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How to fight dew on large-ish RC

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#1 Cedric_r

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Posted 28 January 2022 - 05:44 AM

Hi all,

 

I have a 14" RC, remotely hosted. With the winder, I've run into problems with dew (in fact ice at the moment) depositting on the main mirror. We have 100% humidity nights pretty much every day. As a result, images become very flat, and the autoguiding becomes essentially impossible.

 

Here is a view from inside the telescope this morning:

http://raguenaud.spa...cam5-bright.jpg

 

I have 2 dew heaters around the body f the telescope: one midway of the plain section of the body (it's a truss) and one toward the front of the truss near the secondary. Because of the size of the telescope, they're very ineffective. Hence my problem.

http://raguenaud.spa...am3-heaters.jpg

 

So my questions is this: what solutions have you used to avoid that problem?

 

Thanks.


Edited by Cedric_r, 28 January 2022 - 05:49 AM.


#2 cuzimthedad

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Posted 28 January 2022 - 12:07 PM

Moving to Cats & Casses



#3 HunterofPhotons

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Posted 28 January 2022 - 12:45 PM

Hello Cedric,

The dew heaters need to make contact with the primary mirror.  

The two strategies that I've seen used the most are taping nichrome wire to the back of the mirror or using a standard dew heater wrapped around the outside edge of the mirror.

The nichrome wire strategy requires one to source the proper size wire.  I've read that the resistance for that should be at least 2.7 ohms but you should not take that as gospel.  The current should be regulated.

The standard dew heater strap would be easier.  For your 14" I believe that you would want a 48" long strap and that should be regulated by a variable output controller.

Some RC scopes, such as the RCOS scopes, have the capacity to add a thermistor-controlled circuit.  I wouldn't know about other brands.

It is important that only the absolute minimum of heat is added to the mirror to stop the dew.  You certainly don't want to distort the mirror with too much heat.

I hope that this helps.

 

dan k.



#4 Cedric_r

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Posted 28 January 2022 - 02:10 PM

Thanks a lot Dan. I considered hearing the mirror but didn't think the I've would be as bad as it is. I'll investigate the mirror heater route.


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