August 1, 2008

In Today's One Minute Astronomer...

Reflecting Telescopes

We've been talking about refracting telescopes over the past few months at One-Minute Astronomer. Today, we switch gears to talk about reflecting telescopes, the most widely used type of telescope in the world for both amateur and professional astronomers. Here are the basics.

The Basics

• While Galileo rocked the scientific world with his small refractor, and many other scientists adopted his design, early refractors suffered from horrendous chromatic aberration that degraded the image of bright stars and planets. That's because glass refracts (or bends) blue light more than red light, so all colors can't come to the same focal point.

• But mirrors reflect all colors of visible light exactly the same. So a curved mirror causes no chromatic aberration whatsoever. Niccolo Zucchi made the first reflecting telescope in the early 1600's, but couldn't find a way to view the image without blocking the mirror. His design was not widely used.

• Isaac Newton was much smarter. He used a second small diagonal mirror to direct the light out the side of the telescope to an eyepiece. His immensely practical design, the Newtonian reflector, is still in use today. You might even own one of Newton's inventions!

A replica of Newton's original reflector

• Unlike the lenses of a refractor, only one surface of a mirror needs carefully figuring and polishing. And since no light passes through a mirror, less expensive glass is used. Some homemade reflectors even use glass from the discarded portholes of a ship!

A Deeper Look

• Reflectors are not without their own aberrations that degrade astronomical images. Mirrors ground with a spherical curvature, for example, can't produce a perfect focal point of a star. That's why most mirrors today are ground to a more complex shape-a parabola-that allows near-perfect focus.

• The biggest astronomical lens (at Yerkes Observatory) is only 1 meter across. A bigger lens would sag under its own weight. But a mirror can be fully supported on its non-reflective surface, so sag is not a problem. The largest single monolithic mirror is the size of a small pond… more than 8 meters across!

• For you, the biggest advantage of a reflector is that you can get a much larger aperture-and therefore see much fainter objects-than you could with a refractor of the same cost. A top-of-the line 4-inch refractor costs as much as 12-inch reflector.

A Bit of History

Early telescope mirrors were made of “speculum”, an alloy of copper and tin. But in 1859, Foucault learned how to figure parabolic mirrors from glass and coat them with a fine layer of silver. Today's mirrors are much the same, though aluminum is used for the reflective coating since it's cheaper and more robust than silver.

Personal View

My first telescope was 3-inch Tasco Newtonian reflector on a spindly altazimuth mount. My parents bought it for me when I was 5 years old. But under dark skies over many years, it revealed hundreds of objects and stoked my never-ending enjoyment of the night sky.

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