Quote:
Originally Posted by g__day
I noticed a light fog form on the centre of the eyepiece after I had placed my eye near it and commenced viewing for a few seconds. If I moved away from the eye piece the fog would evapourate
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Nothing to do with breath, or heavy breathing.
Your eye is warm, the eye lens is cooler.
The water at the surface of your eye (it's wet, remember) evaporates and, close to the eyepiece, this water vapour in close proximity to the eyelens has a relative humidity that exceeds the level sustainable at the temperature of the eyepiece. The result is that it condenses on the eye lens, forming the fog you saw. As you move away from the eyepiece the surrounding air has lower humidity and the fog thus evaporates off the eyepiece back into the air.
The easiest solution is to keep the eyepiece warm, e.g. use a heatpad around the eyepiece to keep it warm, or warm it beforehand in an inside jacket pocket, or to use a heated eyepiece case, or blowing warm air on it (hair dryer).
Another solution is to wear spectacles (which i do most of the time). Awkward, but they prevent this happening.