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Round and flat dew shield

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#1 Inkswitch

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Posted 27 January 2021 - 08:20 AM

I have issues with the secondary dewing up on my 300mm reflector.  I know that dew heaters are the preferred method but I like to minimize power in the field so I considered another option.  A flat disk whose circumference is the same as the footprint of the secondary mirror.  Mount this disk above the spider in order to shield the secondary from super cooling due to radiating into space.  This may be less than ideal when working at 45 degrees but I mostly observe at or near the zenith.

 

Thoughts on:

 

Will this work?

 

If so, what would be the best material to construct it from?

 

Any thoughts, opposition or support, are appreciated.



#2 TOMDEY

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Posted 27 January 2021 - 09:18 AM

Interesting concept! It's certainly worth trying, but would then want some kind of comparative results between the two approaches. I did the analysis in a different context a few years ago. Related to how much an ambient, unheated dome protects the scope inside from dew and frost. We have always noticed that domes profoundly protect the interior, but measurements or analysis in the amateur literature were not to be found. Anyway, the solid angle subtended by the dome opening is astonishingly miniscule compared with a scope just sitting outside or in a roll-off pretend observatory. And that entirely explains it.

 

Now, in your creative context... that flat disc won't help much if it is too far from the folding flat... because it will not subtend much of a solid angle. You see, the "cold sky / outer space" can be as much as a hemisphere (solid angle 2 pi). And that is lambertian, not collimated as you might think. It all is consequent of Emmy Noether's Radiance Theorem.

 

The best passive solution might be both your disc and a (removable?) tube extension forward by an extra foot.    Tom


Edited by TOMDEY, 27 January 2021 - 09:56 AM.


#3 Inkswitch

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Posted 27 January 2021 - 09:42 AM

The best passive solution might be both your disc and a (removable?) tube extension forward by an extra foot.    Tom

Very good idea.



#4 Mike G.

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Posted 27 January 2021 - 11:19 AM

I've tried the disc in front of the secondary AND a dewshield for my 12" LB.  while the dewshield provided the best results (the disc was minimal improvement by itself, probably because I do very little viewing at zenith), it was rather a nuisance on the front of the tube being 13" in diameter and 18" long.  lets just say huge dewshields and dobsonian mounts don't always play well together.  in the end, I went with a heater mounted behind the secondary powered by a 9v battery velcroed to the UTA..the heater is designed so it only comes on when needed and does the job perfectly without adding pounds to the OTA or causing the tube to move everytime a bit of wind comes up.  a single battery typically lasts 6 or 8 viewing sessions.




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