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Dew zappers

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#1 zeddie

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Posted 28 September 2020 - 06:25 PM

Does anybody have an opinion on why dew zappers don't cause serious image degradation because of uneven optics expansion? Or do they cause degradation, but you live with it, because the other option is not observing? 



#2 Linwood

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Posted 28 September 2020 - 06:55 PM

The goal of a dew heater is to get a few degrees above ambient, not enough to cause lots of thermal currents, and hopefully to heat around the circumference of the objective or eyepiece, not on one side.

 

That said, asking Mother Nature to lower the dew point is a better solution if you have the connections.



#3 scopewizard

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Posted 28 September 2020 - 06:57 PM

You need to regulate the heaters. All you want to do is have the optics slightly warmer than the surrounding temperature. If you turn these on when you see dew on the optics, it is too late and will require full heat which in turn will degrade the optics. Initially I turn them on low. If the dew on surrounding objects increases I do the same with the heat. Staying ahead of it is the trick. You can spend more money for auto system with temperature and humidity sensors. I use common sense and never had an issues.



#4 zeddie

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Posted 28 September 2020 - 07:02 PM

Thanks for the info. I've seen a heater "pad" to go underneath a primary mirror advertised, as opposed to a circumferential strip. Any pros/cons with that?



#5 Napp

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Posted 28 September 2020 - 07:10 PM

What kind of scope are you using?  It would take tremendously bad dew to require a heater for a primary mirror.  Usually all that’s required for a primary mirror at worst is a fan running continuously.



#6 zeddie

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Posted 28 September 2020 - 07:20 PM

I'm restoring a home built (not by me) Dob, 10.1" f5.45 Coulter mirror. Primary mirror and secondary are out, soon to be sent to be re-aluminized. Just asking about dew zappers in general, I have not had a problem with dew on the primary. Any suggestions about heating the secondary? Where would I need to be most concerned with dew problems? 

Thanks. 



#7 Napp

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Posted 28 September 2020 - 07:49 PM

The secondary is the weak spot as far as dew is concerned.  You can buy a dew heater for the secondary or make one.  One approach is to build a dew shield around the opening that extends out interfering with radiational cooling.  What I am doing so far is draping a towel over the opening (and the eyepiece and finder) when I am not actively observing.  I also have a 12 volt hair dryer handy.  Of course all this assumes either a solid tube or a shroud around a truss tube.  I plan to build dew heaters for the secondaries of both my DOB’s.


Edited by Napp, 28 September 2020 - 08:13 PM.


#8 zeddie

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Posted 28 September 2020 - 08:59 PM

As of now, it's a truss tube dob. I'm considering a tube shroud, sounds like I should plan on it.



#9 Napp

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Posted 28 September 2020 - 09:02 PM

Without a shroud the primary is at great risk of condensation forming unless you live in a very dry location.  Here in Florida it would be a very short night without the shroud.


Edited by Napp, 28 September 2020 - 09:03 PM.


#10 zeddie

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Posted 28 September 2020 - 09:41 PM

I'm in NE Georgia, not very dry here.




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