Dianne: Your situation is a lot worse than mine! I'm in So Cal where it is never too cold, but sometimes wind and condensation on a SCT front corrector are problems. Sky & Telescope Mag from 25 to 30 years ago had an article about observing in a domed chamber with a window of Mylar plastic sheet. The result was surprisingly good. Some slight "smearing" of bright star images was noted at high power, but it was not excessive. It was possible to use the telescope for most "non-critical" uses. The thought was to make the window as thin as possible--.003 inch mylar sheet IS THAT! This eliminates any lateral color due to refraction of thick glass. The mylar was tensioned in a frame to be flat as possible. Art supply or industrial plastics house may have sheet mylar in rolls at an economical price. Also a thin (1/8 to 1/16 inch) Plexiglass or Lexan sheet may work too. I use a thin plexi piece over the front of my 10 inch Celestron's dewcap extension. It is straight from the wrapper, never touched or wiped, just brush-dusted. A good image quality is still there, and it does keep all moisture off the corrector glass coating. Maybe there is a 5-10% loss in transmission due to lack of anti-reflec- tion coating on the plexi, but OH WELL! Something you may try is put a big collar of plastic sheeting around the upper end of your telescope. This sheeting could be made from very thin poly film--painter's drop cloth or such at hardware store. This sheeting would extend out to and be taped to the edges of an obervatory dome's opening or slit. It would provide a barrier to keep warm air in the observatory. You could put an extension tube or long dewcap on the front of the telescope to cut down on the material. Of course, you have to move the telescope and dome in unison. If you go for a sturdy window and can afford it, there is a glass that is not too bad on transmission--"water-white" float sheet glass. It has few of the iron or other chemicals that make window panes greenish. The "float" part refers to the method of making flat glass. It is floated on a pool of molten metal while it is liquid, and cools to a much more uniform flatness than machine ground and polished plate glass. Polishing machines in glass plants use belts, and these leave "waves" in the surfaces. I used disks of this glass in the projection ports of a major planetarium years ago. I don't know of sources or prices anymore. In any case, the heat differential you may have between a "comfortable" interior of 10dC and a winter night's -30dC will give you plenty of air currents to wiggle the light beams. Good luck to you on the "airtight" observatory! Gregg.

• Re: Observatory window? - Dianne - February 9, 1999 - 08:36 UTC