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Which dew-prevention method won't make tube currents?

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#1 bokemon

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Posted 24 January 2021 - 09:34 PM

Hello folks,

I have a carbon fiber Newtonian that is insulated with bubble wrap on the outside.  But I still have problems with the primary mirror getting dew after a few hours, or rather, it seems like a hazy pre-dew that bloats the stars a tiny bit, but never develops beyond that into water droplets.  (or maybe its micro ice crystals)  Anyway, I read about a few different dew prevention methods, but which one is good about not making tube currents that mess up the seeing?  I recognize that you are only trying to barely heat the primary to or above ambient temperatures to avoid this.

a) heated dew shield or front lip of telescope - that seems like it would create heat waves in front of the telescope

b) blow air in thru the back and out the front.  It does create turbulence inside the tube, but maybe that isn't a problem because the air is at a constant temperature throughout

c) evenly-spaced heaters on back of primary mirror.  Maybe if you apply low enough power it won't warp the mirror, nor cause that boundary layer "steaming" in front?

d) heater wrapped around back part of telescope

e) ???


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#2 havasman

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Posted 24 January 2021 - 10:01 PM

Do not heat the primary. Ever. Put a fan behind the primary, blowing on the back of the mirror.

If your secondary dews, active dew control works extremely well. Astrosystems has an exceptional system that meters current to heating elements on the back of the secondary to keep it just a bit above the ambient temp that is monitored by the system.

I'm not sure what bubble wrap may accomplish. Dew on external parts of a Dob is completely insignificant.

Do not heat the tube or any part of it. That'll give you convection currents in the optical path that are destructive to your image.


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#3 Napp

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Posted 24 January 2021 - 10:17 PM

I observe in Florida with a 10 inch solid tube and a 16 inch truss tube with shroud.  Dew is a nightmare here.  It's not uncommon to have water dripping from the scope and anything uncovered soaking wet.  I run the primary mirror fan(s) continuously.  The only times I have had condensation form on the primary mirrors was when I neglected to connect the fan(s) or allowed the batteries to run down.  I put a towel over the the scope opening, the eyepiece and the finder when I am not actively using them.  I also keep a 12 volt hairdryer handy in case the towel is not enough for the secondary, finder or eyepiece.  I am planning to add dew heaters for the secondaries but so far my measures are working well enough.



#4 Jethro7

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Posted 24 January 2021 - 11:27 PM

Hello Bokemon,

I live in Bayou Country here in N.W. Florida. Dew is horrible here. I run a 12" Orion Skyline Dob. I have had no real issues with the primary fogging up I will run the primary fans all night but I had to add a dew heater on the secondary to keep it clear. It does not take much heat to stop the dew just a few degrees above the ambient temperature will do with no issues with creating heat currents. Some nights the dew will build up on the scope body till it is dripping wet. I will need to wipe everything down on occasion with a towel. 

Havasman, made a good point in post #2  dont heat the primary.

 

HAAPY SKIES AND KEEP LOOKING UP Jethro


Edited by Jethro7, 24 January 2021 - 11:30 PM.


#5 bokemon

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Posted 25 January 2021 - 08:55 PM

ok, so fans it is.

The next question is how to prevent dew from forming after I have to pack up the scope and go home?

(I find that dew causes dust to stick on)



#6 Napp

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Posted 25 January 2021 - 09:41 PM

I typically observe at remote sites.  I drive an F150 with a SuperCrew cab.  I keep all dew control measures in place until I break the scope down.  I raise the backseat and pack the scope there.  I run either the ac or heat as appropriate on the hour+ drive home.  Everything is usually dry by the time I get home.  I do pack the eyepieces in hard cases for the trip home but I set them on their sides uncapped under ac or heat until completely dry.  Laying them on their sides prevents dust accumulating.  If I observe at home I leave everything uncapped and uncased under ac or heat until dry.




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