As an aside, it seems not many people remember Bryan Greer's (owner of Protostar) landmark article in S&T some years ago, where he proved that it's the boundary layer of air in front of the mirror's surface, and resulting tube currents, that impair the image- not just a "warm mirror" in and of itself. By blowing a fast moving stream of air across the mirror's face, image degradation from this source virtually disappears, even with a mirror much warmer than ambient air temperature. Alan Adler incorporated the side-blowing fan in his optimized flexed-mirror scope, also published in S&T. A fan blowing on the back of the primary does not accomplish this; it's purpose is strictly for cooling the mirror- which will take time. I noticed a similar effect on my 6" f/8 Newtonian before Greer's article was published. That scope uses a fan behind the mirror, but the back end of the tube is otherwise completely closed-off, so all fan-forced air must blow around the mirror's edge. Enough air apparently spills onto the face of the mirror to break-up the boundary layer before pushing it up the length of the tube, for the most part above the optical path. The effect in the eyepiece is obvious, and striking. Looking at a planet, for instance, through the scope just taken outdoors, reveals typical "slow seeing" artifacts from tube currents. Flip the fan switch, and "poof"! The image steadies instantly. I'm now a firm believer in fans blowing across the mirror's face, and have incorporated a pair of them into my 17.5". As far as seeing is concerned, fast-moving air next to the mirror's face is a good thing. Slow-moving air, such as from a passively-cooling mirror, is a bad thing. On Thu, Jan 8, 2009 at 3:34 PM, <zaurak@digis.net> wrote:
One of the few advantages of cold weather observing, a short cool down period for your optics.