Anti Dew Heater inside Dew Shield

[Jon22]

I am building an anti dew heater cable as described elsewhere in this forum. I plan to install this inside the dew shield for my Celestron CPC925 mainly because I think heat transfer through the dew shield will be poor and I would prefer to not fit it directly to the telescope.

The wattage of the heater cable will be controllable from zero to about 35w and I will paint it black to minimise reflections.

Are there any reasons why I should not do this.

I am aware that the potential exists for air currents to be set up. My main interest is in photography, so I could perhaps use the heater intermittently to minimise this effect.

Thanks

[Astroboffin]

Hi,

Happy new year to you

I personally would not fit inside the dew shield, as that will mean it will be in front of the corrector plate of the scope, this will cause warm air currents to be swirling around and will cause all sorts of problems for imaging, probably OK for visual, but not imaging.

You would be much better putting it on the outside of the dew shield just below the front edge of the scope, the heat transfer will be much better than you think, and won't cause any issues for imaging.

Worst case senario is that you have to turn it up a couple of watts to get the heat transfer you want, as it has to go through the dew shield first, but that is nothing compared to the issues you face by putting it inside the shield and in front of the corrector plate....

Just my opinion, but I have done this myself, so it is from experience

AB

[Gina]

I have mine (on my MN190) between scope and dew shield just under the edge of the dew shield.  This warms the corrector plate nicely and the dew shield provides thermal insulation meaning the less heat is used in the heater.

[laudropb]

Gina's idea seems much better. Inside the dew shield will undoubtedly lead to an unsteady image.

[james_screech]

I built put a DIY heater inside the dew shield on my C8 and didn't notice any problems.

James

[opticalpath]

I have mine (on my MN190) between scope and dew shield just under the edge of the dew shield.  This warms the corrector plate nicely and the dew shield provides thermal insulation meaning the less heat is used in the heater.

I do exactly the same as Gina on my MN190, and I agree about the outside insulation - definitely reduces the wattage required.

Adrian

[michael8554]

Chris Heapy had his just in front of the corrector, inside the front lip of the OTA tube:

http://www.beevo.org/Dew_Heater.htm

Michael

[ChrisLX200]

Chris Heapy had his just in front of the corrector, inside the front lip of the OTA tube:

http://www.beevo.org/Dew_Heater.htm

Michael

Whoa! that takes me back ;-)   But anyway, you have to think how a dew heater works to keep dew at bay, the heat has to go into the front element to raise its temperature slightly above ambient, putting heat in front of the tube does nothing useful at all. If you need to put the dew strap around the outside of the tube (close to the edge of the corrector plate) then by all means add some extra insulation on top of the tape as an extra layer, that will direct the heat inwards rather than losing most of it to external air.

ChrisH