problem it can be if the collimation isn't spot on.

 

As far as weight is concerned, I totally agree. The carbon tube is 1kg lighter.


Edited by nyx, 30 June 2020 - 11:09 AM.


#4 Saganwannabe

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Posted 30 June 2020 - 11:09 AM

But the carbon just looks so cool! grin.gif


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#5 spereira

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Posted 30 June 2020 - 11:10 AM

Moving to Reflectors, for a better fit.

 

smp



#6 SeattleScott

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Posted 30 June 2020 - 11:12 AM

But the carbon just looks so cool! grin.gif

But it scratches easily...

#7 nyx

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Posted 30 June 2020 - 11:13 AM

But the carbon just looks so cool! grin.gif

Yeah, I'm sure all the insects will be amazed seeing a carbon tube telescope :p


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#8 OhmEye

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Posted 30 June 2020 - 12:26 PM

I don't have a reflector but have been researching various 8" and 10" newt options. Weight of the OTA itself is mainly relevant with regards to your mount. If you are pushing the payload limit, shaving off a bit might be worth the price to you. My takeaway has been that thermal properties with regard to focus are not a huge issue under 10". My current conclusion is that the biggest consideration for steel vs. CF tube is rigidity. If the focuser is mounted at the far end of a thin-wall steel tube, the weight of the camera gear attached to the focuser can put a fair amount of force on that spot of tube, and the angle varies with respect to gravity as the mount tracks. The tube needs to be rigid enough to not flex under this variable load. I've read a lot of mixed opinions about this with some people comparing steel tubes to soda cans and flexing to the point of being unusable, others seem to have zero issues. The temperature of the mirror itself has a larger impact to focus than the temperature of the tube. This is why many reflectors have fans for the primary mirror.

 

The same choice still exists in CF tubes, by the way. Some CF tubes are thin, so weight may benefit more than rigidity. I've seen 2.5mm, 5mm, and 10mm(!) thick CF tubes for newts. Also, the baffles may not be the same between tube types, I've seen some cheap steel tubes with very nice baffles and CF tubes with no baffles at all. You have to consider the OTA as a whole, not for it's tube type.


Edited by OhmEye, 30 June 2020 - 12:29 PM.


#9 nyx

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Posted 30 June 2020 - 12:44 PM

I don't have a reflector but have been researching various 8" and 10" newt options. Weight of the OTA itself is mainly relevant with regards to your mount. If you are pushing the payload limit, shaving off a bit might be worth the price to you. My takeaway has been that thermal properties with regard to focus are not a huge issue under 10". My current conclusion is that the biggest consideration for steel vs. CF tube is rigidity. If the focuser is mounted at the far end of a thin-wall steel tube, the weight of the camera gear attached to the focuser can put a fair amount of force on that spot of tube, and the angle varies with respect to gravity as the mount tracks. The tube needs to be rigid enough to not flex under this variable load. I've read a lot of mixed opinions about this with some people comparing steel tubes to soda cans and flexing to the point of being unusable, others seem to have zero issues. The temperature of the mirror itself has a larger impact to focus than the temperature of the tube. This is why many reflectors have fans for the primary mirror.

 

The same choice still exists in CF tubes, by the way. Some CF tubes are thin, so weight may benefit more than rigidity. I've seen 2.5mm, 5mm, and 10mm(!) thick CF tubes for newts. Also, the baffles may not be the same between tube types, I've seen some cheap steel tubes with very nice baffles and CF tubes with no baffles at all. You have to consider the OTA as a whole, not for it's tube type.

The two scopes in question are:

 

https://www.teleskop...tall-Tubus.html

https://www.teleskop...rbon-Tubus.html

 

I cannot say anything about any of them, since I don't have them in front of me. I did start astronomy (visual) with a F/5 Photon from TS and it seemed to be stable enough for visual. The steel wasn't very thin, but I could certainly temporarily "deform" it using my fingers.

 

I understand what you mean regarding weight on the focuser and tube flex. The plan is to mount the EAF on the focuser, along with a 1x coma corrector/flattener and the ASI533MC. Certainly not so heavy compared to a mono camera+filterwheel but still, considerable weight in my opinion. Pointing the focuser towards the ground helps, so I read.

 

The steel tube is 350€ cheaper. Not a sum that breaks the bank, but if (considering the setup) collimation/focus won't cause any problems, I'd rather keep the money in my bank account :)



#10 ChrisWhite

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Posted 30 June 2020 - 02:33 PM

In 6in it won't make much difference. With larger ota's flex is a genuine concern, but 6in steel newts are pretty stiff.

Also, there is likely little difference for thermal considerations.

#11 calypsob

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Posted 30 June 2020 - 07:13 PM

Honestly I dont like the seam on the steel tube newtonians, My tube is not perfectly round because of the seam.  If you have a lazy susan and a dial indicator, its pretty easy to tell how out of round the tube is. I will probably end up replacing my tube with a seamless steel tube from hastings.

 

With that said, a carbon tube is made on a mandrel and is perfectly round.  



#12 Brian Carter

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Posted 30 June 2020 - 07:56 PM

The bigger issue with metal is that it will create tube currents any time you touch the tube.  The thin metal will conduct the heat from a hand very quickly and that will affect what you see at the eyepiece or camera. Wait for the carbon. 



#13 nyx

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Posted 30 June 2020 - 07:59 PM

The bigger issue with metal is that it will create tube currents any time you touch the tube. The thin metal will conduct the heat from a hand very quickly and that will affect what you see at the eyepiece or camera. Wait for the carbon.


Good point. Though, after mounting the tube I have no reason to touch it, especially during exposures :)

#14 TelescopeGreg

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Posted 01 July 2020 - 02:04 AM

For what it's worth, I had a lot less issue with focus drift on my 8" f/5 steel Newt than my 5" f/7 aluminum refractor, but the Newt did flex. It was impossible to collimate it in one position and have it work in another.



#15 Jon Isaacs

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Posted 02 July 2020 - 04:31 AM

The bigger issue with metal is that it will create tube currents any time you touch the tube.  The thin metal will conduct the heat from a hand very quickly and that will affect what you see at the eyepiece or camera. Wait for the carbon. 

Gloves.. or a glove.

 

With a GEM mounted scope, there is little reason to touch the tube.

 

Jon




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