Wednesday, September 21, 2011

In unusual optics, don’t discount the sphere

John Wallace
Senior Editor
Laser Focus World
When it comes to simple, imaginative approaches to optical design, the spherical mirror has not been left out. Or at least that's what I found when I came across two optical systems that, while very different, share the same basic configuration.

Both are designed to collect light from astronomical objects; both can scan large portions of the sky; and both have very large, fixed primary mirrors.

Their spherical primaries are symmetric about a single, central point; this is a higher degree of symmetry than that for a paraboloidal mirror, which is merely rotationally symmetric about its optical axis. Well, obviously, one might say. And so what, one might add.

But this allows the designer to build a system with an optical axis that pivots around the sphere's center -- meaning that the enormous primary never has to move, and that the rest of the optics can have a narrow field of view and yet cover a huge portion of the sky.

In the case of the Hobby-Eberly telescope in West Texas, the optics are diffraction limited. The primary is 11 x 9.8 m in size, with the optics being able to use a 9.2-m-diameter portion of the mirror at any one time. This telescope can access three quarters of the night sky (just imagine the pointing mechanism that would be required for a more-conventional 9.2 m ground telescope that could access that much sky).

Hobby-Eberly telescope (Image: Marty Harris/McDonald Observatory)

In the case of the solar bowl, one 15-m-diameter example of which was built on the roof of a kitchen in the south-India town of Auroville, a solar collector in the form of a rod pivots about the center of the sphere. As long as the rod is pointed at the sun, all sunlight received by the sphere is collected by the bottom half of the rod. On sunny days, the Auroville solar bowl provides steam for cooking for 300 people. In another example, a 20 m bowl was built in Crosbyton, Texas in the late 1970s by Texas Technical University to power a 40 kW steam turbine.

Auroville solar bowl (Image:

OK, enough optical geekiness for now. But if anyone knows of any other optical uses of this stationary-spherical-mirror configuration, let me know. (By the way, the 1000-ft-diameter Arecibo radio telescope in Puerto Rico also was based on this approach.)

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