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DIY'ing Secondary Dew Heater

ATM DIY Dob Equipment Reflector
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#1 Brisby2

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Posted 05 January 2022 - 02:55 AM

I've been taking my semester break to work more on my 17.5" Dob, and am now trying to integrate electronics into the mix. I have a few dew heater straps, but am using one for my eyepiece and one for telrad. I have a 3.1" Antares secondary that sits in a cell rather than just a stalk, stuffed with filler material (cotton, wool, something like that) behind it. I was thinking of lining the back of the mirror with nichrome wire or another resistive element and soldering an RCA plug onto the ends to plug into my dew controller. I am, however, unsure how much nichrome wire to use and what gauge to ensure proper heating without lighting my telescope on fire. Does anybody have any insight on this? Or perhaps a more sensible alternative if there exists one?

 

Please also feel free to share your secondary dew heater solutions that you've done for your own scopes!

 

 



#2 happylimpet

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Posted 05 January 2022 - 03:42 AM

If it helps, I run my secondary heater with ~0.3A at 12v, which is 4W. Never noticed any thermal plumes, and it never mists up.

 

Using P=V^2/R, my total resistance must be (V^2)/P or 144/4=36 ohms. I may have got that wrong as its early, but you get the idea. Presumably the nichrome has an ohms/meter measure or something, so you can work it out from that.


Edited by happylimpet, 05 January 2022 - 03:43 AM.

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#3 OregonAstronomer

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Posted 07 January 2022 - 01:39 AM

I took the easy, no math option. I coiled the nichrome wire back and forth over the entire exposed surface of the back of the secondary and connected the ends to insulated wires ending in RCA type plugs that ran down the OTA to the dew controller. I then adjusted the dew controller from its lowest setting, very slowly increasing the power just until I felt warmth on my hand held on the coiled wire. Then I covered over the wires and back of the secondary with black matboard (this is a stalk type secondary holder). That became my setting for typical nights. For imaging when I'm at or below the dew point I'll increase the setting by about 50%. Has worked fine through months of Pacific Northwest humidity and nothing has melted yet!

Arnie


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