Improving the thermal properties of Newtonian telescopes

by Bryan Greer
Originally posted April 2004.


Telescope Thermodynamics

Sept. 2000 Sky & Telescope magazine companion web site
May & June 2004 Sky & Telescope magazine companion web site
Using fans with a Newtonian telescope
Tips on attaching a temperature probe to your telescope

Optical Miscellany

Try this at home!
How atmospheric seeing affects telescopes with different focal ratios
Animated focal plane illumination map
Properties of various mirror substrate materials
Adventures in collimation

Companion web site to the May & June 2004 Sky & Telescope articles

As described in the May 2004 Sky & Telescope article, the modified thermal star test is capable of revealing any source of wavefront degradation that is between the object in outer space and your eye. This includes atmospheric turbulence, nearby thermal disturbances like body heat, and of course thermal instability within the telescope tube itself. With a little practice, you can learn to tease these overlapping phenomena apart. The videos below are examples of the types of thermal problems that can be observed using this modified star test.

Videos of the thermal startest

Combination of thermal problems in a 15-inch truss tube Newtonian

(3.1 Mb file size; QuickTime format. Click on the thumbnail to launch the video. Right click on the link to save to your harddrive.)

Alan Garcia captured this instructive hand-held video through his 15" truss tube telescope while it was pointed at Jupiter. Several thermal phenomenon are visible simultaneously in this short clip. These types of thermal phenomena are quite typical of what you should expect to see in a Newtonian that hasn't quite reached equilibrium with its surroundings.

Features to look for in the video:

1) The swiftly moving atmosphere. The atmosphere is plainly visible here moving from right to left (from the 4 o'clock to the 10 o'clock positions) across the expanded disk of light. Some people refer to this as the "conveyor belt" appearance. The exact speed and character of the atmosphere will vary from night to night.

2) Counterclockwise rotation of the boundary layer. If you look carefully at the outer regions of the mirror, you will see a slow counterclockwise merry-go-round motion. The rotational motion is induced by the fan running behind the mirror, but the mottled shadow pattern itself is the result of air of differing temperatures mixing together. This is a sure sign that the mirror is still a few degrees above the ambient temperature, and the rotating shadows will disappear once everything is truly at equilibrium (even with the fan on).

3) Heat emanating from the secondary holder. This telescope was fitted with an anti-dew heater on the secondary mirror, and a boundary layer has developed around the slightly warmer secondary mirror and holder. This demonstrates why it's a good idea to use such devices on the lowest possible power setting that still keeps the dew away.

Thermal flows surrounding the secondary mirror

(5.8 Mb file size; QuickTime format. Click on the thumbnail to launch the video. Right click on the link to save to your harddrive.)

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

(3.4 Mb file size; QuickTime format. Click on the thumbnail to launch the video. Right click on the link to save to your harddrive.)

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.

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