Eyepieces in the Cold

Observing in winter weather brings a whole new set of circumstances that affect your telescope. We make it a practice to leave the scope outside for awhile so that the optics have a chance to acclimate to the cold before observing. I had also noted that after placing a warm eyepiece in the scope, it seems to take awhile for the image to hold focus, and I initially concluded that the lenses in the eyepiece also needed a short time to acclimate. However, I recently found out that the culprit here is the eyepiece barrel itself and not the lenses. The warm metal barrel quickly releases its heat to the inside of the cold focuser and causes small currents of air immediately below the field lens of the eyepiece. The solution is to let your eyepieces acclimate to the cold before observing...it's not so much the lenses that need to cool off, but the metal barrels.

Published in the February 2001 issue of the NightTimes