Unfortunately, on most nights that heat battery won’t discharge completely - it’ll continue to radiate warmth long after you’ve decided to call it quits and head indoors. Even open-tube scopes suffer the effects of the thermal boundary layer. So even if you could get all the warm air out of the tube, the mirror will simply replenish the supply. All day it gets charged up, and all night long it discharges heat to warm the surrounding air, which then drifts up the tube. That mass of glass works like a heat battery. That’s what would happen except for one thing: the big hunk of glass at the back of the tube known as the primary mirror. You might expect that because there’s only a little warm air inside your scope it should flow out of the tube in a matter of moments to be replaced by cool night time air. Inexpensive digital indoor/outdoor thermometers provide a means of monitoring the temperature difference between your telescope’s mirror and the outside air. But as Sherlock Homles famously noted, “when you have eliminated the impossible, whatever remains, however improbable, must be the truth.” And the closer you look at the situation, the more probable it seems thermal issues are the #1 killer of Newtonian performance. Until Bryan Greer’s landmark 2004 S&T articles, few fully appreciated how much thermals affect reflector telescopes. True, any of these can rob your reflector of good views, but if you use a mirror of high quality with good coatings, take care of the baffling and collimation issues, and ensure that the central obstruction is reasonably small, you can still end up with substandard images. Scatter in the mirror coatings, mediocre optics, poor baffling, improper collimation, and central obstruction have all been blamed at one time or another. Time and time again, the usual suspects have been rounded up and accused of hindering the reflector’s performance. Is there some intrinsic shortcoming in the design of the Newtonian reflector that makes this inevitable? This disparity is most apparent when viewing low-contrast planetary detail - the images in a good refractors often have a touch more snap to them. Generations of backyard astronomers have debated why, inch-for-inch, the performance of a high quality refractor usually edges out an equal-quality Newtonian reflector. What you need to know when it comes to optimizing your scope’s thermal behavior.
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