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Thermal insulation on OTA outside

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#1 tloebl

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Posted 11 April 2021 - 03:52 PM

I have been searching for information about upside and downside of using easily available bubble wrap covered with mylar for thermal insulation on the outside of a solid metal tube newtonian ota. Is this something that might slow down secondary dewing or would it impede tube cool-down to ambient?

 

Wonder if anyone has had any experience with this. I live in southern New England.

 

Tom



#2 Bill Jensen

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Posted 11 April 2021 - 04:12 PM

I live in the mid-Atlantic (lots of dew during the spring/summer) and don't have dewing issues with my 10 inch tube dob. It does have a fan at the base which I keep running when I observe. 

 

If you have dewing issues with your tube dob, perhaps an easier approach would be to make in effect a long dew shield fit on the top of the tube . I used some craft foam material to make a make shift dew shield for my Telrad. Perhaps you could make something similar, and just try it out . 

 

check out this article for example...

https://skyandtelesc...aling-with-dew/


Edited by Bill Jensen, 11 April 2021 - 04:48 PM.


#3 barbarosa

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Posted 11 April 2021 - 04:19 PM

That bubble wrap, Reflectix or similar has a very low resistance to heat flow. The aluminized face presumably emit less heat by radiation than a painted surface. Most of heat loss from the massy items, the mirrors might be going up the spout by convection and radiation. This simplistic model is not looking good.

 

The SCT owners who wrap have have the same losses moderated a little by the corrector plate.

 

So will an R value near 1help?

 

I usually wrapped my SCTs for two reasons. It did seem to reduce heat plumes somewhat and it did seem to extend the time until dew formed. Some say we have just two seasons in California, and if so we get considerable condensation in both of them. Coastal and bay weather flowing inland and in the winter radiation fog just like the valley.

 

I would wrap if only so that the water condensation was on the Reflectix and not the scope. Bad enough that floor and walls get wet some nights. 

 

3/4 of the year I start the dew heaters when I power up the mount.


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#4 Migwan

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Posted 11 April 2021 - 06:08 PM

Your secondary is open to the atmosphere.  Insulation won't change that.  I would think that a dew shield would be a lot more effective than insulation.

 

Dew shields can be easily constructed from a piece of closed cell foam cut from a camping mat or similar and some duct tape.   If you like fancy, you can add some velcro. 

 

jd


Edited by Migwan, 11 April 2021 - 06:09 PM.

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#5 CrazyPanda

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Posted 11 April 2021 - 10:41 PM

I have been searching for information about upside and downside of using easily available bubble wrap covered with mylar for thermal insulation on the outside of a solid metal tube newtonian ota. Is this something that might slow down secondary dewing or would it impede tube cool-down to ambient?

 

Wonder if anyone has had any experience with this. I live in southern New England.

 

Tom

That stuff is actually a giant scam:

 

https://neo.ne.gov/i.../aug2014.02.htm

 

It has an intrinsic R-value of 1.0 - that is, its ability to slow heat transfer via conduction, which is what would happen if it were in contact with the tube. Claims of higher R-values is based on the achievable building assembly R-value when that product is used in accordance with its design. By itself, it offers very little insulation.

 

Its only *real* use as an insulator is as a radiant barrier, but the design of a system that relies on a radiant barrier requires something other than just wrapping it around your tube and calling it a day. By putting it in contact with the tube, you're defeating its purpose.

 

In fact, the only way to get it to behave as a radiant barrier would be to put it on the INSIDE of your tube, to minimize how much heat is radiated into the cold metal of the tube. But obviously you don't want a shiny reflective surface on the inside of your tube!

 

Another way it would behave as a radiant barrier is if it was on the outside of the tube, and your goal was to stop your radiant body heat from hitting the tube, or to keep the tube cool if used during the day for solar observing. But at that point, the "bubble wrap" part is pointless, and simple mylar wrap (which which can be had for cheaper) would work just as well.

 

As others have said, dew on the secondary is largely from the broad angle of the sky the secondary mirror is exposed to. This lets it radiate away its heat more rapidly than it should. Extending the length of the tube with literally anything that can be fashioned into a tube, will solve that problem for you. Foam camping mats are popular simply because they are flexible and light weight.

 

In fact, if you *DID* want to actually insulate your telescope tube, then wrapping it in foam camping mats would work far better than that Reflectix stuff. How much value that adds to avoiding dew formation on the secondary or how much it would impede cooldown to ambient, is hard to say.


Edited by CrazyPanda, 11 April 2021 - 10:51 PM.


#6 Max T

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Posted 12 April 2021 - 02:18 AM

Hi Tom,

As others mention, a dew shield would better address the secondary dewing up.   I read somewhere that the length of de shield should ideally be the aperture*1.5 (e.g. an 8" scope should have a 12* dew shield extending out).

 

The great thing with dew shields is that you can easily make a couple of baffles with some black plastic and compass and scissors, and stick the baffles into the dew shield.  

Although the baffles will slightly vignette the image a little, they will also give a nice contrast boost, since they reduce stray light getting into the scope; the best way to remedy stray light is to stop it at source after all.  

 

To test this, tray looking up the inside of the scope with the primary removed and to a before and after dew+baffle rings.   You'll be quite surprised by the halo glow at the mouth of the dob before, and the darkness after with baffles! 

 

As for wrapping the OTA with some insulating material, that will slow tube cool down to ambient, but more importantly if you view for say 1+hrs, the external insulation will prevent the tube cooling *below* ambient during the night and so prevent tube currents forming.  

This will improve your image viewing as the temerature deltas don't interfere with the lightwaves, so a 1/8pv primary can start producing 1/8pv images and not say 1/4pv images.

 

But by far the best way to insulate a solid tube dob is with 1mm polystyrene roll used to insulate walls in houses (https://www.google.c...olystyrene roll).

Apply contact cement (https://tinyurl.com/5c5bmam) to the polystyerene roll and to the inside of the tube, stick together and then apply flocking over the top.

 

Doing this will also slow the secondary from dewing, since the sky-side of the upper dob's metal tube won't be supercooling below ambient and dropping cold air plumes onto the secondary as to chill it and hasten dew forming.

 

More importantly, polystyrene will make the solid tube *thermally inert* which means it vanishes from the view- no impact whatsoever.   And with a boundary fan cooling the mirror to ambient and therefore no thermal deltas within the tube, and no body heat plumes at the mouth of the tube thanks to the dew shield...

... You'll get tack sharp images as the highest strehl possible by the mirrors is delivered with no interference from heat or dew.



#7 dweller25

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Posted 12 April 2021 - 03:30 AM

I’m a big fan of wrapping cats and maks with reflectix.

But for your Newt the best solution to prevent secondary dewing is a long dew shield.


Edited by dweller25, 12 April 2021 - 03:31 AM.

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#8 tloebl

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Posted 12 April 2021 - 09:27 AM

Thanks all, I was talking about Reflectix and looks like my best approach is the extended dew shield at this point. Reflectix is also pretty light so I may use it for the dew shield by spraying the inside surface with anti-reflective black paint to minimize additional stray light.


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