Dew heater

[Geoff]

Hi all, I have just put dew heaters on the finder scopes, a removable one on the eye piece and a heated box for the other eyepieces on my 10 inch DOB.
They work well, but I need one on the secondary mirror and was looking for ideas first. I was not sure if resistors behind the mirror, or a small fan would work. I'm open for suggestions.
Cheers,
Geoff :)

[sheeny]

Geoff,

I'm sure either the resistors of the fan could be made to work, but hopefully some people with more experience with Newts will comment. When I had a newt years ago I just suffered any dew...:( I didn't have any real dew solutions.

I would probably tend towards a 12V hair drier to clear the secondary as required rather than fixed resistors that may introduce convection currents to the optical path or a fan that might vibrate the scope. It depends how much you want to push your optics to the limit.

I'm sure there's plenty of Newt owners here who can give good advice though!

Al.

[astronut]

Hi Geoff, Do you have a dew shield on your scope? if not try this first as it will dramatically cut down on dew cutting short your viewing.
I have a 12" LB and without the shield the secondary dewed within an hour.
With the shield, on a very dewy night with the scope deliberately pointing straight up, NO DEW!!!!
The other thing you need is a fan on the base of your Primary, keep this going all night and it will keep both mirrors from fogging up. Cheers, John.:thumbsup:

[rmcpb]

Geoff,

Try the passive methods of controlling dew. We spend an awful lot of time getting our scope at ambient temperature so why would you add a heater if you don't absolutely need to. Try a tube extension/dew shield and a fan pushing the air up the tube. Since using both these methods I have not had any dew problems.

Give them a go......

[Geoff]

Thanks, for that.
I am fitting a fan and building the dew shield at present.
Geoff

[asimov]

Years ago, when they used to make real telescopes ;) They always made sure to make the tube long enough. IE: the diametre of the tube sticking out past the secondary mirror minimum! The 2 newts I've made, I stuck with the same principle of design. I have never had dew on a secondar mirror in nearly 30 years.

The dew shield should be sufficient.