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  #21  
Old 13-10-2018, 08:56 AM
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mental4astro (Alexander)
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Tube currents and preventing them is not the only part in this. Not letting your scope get hot is better, HOW your scope cools is another, and if you keep your scope in an observatory then how it is constructed and painted are other considerations. Even where you set up has a major impact!

Professional observatories don't have these issues as they are designed not by the astronomers, but by engineers, opticians & technicians whose job is to anticipate these and design so that these issues are not problems for the final scope and structure. Only us amateur astronomers do it all ourselves...

What all this has to do with spotting the Cassini Division? Little and a lot. It is all just a part of instrument handling before you even put your eye to the eyepiece.

But don't get too anal about this. It is very easy to get very pedantic about everything being "perfect". Getting too carried away with the "tech" part can take away much of the fun of astro for many of us. If you are new to astro, just get under the stars and run up some miles. This is more important first up. Then you can look at measures that suit you and where you want to go with astro.

Alex.
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  #22  
Old 13-10-2018, 03:32 PM
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AG Hybrid (Adrian)
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Alright Alex. I'm intrigued by this insulation idea. If it can turn my MAK into a grab and go Ill give it a try. I saw the original post on CN but I didnt go deep into it. I'm thinking EVA 75 again. How thick are we talking here? 6mm? 12mm? Do I just get a yoga mat from Clark Rubber and start hacking at it with a box knife to fit?
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  #23  
Old 13-10-2018, 03:44 PM
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Quote:
Originally Posted by AG Hybrid View Post
Do I just get a yoga mat from Clark Rubber and start hacking at it with a box knife to fit?
Yep.

Real hard, eh!

There's no exact science to this. Really it's trial and error. The aluminium tube of my Intes Mak is 3mm thick, the EVA mat doesn't need to be too thick. For an SCT, the tube being so thin it may need a thicker mat.

I'd start with 10mm.

Alex.
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  #24  
Old 13-10-2018, 04:01 PM
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Atmos (Colin)
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One of the most entertaining views I've had the stars was with a 12" dob that had just been taken outside and turned on the fan. Watch a bright defocused star and get sucked into the VORTEX!
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  #25  
Old 13-10-2018, 08:00 PM
Wavytone (Nick)
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Do I just get a yoga mat from Clark Rubber and start hacking at it with a box knife to fit?
A sheet of black 2.5mm CoreFlute from Bunnings is even cheaper and also effective.
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  #26  
Old 13-10-2018, 11:44 PM
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A sheet of black 2.5mm CoreFlute from Bunnings is even cheaper and also effective.

Really? Does it roll up easily?
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  #27  
Old 14-10-2018, 08:48 AM
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Originally Posted by AG Hybrid View Post
Really? Does it roll up easily?
If you slit every second “flute” with a craft knife - as I did- it becomes very flexible and rolls up easily, it will wrap even a 50mm tube.

I made mine using white and lined it with black foam rubber before realising Bunnings do stock the black stuff. Wrapped a strip of double sided Velcro to hold it in place. https://www.cloudynights.com/topic/6.../?fromsearch=1

Last edited by Wavytone; 14-10-2018 at 10:53 AM.
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  #28  
Old 14-10-2018, 09:57 AM
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Would this insulation work on a newt to help with tube currents?
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  #29  
Old 14-10-2018, 10:23 AM
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Chris, no this system won't work with a Newt because it is an open tube. If you want to prevent tube currents in a Newt, have a read through the article I wrote on the topic:

Article: Cooling of Newtonian Optics

Alex.

Last edited by mental4astro; 14-10-2018 at 12:00 PM.
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  #30  
Old 14-10-2018, 10:54 AM
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Cheers Alex thanks for the link and saving me a trip to Bunnings. But just been to Russia and didn't find any under bed !!
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  #31  
Old 14-10-2018, 11:00 AM
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Quote:
Originally Posted by Wavytone View Post
Carbon fibre isn't much better as it is thermally conductive - and black.
Ooooh.... careful there.

Whilst carbon is an exceptional thermal conductor, the resin that it is embedded in is an exceptionally poor thermal conductor.

The thermal properties of carbon fibre tubes, therefore, are not uniform.
Aligned with the direction of the weave, the thermal conductivity is a couple of percent that of aluminium. Perpendicular to it, the figure is even less (by a factor of 10).

I suspect the angst directed to carbon fibre is based on assumptions rather than direct measurement.

fwiw) Professional observatories take a completely different approach to thermal management. They pre-chill the telescope prior to an observing session... as long as you are above dew point when the dome is opened, you're good to go. Cold primary mirrors do not induce a turbulent boundary layer.

As to seeing objects smaller than Dawes limit, etc)... you don't need to resolve an object in order to perceive a contrast minima (or maxima).
Much like you don't need to resolve a star in order to see it.

best
~c

Last edited by clive milne; 14-10-2018 at 11:40 AM.
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  #32  
Old 14-10-2018, 05:57 PM
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Professional observatories take a completely different approach to thermal management. They pre-chill the telescope prior to an observing session...
Sure. Thermals are produced by a temperature gradient and the thermals are specifically the result of convective cooling.

Until recently its quite safe to say that everyone was of the traditional view that the only way to stop these thermals was to eliminate the temperature gradient, ie everything had to be close to the ambient external air temperature.

This is certainly true for newtonians and observatories - because they're invariably using large reflectors with open tubes and under a dome with a huge open slit. There is however at least one I know of in the arctic circle that doesn't cool theirs - it uses a high-grade plastic film across the dome slit - like glad-wrap but better - to retain warm air in the dome despite the outside temps being subzero.

However there is a second way - stop the flow of heat using insulation. This only applies to scopes with closed tubes that display a tube current - specifically SCTs and Maks and I suggest that has always been obvious in these threads and those on CN. Insulating the OTA stops the convention, very effectively, as does insulating a house.

Last edited by Wavytone; 14-10-2018 at 06:31 PM.
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  #33  
Old Yesterday, 11:51 AM
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Just a thanks to Alex. Your experience is a great benefit for us here. I live 350meters up, and dewing up of my C8 a few years ago made me sell that scope and go back to a newtonian. I loved the ergonomics of that scope on my eq mount. Waiting for GSO classic cassegrain to arrive in OZ. I could be thinking of a C8 again if insulating works. Doug
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  #34  
Old Yesterday, 05:40 PM
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Quote:
Originally Posted by Wavytone View Post
If you slit every second “flute” with a craft knife - as I did- it becomes very flexible and rolls up easily, it will wrap even a 50mm tube.

I made mine using white and lined it with black foam rubber before realising Bunnings do stock the black stuff. Wrapped a strip of double sided Velcro to hold it in place. https://www.cloudynights.com/topic/6.../?fromsearch=1

Thanks for that. I appreciate your assistance. I have an opportunity to get some tonight and get started on it.


Also. How much of the OTA did you wrap. Only the area around the corrector? No down to the primary?
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  #35  
Old Yesterday, 06:35 PM
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A bit more than half the length of the OTA - made a huge difference on my mak.

All of it would be better though IMHO particularly for an SCT, but covering the back isn't necessary
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  #36  
Old Yesterday, 07:15 PM
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A bit more than half the length of the OTA - made a huge difference on my mak.

All of it would be better though IMHO particularly for an SCT, but covering the back isn't necessary
OK.

I couldnt get any of the black stuff. But, I have 2 bottles of Black 2.0.
Lets see how they go.
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  #37  
Old Yesterday, 08:23 PM
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Thanks Doug

This thread has certainly developed from when I revived it in post No. 9! I'm glad the thread is being found most useful in its resuscitation

All of this has been a learning experience for me too. Starting with blind faith, to questioning, to new realizations - and then try to convey this the best way I can. It's a process. I've learned a hell of a lot from 2012 when I first made a post in this thread. And I'm not doing this on my own. I have had a lot of help too. And I'm sure I'm going to learn a hell of a lot more too as time goes on.
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  #38  
Old Yesterday, 08:31 PM
Wavytone (Nick)
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Adrian, Black 2.0 is very fragile - I have a bottle and tried a bit last week - it won't stand any wear and tear so really only suitable for the inside of an OTA.

Coreflute being plastic, I wouldn't expect paints to adhere to it well - but spray contact adhesive did, well enough, and failing anything else I'd suggest bonding something to it before you cut it up.

Things I'd consider range from black paper (art suppliers such as Eckersly's) to a black fabric (felt, linen or even silk - from Spotlight).

I resorted to 3mm black foam only because I had a roll lying around.
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  #39  
Old Yesterday, 08:55 PM
alval (Alan)
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You can make an insulating jacket from a car windscreen heatshield from supercheap auto. You get two from one shield. I’ve covered my five inch mak and home made aluminium dew shield the full length with cut outs for the finder brackets. It folds flat and is easy to wrap around and fasten with velcro dots, then add finders etc. I havent been dewed out yet. One benefit is when everything else is wet you take the jacket off and the ota is dry.
Al
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  #40  
Old Yesterday, 09:05 PM
Wavytone (Nick)
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Agreed, this did occur to me too. Good to know it works.
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