How do you prevent fogging of eyepieces?

[Don Pensack]

I suggest the following:
1) When fogging or dewing is likely (high dew point and cold weather), remove the eyecups from your eyepieces. It will allow more air to circulate between the eyepiece and your warm, moist, eye.
2) If viewing with, say, the right eye, breathe through the left corner of your mouth and always blow away from the eyepiece when exhaling.
3) If fogging is really tough to avoid, rotate your head and view with your head at such an angle that the exhalation from the opposite corner of your mouth is at or above the eyepiece height (because warm air rises). Never, therefore, breathe through your nose or exhale UNDER the eyepiece--it will guarantee fogging.
4) Don't use an observing hood in the winter. It will just guarantee fogging.
5) If it's just a little fogging, wave your hand or mitten at the eyepiece to circulate some cold dry air around the lens and the fog will evaporate. If that fails, or it happens to be frost because of a high humidity, then a small 12V hair dryer will work. This isn't a bad thing to have handy, anyway, because many secondary mirrors are too close to the upper end of the UTA and may themselves dew up.

I can't see keeping eyepieces in a vest, because in cold weather that vest would be on the outside of 3 down parkas and my body wouldn't keep them warm at all. Plus, my eyepieces weigh at least 12-13 lbs, and I can't see keeping that much weight in pockets.

[Dragan]

I suggest the following:
4) Don't use an observing hood in the winter. It will just guarantee fogging.

Using a hood in winter is just fine. I do it all the time. Takes a little practice but using a hood doesn't necessarily guarantee your eyepieces fogging over. Besides, just the heat of your eye and face can be enough to fog things over. What do you do then? Not observe?

The trick is to keep the eyepieces warm just enough and there are options to do that.

I can't see keeping eyepieces in a vest, because in cold weather that vest would be on the outside of 3 down parkas and my body wouldn't keep them warm at all. Plus, my eyepieces weigh at least 12-13 lbs, and I can't see keeping that much weight in pockets.

The stitching can handle it just fine so don't get hung up on weight. You're only placing one eyepiece in one pocket at a time (I would hope). The heaviest Televue EP (21E) is 2.25 pounds. That's well within reason for a pocket to handle. Its a static load so there is no issue whatsoever. With 3000 vests in the field, we have yet to have a stitching fail. Not one. That should say something. Load em up. Just don't do any handstands and you'll be fine.

Depending on air temperature, I'll sometimes where my vest underneath my outer garment. There is plenty of body heat to keep all your eyepieces toasty. Yes you have to zip and unzip each time but that's a small sacrifice pay to ensure warm eyepieces.

If that isn't to your liking, a handwarmer in the bottom of each pocket is plenty to help keep things warm.

Just takes some forethought to ensure fog free eyepieces.

[Don Pensack]

Perhaps I should have said, "It will just guarantee fogging if your eyepieces aren't heated. Not allowing heat from your face, breath, and warm moist eye to fog the eyepiece usually, in my experience, requires allowing ventilation between the eyepiece and your eye and that is the opposite of a hood that extends forward on either side of the eyepiece. I very rarely ever observe at a place that is warm enough at night to allow the use of a black cloth over the head or an observer's hood. With either, one breath would cause instant fogging of a cold eyepiece. I don't have a problem with fogging because I don't breathe on the eyepieces and I have sufficient eye relief that my eye doesn't cause fogging.

I have used an occasional eyepiece that required complete blockage of peripheral light to prevent visible reflections off the eye lens (one example: 9mm 120 degree ES). A hood or black cloth would work great on an eyepiece like that (easier than cupping one's hands on either side), but I would expect the eyepiece to fog up. You can always use heater cords on the eyepieces, but I prefer not to do that. I'm fortunate enough to observe in arid, non-humid, environments.

An observer's vest to hold eyepieces sounds just great, but I have about 12+ pounds of eyepieces and PowerMate in my case, and I just couldn't see carrying that in pockets, even if the stitching were strong enough. But, then again, I don't use a ladder with my scope. A vest for eyepieces would be quite practical in that environment.