The Lake County Astronomical Society

Cold Eyepiece Fogging?

If you have a problem with an eyepiece fogging up on cold winter nights, it could be that your eye is too close to the eye-lens of the ocular. This is generally the case on short focal ratio eyepieces used to achieve higher magnification. As noted by Minnesota amateur Phil Lefever, "The biggest problem I have found with short eyepieces is that the eye relief is too tight causing eye lens fogging in the winter. Eyepiece designs that lengthen the eye relief are a plus especially when used with an external Barlow for high magnification." So when you want more power without the ocular fogging up, there are two ways to get it:

1. Use a short focal length eyepiece with lots of eye relief (the distance from the eye-lens that the eyepiece comes to focus). Pentax XW and Tele Vue Radian eyepieces are especially good in this regard, plus they have adjustable eyecups that help position your eye at the optimum distance. Conversely, short focal length Orthoscopics and Plossls typically have short eye relief.

2. Use a longer focal length eyepiece (which will have longer eye relief) along with a Barlow lens. A Barlow will increase the magnification with minimal effect on the eye relief of the original eyepiece.