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I've been reading through a number of posts on the forum and "Dew shield" is mentioned a number of times. From what I understand, this is a semi-flexible extension to the reflector that (I assume) reduces the build-up of dew on the mirrors.

Are these important? I see that they can cost around £40 but I also note that they look rather like rolled up camping matts that are cut to the right length!!

Is it really that simple to make one or am I missing some crucial element? And what impact would this have on my 200P dob

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You have it right. Most people make their own using a camping mat cut to size. They do work well all depening on the level of dew in the air at your location. Well worth the few quid to make your own. There are other methods to prevent dew: heated dew straps, or cooling fans (for Dobs).

Hope this helps.

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Here's mine. Pure camping mat and package tape. Beautiful, aint she? ^_^

post-26290-0-28783500-1353085314_thumb.j

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I'd recommend a dew shield. They aren't essential but they do roughly double the amount of time it takes dew to form, which on particularly bad night can mean 2 hours instead of one. Imagine the secondary dewing up just as the primary becomes cooled... :(

They're easy to make, just a rolled up mat with some tape or sticky velcro. I found mine would sag/deform over time so i reenforced it with 2 embroidery rings.

The general rule of thumb for length is 1.5-2x the diameter, so 300-400mm for the 200p.

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The general rule of thumb for length is 1.5-2x the diameter, so 300-400mm for the 200p.

Good point and worth knowing. Although it seems that the length it extends out from the scope really does not matter once the shield does not sag or droop infront of the OTA and block incoming light. But yes.......................1.5-2x the scope diameter is more then enough. I used double sided sticky back velcro on mine.

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So, a sheet of dark foam that's 2pi x 100mm (620mm) x 400mm should be about right? Will this also help with incidental light getting in the tube / affecting the secondary mirror?

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So, a sheet of dark foam that's 2pi x 100mm (620mm) x 400mm should be about right? Will this also help with incidental light getting in the tube / affecting the secondary mirror?

For a solid tube 200P Dob................you want it to be 200mm (8") to fit around the scope tube and 300-400mm in length to extend out from the tube. With a solid tube Dob it doesnt really act as a light shroud (accidental light getting in), but it will do that also to a degree. I have an open truss 130mm Dob and my dew shield acts as a dew shield and also as a light shroud for the open truss design of the scope.

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I made mine from a card board box, cut it down to the right size and colored it black, works excellent, forget the extra expense, unless you really have too :grin:

Ronnie

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My scope is a Nexstar 4SE so a good size cereal box did the trick :tongue:

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A dob has the optics at the back so you aleady effectively have a dew shield - the tube itself.

If you're experiencing problems with dew on the primary or secondary then go for it but if there's no problem to solve then why bother?

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The secondary is the main problem on dobs. I've yet to see dew on the primary, but the secondary bites it every single session.

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DIY camping mat version has helped a lot with our Dob. Specs as per Luke Skywatcher's post above.

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Hi I made one out of a cheap thin door mat £2.99 bought from a local store, rubber on one side hard ribbed mat on the other, put the rubber side out so the water runs off and doesn't soak through and the mat inside so it might soak up some moisture, as bought its width was 360mm which is 3X the 120mm lens which is what a lot folk recommend, I just wrapped it around the existing metal shield and glued it together so it comes off if not needed, it is also very light but very hard and wont distort with damp or time

gordon

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A dob has the optics at the back so you aleady effectively have a dew shield - the tube itself.

If you're experiencing problems with dew on the primary or secondary then go for it but if there's no problem to solve then why bother?

Even if the secondary isn't dewing if your observing site has some stray light (street lights, neighbour lights...) then a dew shield can act as a light shield to prevent stray light getting to the eyepiece/reflecting off unflocked tubes

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I tend to use my home made dew shield as a kind of light block from the street lights, does not get rid of it all but it helps

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