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Preventing dew on my scope ?


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Dew on the tube or mount will not do any harm. The tube of the dob is its own dew shield. A few tips are:

- Don't leave the scope pointing at a high angle when not in use - ie you are deciding up what to look at next

- Keep the finderscope caps on when it's not in use

- Keep EPs in your pocket until you need them

Mike

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I believe that's true to a degree (not as effective as actual heating obviously).
Yes, it stops the scope cooling down so fast, so delays the onset of dewing.

However dewing in a (closed tube) Newtonian is not usually a serious problem. The primary mirror will hardly ever dew up in use, and the secondary is more easily dealt with by using a small 12V "hair dryer" when it does. Extending the tube upwards with cardboard or foam (closed cell camping mat material is excellent) will make even secondary dewing very rare indeed.

The short metal "dew caps" often fitted to refractors are almost useless - far too short to be effective, and conductive material is poor because it cools below ambient (by radiation) & therefore chills the air in the vicinity of the objective. To be effective, a dew cap needs to be at least three times as long as its diameter, and to be constructed of an insulating material.

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Dew on the tube or mount will not do any harm. The tube of the dob is its own dew shield. A few tips are:

- Don't leave the scope pointing at a high angle when not in use - ie you are deciding up what to look at next

- Keep the finderscope caps on when it's not in use

- Keep EPs in your pocket until you need them

Mike

I'll testify to this advice - it's exactly what I do and it works. :icon_eek: Except one has to be careful not to leave fingerprints on the EP as you scrabble around in your pocket! If you are going to put EPs in your pocket, it's best to make sure it is fluff-free! Details, details...:)

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Hi is it true that if i wrap a blanket around my 8" dob it will help prevent dew ?

It will prevent dew forming on the outside of the tube - but dew on the tube is not a problem (I've frequently had it freeze solid on mine).

Dew on the primary and secondary mirrors are presumably what you're asking about, and wrapping a blanket round the scope won't prevent that.

Boradly speaking, dewing happens when glass or metal is exposed to the sky: it's the top of the scope tube that dews, not the bottom. In other words it's all about radiant heat, not conduction. So the solution is shielding, not insulation.

The simplest way to prevent dewing of mirrors is to minimise exposure to the sky. For the primary, the tube itself is usually enough, but the secondary is exposed to more sky, being at the top of the tube (even though it doesn't actually face the sky - I did say "broadly speaking").

To reduce the secondary's exposure you wrap a dewshield (made from a camping mat) round the top of the tube. You want it as long as possible, but for an 8-inch scope a standard mat is perfectly adequate. Just make sure it doesn't sag into the light path.

For the finder, a bit of cardboard held on with a rubber band is sufficient.

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I live in an area that is EXTREMELY bed for dew.

I have active dew control for the two finders and objective on the Refractor.

I have no dew control what so ever on the Newt. I don't recall the primary or secondary ever dewing up (EDIT: At home) - but it's only a 6"... maybe size matters :icon_eek:

But at Kelling last year both the primary and secondary dewed up within an hour or so :) It was truely awful!

Ant

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I live in an area that is EXTREMELY bed for dew.

I take it you mean Britain.:icon_eek:

I observe not very far from Kielder and my only anti-dew measures are shields and a hairdryer. I only occasionally need to resort to the hairdryer. If I leave my eyepieces sitting exposed on the rack on the scope they dew up in moments.

Size does matter. It's usually recommended that a dewshield should project beyond the tube end by at least 1.5 times aperture. That's feasible up to 8-inches but gets difficult for bigger scopes. I only needed to buy the hairdryer once I went from an 8-inch to a 12-inch scope.

I did find the primary of my 8-inch sometimes dewing, and I now suspect it was from leaving the scope aimed upright for too long.

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I live on Canvey Island, which strangely is an island, and I believe that we are 1m above sea level.

But being surrounded my water dew is a major problem. Anything placed outside is dripping within about 5 minutes...

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