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A lot of dew on EP's

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A lovely clear night last night here in west wales and very dark :eek:

After about an hour or so, the OTA was very wet with dew, but the mirrors looked fine, but when i went to change an EP i could not see a thing, tried another one and the same, after taking a closer look at EP's dew was quite bad on them, and had to take some soft tissue to them to be able to use them, :)...not good.

Then when looking into my flight case (which holds ALL my kit) everything was soaking! my filters very bad (even in there cases) my OIII had water on it, so concered about that now.

How can i prevent this again? and is there a safe way to clean them and the filters? these have water marks on them now:(

Edited by bcfcciderhead
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Hi Dave - I'm not too hot on cleaning optics never having done it myself. But it sounds like you definitely need a dew control system for the ota.

Far as eye pieces go - I keep my flight case closed when outside, and that's usally enough to stop the dew forming. Whilst using one eyepiece, I have the next one in my pocket for 5 mins to keep it warm and dry.

If that isn't satisfactory you can get dew bands for eye pieces once you have dew control in place. Ensure you get a system with two outputs at least. Bear in mind when the dew is very heavy, sometimes the only alternative is to pack up for the night. Sorry I can't offer more but hope that gives you a few ideas for the future. :)

Edited by brantuk
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Thanks Brantuk, any advice is good to me.

The OTA looked ok, nothing on the mirrors, my main concern was with the EP's but unlike yourself i had my case open...lesson learnt.

I just need to clean the filters and the odd EP as they have dried with a water mark on them.

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brantuks idea about keeping EPs in your pocket is a good one

i had my case closed last week but the eps in there still fogged up but the one in my pocket was OK

(bcfcciderhead....anything to do with the mighty robins from the gate by any chance ??)


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Dew heaters and straps are an expense that we all try to put off until the inevitable happens. In your case this might appear further down your list of priorities as you have a reflector but the secondary, if very near to the top of the scope, will pick some of this up sooner or later. The easiest route for you is to make a dew shield to extend the front of the scope thereby effectively placing that secondary further back in the scope. Regarding eyepieces, I would always cover them with an empty film canister or eyepiece container every time you're not observing, even if it is for a couple of seconds, likewise with the the finder scope too.


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Optics dew if they are exposed to the sky, allowing heat to radiate from them. As long as they are shielded from the sky they are unlikely to dew. Eyepieces kept inside a closed case, or in a pocket, or capped, should be fine. If an eyepiece dews, put it inside the case and leave it there for a few minutes - it will go clear.

Surfaces that are vertical are less prone to dewing than horizontal, sky-facing ones (cars mostly dew on their roof and bonnet, not the sides). Keep the scope horizontal when not viewing through it, and don't leave eyepieces exposed on a rack. A dewshield works by shading the secondary (to an extent) from the sky. Any flexible material will do: camping-mat is convenient, though cardboard is just as good for small apertures (I use cardboard for my finder dewshield).

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Thanks guys, helpful as ever.

I have a home made dew shield from a camping mat which seems to work quite well, but after reading the replys i have had the dew on the EP's was all my fault :iamwithstupid:the one's i was using and kept changing around i just put down on the table, not back in the case with the covers on so lesson learnt there,

and yes my name above is refrence to the mighty (well sometimes) Bristol City Fc........for my sins

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