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HomeTelescope ThermodynamicsSept. 2000 Sky & Telescope magazine companion web siteMay & June 2004 Sky & Telescope magazine companion web siteUsing fans with a Newtonian telescopeTips on attaching a temperature probe to your telescopeOptical MiscellanyTry this at home!How atmospheric seeing affects telescopes with different focal ratiosAnimated focal plane illumination mapProperties of various mirror substrate materialsAdventures in collimation |
While the primary mirror is the biggest source of thermal problems, even a Newtonian's secondary mirror can develop its own boundary layer under some conditions. This video focuses in on the energetic sheath of warm air surrounding the secondary mirror and holder. The telescope had just been taken from room temperature out into a 0 ºF night. Fortunately, thermal problems associated with the secondary mirror are short-lived, as the much smaller mass of the secondary can only store a fraction of the heat the primary does. The light source for this video was a 150 Watt halogen fiber optic about 400 feet from the telescope. Features to look for in the video: 1) The cyclonic eddies of warm air. In this video the left side of the mirror is the highest point, so the warm air rises across the slanted surface from right to left. Yes, this video is running at normal speed! 2) The delicate nature of thermal boundary layer. Halfway through this video, I wafted the layer away by merely waving my hand near the secondary spider. Watch the turbulence briefly clear, then rush back onto the surface. Boundary layer on the primary mirror
This video shows the appearance of the modified star test for a warm 8-inch Newtonian. In this case, the highly defocused source of light is Venus. No fan is running for this clip to make the boundary layer structure easier to see, and there are gusting winds which periodically kick the boundary layer into motion.
Features to look for in the video: 1) Notice the ever changing boundary layer. At the beginning of this video when delta T is 22 ºC, the boundary layer repeatedly sweeps across the face of the mirror taking on a variety of structures. When you see patterns like this, it's a clear sign the telescope is far from being in equilibrium. 2) External disturbances change the structure of the boundary layer. Notice how at one point the layer seems to disappear, then reappear a second later from the outer edges of the mirror. This was caused by a gust of wind from behind the scope. Once the mirror has reached equilibrium the telescope is immune to these effects, as the boundary layer will disappear completely. Return to Telescope Optics Topics. |
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Copyright 1997-2024 Bryan Greer
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