Optics in Cold Weather

Telescope optics must equalize with the outdoor temperature, otherwise the image cannot be brought to a sharp focus. The glass expands and contracts ever so slightly as it adjusts to the temperature change. Once the glass has stopped "moving", the telescope can be focused. This is especially true with larger telescopes, owing to the greater volume of glass. But small telescopes also need time to acclimate. When taking the telescope from a 65o house to the outdoors at 10o, my 4" refractor requires at least 20 minutes before it can achieve a sharp focus. I've also noticed that even eyepieces need a short acclimation time for a substantial temperature difference. The focus seems to "drift" just a bit for about the first 5 minutes when an eyepiece is brought outdoors from a warm house.

Published in the January 1998 issue of the NightTimes