Jump to content


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


diy dew heater

This topic has been archived. This means that you cannot reply to this topic.
7 replies to this topic

#1 Wouter D'hoye

Wouter D'hoye

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1553
  • Joined: 27 Jun 2003

Posted 25 September 2010 - 02:22 PM


I'm considering making my own dew heater and controller.

here's they idea i'm working on:

for the dew heater elements i would use regular resistors in heat shrink tubing. each heater would have 3 series of resistors.

The controller would consist of just three swithes controlling a singe serie in each element. it would be a binary coded system. like 1-2-4 ging me 8 levels of power.

Now i just wonder how much power each element should have.. so what would be the power required for a telrad, for a 60mm finder?

and would self adhesive velcro be a good material to put the isolated resistors in?

I appreciate your thoughts,


#2 ischua


    Under the Radar

  • *****
  • Posts: 3663
  • Joined: 17 Jul 2008

Posted 25 September 2010 - 03:26 PM

After trying this myself and spending lots of cash on stuff that never worked I found this site Homemade dew heaters

#3 Gene7


    Viking 1

  • -----
  • Posts: 877
  • Joined: 10 Jun 2010

Posted 25 September 2010 - 05:51 PM

The major reason for Dew formation is scope exposure to the 50 degree below zero sky. Cut that 80 % or so by adding a mylar (or aluminum) reflective shield to the top (only) of your scope. Allow an air space below also. Prevent the scope from cooling below air temperature. You will have a lot better scope stability if you do not try to heat sky and then upset it more by adding heat. Do not add insulation, that is not the problem. Gene

#4 Olivier Biot

Olivier Biot


  • *****
  • Posts: 24977
  • Joined: 25 Apr 2005

Posted 25 September 2010 - 05:54 PM

Wouter, you may want to read this article.

I'd consider combining this with a boundary fan solution as well.

#5 a__l


    Viking 1

  • -----
  • Posts: 721
  • Joined: 24 Nov 2007

Posted 25 September 2010 - 07:58 PM

For heating big 2" of eyepieces(plus paracorr) I use only belt from AstroZap. Resistance 42 ohm. The controller is not necessary. Work very well. The battery 17 amp/h (12 v) it is enough for 2-3 nights (plus a heater secondary mirror, fan, Sky Commander, 2 encoders).

#6 GlennLeDrew


    Voyager 1

  • *****
  • Posts: 12753
  • Joined: 17 Jun 2008

Posted 27 September 2010 - 05:51 PM

I make my heaters to run at one power setting only, by being run continuously when needed (no 'controller' other than turning on or off.) And I do use thin foam--if it can be fitted--as a 'blanket' to increase efficiency. Why lose more heat than necessary to the environs?. I try to get best thermal coupling by NOT allowing more than heat shrink tubing between the heater and the surface to be warmed. That is, no fancy stuff like Velcro or other cloth separates the heater from the surface.

I've found that 3W, or 48 Ohms works well for 2" eyepieces, and 2W (72 Ohms) for 1.25" eyepieces and 50-80mm finder objectives. (More heat is needed for eyepieces because of the close proximity of the warm, moist eye.) Your 4" refractor should stay dry with no more than 3W. (I've fitted out 8-10" SCT's with no more than 4W, by placing the heaters on the inner surface of the corrector cell. Compare this to some commercial heaters which can squander near to 30W--albeit on a timed duty cycle.)

#7 Wouter D'hoye

Wouter D'hoye

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1553
  • Joined: 27 Jun 2003

Posted 28 September 2010 - 09:36 AM

Thanks for the replies. :)

very helpful information..



#8 Achernar



  • *****
  • Posts: 9795
  • Joined: 25 Feb 2006

Posted 28 September 2010 - 06:03 PM

I made my own heaters and controller, and they work well for the most part. However, I wired the heaters in parallel, not in series for two reasons. One is to prevent them from overheating and even damaging something, resistors in series can get very hot. A single failure among them also means your heater fails, but a parallel circuit generates a controllable amount of heat and it will keep working if one resistor fails. I use 330 Ohm, half watt resistors to make mine, and they do a good job in the dew here.

The controller is modeled after a single phase electrical panel, using two terminal strips as "bus" bars, one for ground or common, the other for the hot or line side of the circuit. I made the connections to the strips using crimp on lugs and a crimp tool, both of which you should be able to find in an electronic supply store. I installed a fuse between the positive or line side terminal strip and the battery to prevent a short circuit from damaging my equipment or even causing the battery to explode. Never build a device like this without a fuse, it's there to protect your equipment. I control the amount of heat using variable resistors in between the switches and the RCA jacks the heater plugs into. On the load side of the switch, I connected a red LED which then goes to the ground terminal strip so I can tell in the dark if the heater is on or not. A couple of jacks do not have a variable resistor in line with them so I can power my digital setting circles with 12-volt power.

For some instructions on how to make dew heaters, go to www.dewbuster.com, which is where I got the information to make my own.


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics