More Effects of Temperature Change on Optics
I had to throw some more into the pot on the issue of optics acclimating to temperature changes. There are also problems in going back from the cold into the warm. I'll keep plastic bags at the ready, in order to enclose the optics before bringing them into the house after "cold soaking". This helps prevent condensation from forming on the optics, cameras, eyepieces... While the condensate will be distilled water, and should be harmless, it could be a potential problem if it dissolves a contaminate.
I ran into the condensation problem back during the Shoemaker-Levy impacts, dragging the scope from the cool air-conditioned basement into the warm summer evenings. I was dewed up before I could even start. I allowed about 45 minutes for acclimating and de-dewing the scope. Ever since then I've gotten around the problem by storing my scopes in the detached unheated garage. Dust and condensation during storage can be a problem, but at least they're pretty far along to being thermally acclimated when I go to set them up.
I've employed a somewhat different trick to minimize moisture condensation on optical surfaces. I make liberal use of silica gel packets stowed inside the mirror covers of my reflector and inside the boxes containing smaller telescopes and eyepieces. The silica gel makes the moisture disappear fairly quickly. During an observing session, fogged-up eyepieces can also be "undewed" by placing them back in the eyepiece box (with the silica gel packet) for awhile. - J.K.Published in the February 1998 issue of the NightTimes