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Disaster!


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All set up to show my wife Saturn tonight -beautifully clear and apart from a very bright moon everything seemed perfect. I even put the scope out ready way in advance all set up with the right ep. Tonight, at last, she would be spellbound... until conden-blumming-sation ruined everything, had a better view just with thine eyes thanI could get with the scope!

Now then, is this a problem with spring/summer viewing, with the big drops in overnight temperature? And is there anything I can do...? Also once mirrors and eps have got wet is it better to let them dry out before replacing dust caps - or better to prevent the dust and hope the warmth of indoors evaporates the water beneath the cap?

Humph!!! :)

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Let the scope dry out before replacing the caps otherwise you can get fungus growing on them.

When observing you can use dew shields (tube made of camp mat)/dew heater straps or a hairdryer on coolish down the tube. Careful with mains out as the dew will collect on any extension cable etc you have out too.

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Just an idea - with the mirrors covered, how about letting it cool down outside under a parasol/gaxebo etc. to minimise condensation getting anywhere near it. Or let it cool in the garage and move it outside just before viewing..

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You can buy dew shields which seem rather overpriced, or make one yourself, many people recommend using camping mats. I made one out of an offcut of radiator reflective foil and some duct tape - very easy (and I'm normally a DIY numpty) and effective, my sope was out for 5 hrs this evening with no problem - let me know if you want the "recipe"

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the dew was dreadful tonight. I did a silly thing, set up too early.. by the time i got outside i needed the hairdryer to gently remove the moisture. The eyepiece was ringing wet too! Was going to try imaging but gave it a miss. Like the idea of the umbrella.. maybe that would work? I guess its just we had rain this morning which made the conditions worse tonight.. Oh well just got to wait for another clear night maybe tomorrow will be better! Summer can be dewy too just have to check the humidity levels.

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Really bad luck with the dampness :( , ironically for me its been a night free of it for a change, the sct has now been out for over 5 hours and not a sign of it on the tube or the lens shade, or misting up of the corrector so far, still time for another hour or so. :)

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There seems - in my experience - to be two types of dew:

  1. The type of dew that plagues astronomers most; seemingly limitless condensation that forms on equipment in the early hours of the morning and can be so bad your equipment can be dripping like a tap
  2. The one-off mild saturation that happens once, shortly after sunset. This stuff actually falls silently like ultra-fine rain, and only affects items that are exposed to the sky.

From the time of your post, your problem sounds like (2). You can solve this simply by keeping eyepieces etc covered with a cloth, and not having the scope pointing upwards at the time when dewfall happens. Once dewfall is over, you can uncover things, and not worry where the scope is pointing.

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Once you have it on the lens there is a fairly quick fix. Have a towel on a radiator inside, warm and dry; place it over the scope, not in contact with the glass, and in a few minutes it will absorb the humidity and your lenses will be clear.

Camping mat dewsheilds are superb and cheap.

Olly

Edited by ollypenrice
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Thanks for all the tips, the main problem seemed to be the eps and I kept them capped until I wanted to use them. Although the secondary mirror may also have been wet, but I couldn't tell. The primary looked okay. Are dew shields effective on Newtonians or are they more for refractors and scopes with front lenses?

Regarding the eps, is it best to keep them indoors til the last minute or let them cool too but maybe in a box or something to prevent dewing?

I like the idea of cooling in the garage Mike, I'll try that in future.

Thanks again for all the help, it's all these little bits of knowledge sharing that make this such a great forum for beginners.

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Once you have it on the lens there is a fairly quick fix. Have a towel on a radiator inside, warm and dry; place it over the scope, not in contact with the glass, and in a few minutes it will absorb the humidity and your lenses will be clear.

Nice one Olly - hadn't heard that one before - thanks

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Regarding the eps, is it best to keep them indoors til the last minute

Too much faff really.

Some people keep them in their pockets for warmth in order to prevent your eye misting them up during use, but I find if you just keep looking, the heat from your eye warms up the eyepiece after a minute or two anyway and the mist clears.

Important point with eyepieces though: If you've been out for a long while, don't uncap them the moment that you get indoors again, or they'll immediately get VERY damp indeed from indoor condensation. Let them warm up to room temperature over a couple of hours and inspect them for cleanliness etc. later in the morning.

Edited by great_bear
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I must look into dew formation in more detail. I wonder if it rises from the damp grass also.

My tripod is on the lawn, and I never really get hit by dew at dusk. Maybe I'm lucky, or maybe its the routine, or the location......

....I'll read up on dew point and other related matters.....

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The wikipedia page on dew seems quite good.

The main thing with dew is it forms on the coldest surfaces. The surfaces which are coldest are those which are radiating their heat to space most effectively (and aren't getting heat conducted into them from elsewhere). A dew shield works by reducing the range of angles which the secondary mirror can radiate to space. For those angles you are basically replacing cold space with (relatively) warm camping mat. All being well something else (probably the top of your scope) will cool down more than your secondary mirror and the dew will form on that instead.

Since I usually drive out to a car park to observe I keep my eyepieces on the front passengers seat. I wind down the window and just reach in an pick up the one I want. A box will work as well. In fact just putting a 'roof' above them so they are not seeing cold space should be enough to stop dew.

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Seems I wasn't the only one thwarted by dew last night. After a beautiful day, tonight it is the more familiar cloud that will spoil the fun. Oh well, such is life. Today hasn't quite gone to plan, especially events in Dublin.

Thanks for all the advice once again, seems like everyone has there favorite was of combating dew. I have a garage and an old camping mat and some perseverance so I'll beat it in the end.

Night all.

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