Observation...

[vulcan]

It occured to me that a lot problems with 'dew' could actually be caused by breathing on the eyepiece. Difficult to avoid! Breathing on a cold piece of glass will cause condensation. It happened to me so it must happen elsewhere too! The trick here is to keep it cold if you are going outside. Fog is another story but where, oh where, can you get 'clear sky' aerosols (of the environmentally friendly type)?

[perrin6]

I try not to let my eyepieces get too cold by bringing them indoors every time I consult my sky-software and sometimes leave them near an electric fire (careful now!). I have made the mistake of leaving them until they got quite hot - luckily with no apparent damage except for slight eyecup warping on my nagler. Am also well in the habit of breathing out the side of my mouth to minimize misting.

I also have a hair-drier plugged into an extension cable ready for secondary de-fogging.

[John]

I always keep my eyepieces a little warmer than the outside air - and capped when not actually in the scope. It's a little chore but it does help reduce at least one misting problem.

Interestingly, some eyepiece designs are more prone to misting than others - usually those where the eye lens (top lens) is not recessed much because the eye relief is short and the eye needs to get close to the lens to see the whole FoV. A plus for long eye relief eyepieces perhaps ?.

[perrin6]

Interestingly, some eyepiece designs are more prone to misting than others - usually those where the eye lens (top lens) is not recessed much because the eye relief is short and the eye needs to get close to the lens to see the whole FoV. A plus for long eye relief eyepieces perhaps ?.

I'm sure the water on my eyeball sometimes mists up the eyepiece and this gets to be more of a problem on very cold nights especially if there is a breeze as this makes my eyes water. So this would be more likely to happen with eyepieces of low eye-relief as the eye is closer.