Dew shield length and dew prevention

[boomboom]

As the title says.

A question on the length of dew shields. Mine is 3x the objective diameter which looks great blocks stray light and so far no dew problems.
Does the longer dew shield prevent dew forming? My experience so far says yes, but (to my mind) a slightly misinformed person on the Aussie forum IIS, thinks that as his Dob gets dew on the mirror and the 1200mm tube acts as a dew shield, then any length dew shield isn't going to work. Personally I think a dob with an open end tube at the mirror end, and being close to the ground is the difference. I have had my Tube wet with dew but nothing on the objective. Am I correct in thinking that? I have never owned a dob so I can't compare. Any one done a comparison like this?
I'm enjoying the freedom of the 1877 Gaunt mount, and the f12 refractor…not an electric cable in sight!
Matt

[mikey cee]

It's hard to tell really what a dob owner actually sees if anything! You are correct any open trussed dob will get moisture down next to the ground. That's why some use those nylon hose contraptions over the trusswork. That and the dropping of collimation tools constantly being used plus scattered light. Mike

[boomboom]

Thanks Mike. Knew I could count on you to give a straight answer

My next scope is going to be an 8" f12 R35, now I have a mount that can take it.

Matt

[Mike]

If you bought a 204-12 R35 TCR the mount would not be as big a concern There is a certain amount of polar moment with an object that long even at lighter weights though. The big issue would be the $1000 (shipping charge) it would take for me to ship it down to you.

Mike

[glend]

Which is why you should build it boomboom.

[boomboom]

Mar 26, 2014 9:23:37 GMT -5 Mike said:

If you bought a 204-12 R35 TCR the mount would not be as big a concern There is a certain amount of polar moment with an object that long even at lighter weights though. The big issue would be the $1000 (shipping charge) it would take for me to ship it down to you.

Mike

Thanks Mike!

We have a Prime Minister in this country who when in opposition had a few slogans one of which was …'axe the tax'

He was referring to a mining tax, but I think it should be applied too import tax!

It will have to be another ATM…when I have a dark sky site to permanently mount it, the wheels are in motion

Matt

[jimcurry]

Hey Boomboom:

I too build with 3:1 dewshields. Never had an Istar dew up on me even when the OTA was dripping. Looks good, too.

Jim

[boomboom]

Had two nights with a very dripping wet OTA. 70mm finder scope lens..fogged up, Istar lens clean and clear.
Well that answers that. A 3:1 dew shield works.
Now to build a dew shield for the 70mm finder.
Matt

[kevinbarker]

Apr 15, 2014 19:27:24 GMT -5 boomboom said:

Had two nights with a very dripping wet OTA. 70mm finder scope lens..fogged up, Istar lens clean and clear.
Well that answers that. A 3:1 dew shield works.
Now to build a dew shield for the 70mm finder.
Matt

Matt
With the finder try a smaller dewcap of say 1.0 to 1.5 times the objective diameter past the lens. Then wrap around some tinfoil around this dew cap. Obviously so the foil does not reflect light into the optical path. I bet it keeps the dew at bay.

It used to work for me in winter dewy conditions in the South island!!

Kevin

[jimcurry]

Apr 15, 2014 19:27:24 GMT -5 boomboom said:

Had two nights with a very dripping wet OTA. 70mm finder scope lens..fogged up, Istar lens clean and clear.
Well that answers that. A 3:1 dew shield works.
Now to build a dew shield for the 70mm finder.
Matt

Or put the lens cap on the finder between uses :>)

Jim