How good are dew shields?

[gobes]

Just wondering how good dew shields actually are. I've seen the ones that appear to simply wrap around the end of the tube, but I can't see why this would be hugely effective.

How do they work, and are they any good?

Thanks.

[resonator77]

Depending on the scope, for SCT and Mak dew shield is very important, as it stops the secondary mirror from dewing up result of which is blurred image as if you were looking through thick fog. Dew shield alone may not stop dew forming during long viewing/imaging fully that is when a heater strap will come in.

For the most of the time dew shield is sufficient and if you have 12v portable hair dryer you can blow away the dew as it starts to form.

[Merlin66]

Yes they certainly work.

For a refractor/ SCT the ideal length would be about twice the diameter of the aperture.

They can be made from cardboard sheet, camping mats, plastic etc etc.

The object of the exercise is to prevent ( or minimise) the amount of dew which falls on the lens. ( Dew forms in the atmosephere when the temperature drops below "dew point" and falls vertically)

Ken

[Demonperformer]

Would agree that they are vital, but they do not have to be elegant. Mine consist of stiff black card (£2.64 for an A3 sheet from Hobby Craft), folded into a loop, stuck with masking tape, which slide over the front of the scope.

"Simples."

[yeti monster]

Mine is certainly vital, as it replaces the lost weight of the corrector plate cover and thus aids balance on the Dob mount. Also it helps to keep dew off the corrector and stray light from entering the scope.

[russ]

For larger apertures a camping mat makes a good material for a dew shield. I run a strip of Velco down the joint. My local camping shop had some mats that were dark blue on one side and black on the other. Which looked okay with the Meade.

My location is dew magnet, so i need to use a dew shield in combination with a heated dew strap. Or if it's not too bad, the dew shield and occasional blast from a 12v hairdryer.

Also use the dew shield with the 8" dob. This reduces the chances of the secondary dewing over and also acts as a light scatter shield.

[gobes]

Cheers guys.

Sounds like I'm better off knocking my own up then, and avoiding shelling 25 quid for no reason.

How far out from the rim of the scope would it ideally need to extend?

[Merlin66]

As mentioned above:rolleyes:

x1.5 to x2 the objective diameter

[russ]

Twice the aperture (2 x 5") is the length to aim for. Perhaps 8-10".

Ah beaten to it by Ken.

[gobes]

^ Ooops... thanks guys. I must read replies fully before jumping back in ;-)

[jgs001]

I use the camping mat approach, which also works well on camera lenses, although I found for my 50mm, it had to be shorter than the 1.5 to 2x

[ollypenrice]

Agree on camping mat. It has the right thermal properties, is light and hard to destroy.

Olly

[acey]

Dewshields work by physically screening your optics from the sky, thus reducing radiation of heat away from the glass, which is what causes dewing. Other than physically warming the glass, the best strategy for preventing dewing is to expose it to the sky as little as possible. So you want a dewshield to be as long as you can reasonably make it, and you don't want to leave your telescope pointing up at the sky unless you're actually looking through it. 1.5x aperture should be considered a bare minimum, but the longer the better. Similarly you should cap or cover eyepieces when not in use. A dewshield can be a problem in breezy conditions, but you're unlikely to get dewing then anyway, so leave it off. Whether you use camping mat, cardboard or anything else, it's best to have the inside of the dewshield matt black, as it also serves to reduce stray light. Thermal insulation plays no part, so the only requirement for dewshield material is that it holds its shape, doesn't sag into the light path, and can withstand the dew that will form on it.

I use camping mat for my 12" dob, held on with velcro, and a bit of cardboard for the finder, held on with an elastic band.

[lguise]

Yes camping mats work well. The outdoor shop 'Blacks' sell a nice black version for around £12 I have made two or three dew shields from one of these with some black elestic sewn into bands to hold in place. I tend to cut to a dead fit so there is no messy overlap. With the elestic in place looks and works very well.

Best wishes to all,

Linton

[marcdearn]

I made one on the second day of SGL5 as I had suffered on the first night

It isn't going to win any beauty prizes but did the job very well...

Camping mat and stickyback velcro, a stanley knife and voila!

[Glider]

Camping mat and stickyback velcro, a stanley knife and voila! Yes exactly what I did. Worked a treat, ice on the outside of the OTA but the mirrors where clear as a bell.

[LukeSkywatcher]

I live on the east coast of Ireland and i know that during the summer months that dew is a HUGE problem.

I have constructed dew shields for my scope and bins out of a campimg mat and velcro.

I am hoping they do their job.