dew shield - why

[steelfixer]

morning all.

sorry but I have yet another question for you all.

what is a dew shield for?

Might sound a stupid question and it probebly is but I cannot see the reason behind putting a tube on the end of a tube.

If it is as the name suggest to prevent a build up of condensation surely by increasing the surface area you only increase the amount of condensation.

thanks

graham.

[brantuk]

Hi Graham - it's to help stop the dew on the mirrors or lenses. On a newtonian it protects the secondary from dew, on a refractor it shields the objective lens, on an sct or mak it protects the corrector plate.

It serves also to reduce any stray light entering the tube from the sides (eg street lights. Hope that helps

[steelfixer]

I asume then it would be made from a material that will condense the moisture in the air faster than the tube material and it will have some sort of drain point to allow the collected dew to run off down the outside of the tube.

[Merlin66]

When the temperature drops and reaches the "dew point" the moisture from the atmosphere drops out. This moisture usually falls vertically to the ground, so an extension tube ( if long enough) on the front of the objective effectively shields the optics from this "rain".

As the telescope continues to cool, condensation can form on all the exposed surfaces, including the optics. Low watt heaters (dew heaters) positioned close the the objective can help to reduce the amount of condensation.

HTH

[brantuk]

Yes the material makes little difference - some folks use a cardboard tube or camping mat rolled and velcro'd. If the dew is heavy it'll all get misted up anyway and a dew heater system may be needed. Sometimes though nothing works and the only choice is to pack in for the night if it's particularly bad.

[steelfixer]

question answered thank you for your help. better start making one.

Any thoughts on the best material to use.

I figure Aluminium as it has high heat transfer properties which should pull out the moisture from the air before it enters the tube and condenses and it is light enough not to upset the balance of the scope.

What is the normal length of protrusion from the end of the tube.

[Demonperformer]

Graham

All my dew shields are made from an A3 sheet of black card from a local hobby shop. Never had a dew problem since (even in conditions where others have had no end of problems). The main thing is to make sure they are long enough (rule of thumb: overhang should be 2x diameter of the OTA).

HTH

[steelfixer]

Hell you guys are good you even answer my questions before I have time to post them.

[Demonperformer]

Well, people are always telling me that I am psychic ... no ... sorry ... the word they use is psychOTic

[Mr Spock]

Never use a dew shield. I've only had dew once...

[Telrad]

Hi Graham - it's to help stop the dew on the mirrors or lenses. On a newtonian it protects the secondary from dew, on a refractor it shields the objective lens, on an sct or mak it protects the corrector plate.

It serves also to reduce any stray light entering the tube from the sides (eg street lights. Hope that helps

Kim has answered it. I mainly use mine for stray light too. I made mine from neoprene and velcro - works a treat

[steelfixer]

So rearly it is not an essential piece of kit ?

[lukebl]

So rearly it is not an essential piece of kit ?

Technically, not essential. But fairly essential if you want to prolong your observing on damp nights. You have nothing to lose by having one particularly if, like me, you just use a old piece of cardboard which you can throw out when it's worn out. Here's my high-tech one (note the similarly high-tech one on the finderscope)

[steelfixer]

very nice piece of kit. the telescopes none too shabby either. (lol)

[Moonshane]

for newts dewshields are not essential but very occasionally they are needed but you realise when it's too late. unless you live in a place totally secluded from neighbours (who will turn lights on!) then a dew shield on a newt is essential but more as a 'light shield'. it does increase the contrast a little too and is in a way an alternative to flocking the tube. I have done both. once my secondary dewed up when I left the plug out of the focuser for an hour (I don't leave eyepieces in the focuser when not with the scope).

for SCTs and Maks although I have never used one, know that a dew shield is essential unless you like to see nothing on those all so dew filled nights!

[steelfixer]

Isn't it great knowing the answer to all your questions is only a couple of clicks away.

Thank you all for the help

[Demonperformer]

What sort of scope do you own? As mentioned in some of the posts, their necessity is in part determined by this. If you have a refractor or SCT/Mak, you will need one because of this big piece of glass exposed to the air. Newts are not quite so critical.

[steelfixer]

For my sins I am the not so proud owner of a seban

But I am going to collect my new scope on saturday.

It is an 8 inch helios (now called the skywatcher 200 i am reliably informed by the good people on here).

I cannot wait to get it home to see what a real scope is all about. The six hour or so round trip will be well worth it.

[Demonperformer]

OK, a newt. Therefore I would agree with Shane's comments.

[acey]

question answered thank you for your help. better start making one.

Any thoughts on the best material to use.

I figure Aluminium as it has high heat transfer properties which should pull out the moisture from the air before it enters the tube and condenses and it is light enough not to upset the balance of the scope.

What is the normal length of protrusion from the end of the tube.

A dewshield works by physically shielding the optics from the sky, thus reducing the tendency of heat to radiate from the surface.

Think about where dew forms on a car: mostly it's on the skywards facing parts (roof, bonnet, windscreen etc), more than the sides. Put a roof over the car and its top is less likely to dew.

So the material used for making the dewshield is irrelevant, except for practicality (lightness, flexibility, ease of cutting, ability to withstand getting covered in dew). Camping map is convenient and durable, and I find cardboard fine for small apertures.

Length should be as much as possible. Consider 1.5 times aperture as a minimum. And as a further anti-dewing measure, make sure you don't leave the scope pointing up into the sky when you're not using it.

How much you need a dewshield depends on conditions at your site, length of time you observe, aperture of the scope, and in the case of Newtonians, how close the secondary is to the top end of the tube. With an 8" Newt I found I rarely got dewing but with my 12" the shield is essential. With both scopes I also experienced dewing of the primary, which people say rarely happens, but I suspect it's more common than people realise - they just don't notice it happening. I deal with that using a 12V hairdryer.

Dewing is only likely to occur when the air is still. In windy conditions a dewshield on a large scope can make the scope more prone to vibration (or can blow off!), but is not actually needed. So if there's a strong breeze I dispense with the dewshield.

One further point - make sure the interior of the shield isn't bright, as this will scatter light into the optics. I noticed this with the first camping mat dewshield I made, which was light blue. I tried painting it matt black but the paint flaked, so I got a camping mat that was black on one side.

[damnut]

The seban has done its job and got you hooked ! Agree with comments above re dewshield and newt. I rarely do not get started without putting the dewshield on my sct, especially after one night where i did not bother and the corrector plate looked like niagara falls...

[steelfixer]

ok folks all taken on board. sorry for the time delay in responding but i am at work and only get time to look at this in between welds.

thanks again for all your help.

[swampy]

Thanks for the info. There are so many questions I want to ask but don't need to because it's already on here somewhere!

[Telrad]

I love this forum, it's so helpful and easy to navigate. I use search a lot, usually someone has asked a question I would have asked, and the replies are so helpful. It's a great resource

[spaceboy]

For my sins I am the not so proud owner of a seban

But I am going to collect my new scope on saturday.

It is an 8 inch helios (now called the skywatcher 200 i am reliably informed by the good people on here).

I cannot wait to get it home to see what a real scope is all about. The six hour or so round trip will be well worth it.

I really can't wait to hear how much better you find the Helios Make sure you don't get tempted to clean the mirror with anything other than a bulb blower. Safe journey getting the scope.