Thermal Boundary Layers in Newtonian Reflectors

Thermal Boundary Layer
These video images of a defocused artificial star were taken through the author's 8-inch Newtonian reflector and vividly display the optical effects of the primary mirror's thermal boundary layer. The dark spot at center is the shadow of the secondary mirror, which is held by a curved-vane spider whose shadow is also visible, though less distinctly. At the time these frames were captured, the temperature difference between the mirror and the air was 5°C (9°F). These images were shot within 10 seconds, illustrating how the structure and scale of the boundary layer changes quickly. The top of the mirror is up.
Courtesy Bryan Greer.
In my article "Improving the Thermal Properties of Newtonian Reflectors — Part 1" (Sky & Telescope: May 2004, page 128), I describe how to detect the image-degrading thermal boundary layer that results when your reflector's primary mirror is warmer than the ambient air. The two short video clips presented below utilize a modified star test (described in the article) to illustrate what to look for. To view the videos, you'll need Apple's free QuickTime Player, which is available for both PCs and Macs.

The first clip shows the appearance of a warm 8-inch Newtonian primary mirror. To make the boundary layer's appearance easier to see, no fan was running when this sequence was shot.

. GreerThermals1.mov (3.4-megabyte QuickTime movie)

Features of the video:

1. The ever-changing boundary layer. At the beginning of this video the temperature difference (?T) between the mirror and the ambient air is 22°C (39.6°F). 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 that your telescope is far from thermal equilibrium.

2. External disturbances easily and frequently change the structure of the boundary layer. Notice how at one point the layer contracts in 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 boundary layer disappears and the telescope is immune to these effects.

Alan Garcia captured the second clip with a handheld video camera through his 15-inch Obsession Dobsonian telescope. Several thermal phenomena are visible simultaneously.

. GreerThermals2.mov (3.1-megabyte QuickTime movie)

Features of 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 position) across the illuminated primary mirror. Some people refer to this as the "conveyor belt" effect. The exact speed and character of this behavior 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. This is induced by the fan running behind the mirror and tells us that this mirror is still a few degrees above the ambient temperature.

3. Heat emanating from the secondary holder. This telescope was fitted with a dew heater on the secondary mirror. This video demonstrates why it's a good idea to use such devices on the lowest possible power setting that still keeps the dew away.

Once you've discovered the extent and duration of your scope's thermal problems, you can take steps to fix them. That's the subject of "Improving the Thermal Properties of Newtonian Reflectors — Part 2" in the June 2004 issue of Sky & Telescope. You may also be interested in my earlier article, "Understanding Thermal Behavior in Newtonian Reflectors" (S&T: September 2000, page 125). This article is available in PDF format in the S&T Magazine Archive. PDF downloads are free to archive subscribers and cost $2.95 otherwise. To read the PDFs, you'll need Adobe Reader, which is available at no cost for most computers.

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"Understanding Thermal Behavior in Newtonian Reflectors" (565-kilobyte PDF)

For more video clips and information, visit my Web site.