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pjmaciw

Do reflectors need dew shields

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Title says it all. Do reflector scopes need a dew shield. I have noticed that all the dew shields for sale refer mainly to SCT's and refractors. I recall reading somewhere (probable on here) that reflectors do not usually dew up due to the primary being at the bottom of the tube and the secondary facing down the tube rather than being exposed to the sky.

I have not been out on a cold night yet but on one night the scope was covered in dew but the optics were still clear.

If needed I may make one as the commercial ones are rather expensive for what they are.

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Hi.

In short yes.

It's normally the secondary mirror that dews up.

Regards Steve

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I use one on my 10" newt. It's mainly there to stop unwanted light from entering the top of the tube but also prevents the secondary from dewing too I guess. Mine extends about 12" above the top of the tube.

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the main reason I use them is also as a light shield rather than a dew shield. in this guise, you just need to look through the empty focuser and see now how above the opposite side tube wall top you can see. this is the minimum length for your shield. for dew then in theory it's 2x aperture but a 3 foot one for my 16" is pushing the realms of nutcase.

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i use mine as a light shield too for the most part anyway, never had much an issue with dew on my mirrors, now i cant say the same for foggy EPs!, that annoys

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the only time my secondary ever dewed was when I left the focuser empty with no eyepiece or plug in. the dew fell straight on my secondary. the primary has never dewed (other than when brought inside).

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Thanks peeps. Good replies.

Now what can I cut up for a dew shield ........... another raid on the garage is called for.

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go to hobbycraft and buy some sheets of 2mm black foam, either self adhesive or not and make one from that and join with velcro. works a treat.

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The secondary on my 150P is quite prone to dew when imaging. I have a 10" long dew shield made from a section of black foam camping mat from Asda.

Not really had a problem with dew since other than the Saturday night at SGL6 when the secondary and primary both fogged up as did my glasses and the lappy screen.

I have a 12" long one on my 250PX and have never had a problem with dew. As Shane says, any longer looks a bit bonkers and catches the wind too much.

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go to hobbycraft and buy some sheets of 2mm black foam, either self adhesive or not and make one from that and join with velcro. works a treat.

Thanks. I want to make a shield for my finderscope so will give it a go.

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On another note: Heat radiation!

I have a friend who is a metorologist, and when I asked him about dew forming he promptly asked the color of the Explorer 250P. When I told him it was black and made of fairly thin "tin", he immediately suggested wrapping the whole thing in some insulating material to prevent heat radiation.

When the dew-point is close to the ambient temperature, say a degree or two apart, the heat lost in radiation takes the scope itself down to the dew-point and you get dew accumulation on the primary as well as the secondary. Do not assume that dew "falls down" from the sky; it forms when condensation takes place on a particle, like the surface of your mirrors.

Naturally (he is just about never wrong, be it green house stuff or Al Gore) I took his advise and wrapped the entire Explorer in foam camping mat cut to fitting pieces.

I have tried it out and it appears to work wonders! If you manage to get still-standing air in the tube and you have insulation, the chances of getting dew are small. Adding a normal dew shield is not bad either. It keeps the air column in the scope at a stand-still and thus prevents heat exchange with the colder outside.

Now, as for dew falling down, that statement is not entirely untrue. The air is full of particles, and water vapor condensates on the particles when the conditions are right, and these dew drops follow the laws of gravity and can land on your scope. I have spent twelve years (not the whole time) on polar expeditions in the high Arctic, and the condensation on particles can be observed very clearly up there:

1 - Bring a warm cup of tea outside - no visible "steam" coming up.

2 - Light a normal cigarette lighter one or two feet upwind from the cup: bang, steam visible!

The reason, of course, is that the air is super clean up there and the water vapor just doesn't have anything to condensate on until the invisible smoke from the butane burning of the lighter comes by... Just to prove a point :rolleyes:

Anyway:

Black or dark scope: pad it with insulation and add a dew shield as well

White scope: add a dew shield

Well, that turned into a rant, didn't it ;)

All the best,

Per

insul.jpg

Edited by perfrej
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fascinating post Per! I may try this myself. it may also help with tube currents I suppose? flocking a scope may also have the same effect?

I wonder if imagers suffer from dewing more than visual folk as imagers by the nature of them leaving the scope in the same position for long periods (at worst at the zenith).

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Fascinating post from Per there.

I dont need a dew shield - I have a hairdryer that I rush out with on a long extension cable !

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fascinating post Per! I may try this myself. it may also help with tube currents I suppose? flocking a scope may also have the same effect?

I wonder if imagers suffer from dewing more than visual folk as imagers by the nature of them leaving the scope in the same position for long periods (at worst at the zenith).

You may have something there. My little scope dews far more when imaging than when looking through. I figured it was absorbing a bit of heat from the close contact at the eyepiece?

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I don't insulate the tube, but t makes sense to me. However cold it may feel here, it is colder out in space, so the tube will be radiating away heat. I have heard that even putting an umbrella above the tube (not blocking the view of course) would help. Never tried it though.

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hi pj well i have a 12" reflector and so have the astro zap dew shield ,and it is great for a few reasons stray light and dew ,in a couple a years dew as only been a prob a few times,i tried to make one out of a cheap black roll matt for camping aprt from it getting more dew on than the scope ,i got the astro zap one its flocked on the inside which really does stop the dew my scope is flocked fully on the inside as well

last night i got out for a few hours the scope on the out side was wet ,every thing else was dry as a bone

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Scope are normally put outside for ~ 1 hour to climatise\cool down.

Does this not induce dew if the relative humidity is high.

Is observing recommended when relative humidity is high

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I dont bother when the humidity is up, the seeing is rubbish, its like looking through boiling water

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The rule of thunb I always used was if I was getting dew on the tube or mounting then the secondary on a Newtonian would be dewing up as well.

Not perfect, but it worked fairly empirically for me. Especially useful if you are doing long exposure photography, as its so frustrating to set up everything then come back and find half the esposures useless as the secondary dewed up...

I always used a shield on mine to stop stray light getting into the tube (I observe from a back garden, lots of stray light)/

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Thats good to know, And is 12" enough for the 300p? I have a camping mat I could cut up..

That's how I made mine, for my C8N. It's about 14 inch long, I have cut notches for the spider screws and it sits up to the focuser. icon14.gif

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I purchased a 1 metre square sheet on the 5mm Cell Foam from these people, i cut it in half and this made 1 Dew Shield plus enough for another, this fits a Skywatcher 250, black Ducktape on the outside, Ordered on a Saturday evening arrived on the Tuesday Cost £11.30 including shipping, they also do a 2m by 1m sheet so bigger scopes won't be left out in the dew..:)

EasyFoam: Closed Cell Polyethylene Foam

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