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Dew on refractor lense


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Hi,

I'm a new member here. I'm wondering what is the best course of action when you come in from observing and the lense steams up with condensation? Is it best to leave it with the lense cover off untill the condensation has evaporated? I'm a little reluctant to leave it uncovered overnight when 'stuff' might settle on it but maybe this is needless paranoia!

Thanks,

Malcolm

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It’s the only solution really. I leave mine uncovered until it’s clear, overnight if necessary. You could quicken it up with a hairdryer if you want, just don’t wipe it.  

Edited by Jiggy 67
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Leave the lens uncovered overnight, assuming the 'scope is left on its mount, swing it so the lens is downward to allow any water to drip away from the important bit, and not have any dust settle on it. You could leave the diagonal uncapped and rotated so the eyepiece fitting part is pointing downwards too (to avoid dust but release any damp) if there's a huge amount of moisture. My refractor got caught in a very sudden rain shower for a few moments one night , I rushed it indoors, dried the outer tube off with a microfibre cloth, racked the focuser right out to reduce the chance of any damp being trapped between the tubes, and left it overnight , caps  off and aperture(s) down as I describe above . It shows no bad effects of it's adventure . 

Just don't touch the front lens , wiping it is a really bad idea !

Heather

 

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If you take a warm, dry towel from the airing cupboard and drape it over the front of the tube (not in contact with the lens but resting on the extended draw tube) it will speed up the demisting process. The fabric quickly absorbs the humidity.

Olly

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If your lens gets condensation on it outside then all of the above is correct. However you said the lens steamed up when you brought the telescope inside. The way to stop that is (apart from don’t bring it inside!) cap all the lenses and put your scope in a bag or case before bringing it in. Everything will warm much more slowly and condensation won’t form. 

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  • 2 weeks later...
On 21/09/2021 at 08:38, robhatherton said:

If your lens gets condensation on it outside then all of the above is correct. However you said the lens steamed up when you brought the telescope inside. The way to stop that is (apart from don’t bring it inside!) cap all the lenses and put your scope in a bag or case before bringing it in. Everything will warm much more slowly and condensation won’t form. 


+1 for this. The following morning when the scope will be at inside temp uncap everything as above, no new moisture will form.   Allow to thoroughly dry out, cap and safely store.

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Ref the above posts. Cold air can hold much less moisture than warm air.

If the lens is dry at say 5C and you trap 5C air between the lens and cap, it is still dry after warming to 20C overnight.

Your 20C house contains a lot of moisture. Breathing, cooking, etc.
When you bring in your dry 5C scope, the moist 20C air is chilled over the lens and body so you get condensation.

A quick waft over with a hair dryer will quickly bring a scope up to room temperature to discourage condensation.

If your (not dewed) scope is in fairly airtight storage, some silica gel will keep things dry.
Discouraging tarnishing on the bright and shiny metalwork, and discouraging fungus growth.

HTH, David.

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