Correcting a professor’s homework solutions

Posted by barb on Mar 24, 2005 in Science Musings |

While answering questions this week for Ask a High Energy Astronomer, I came across a set of homework solutions for an astronomy class at a community college. I skimmed over it, and found the following answer to a question about the advantages of putting telescopes on the moon:

Perhaps the greatest advantage of observing from a place that has no atmosphere is being able to observe every conceivable wavelength of electromagnetic radiation equally well. Significant amounts of visible light waves and radio waves are able to pass completely through Earth’s atmosphere, but very little else can. From the Moon, or from space, the same telescope could be equipped to detect gamma rays, X-rays, ultraviolet, visible, infrared, and radio. From the surface of the Earth, a telescope is mostly limited to observing visible and radio wavelengths.

[Emphasis is mine]

While the main emphasis of the answer is correct — since Earth’s atmosphere absorbs certain wavelengths, if we put a telescope where there is little/no atmosphere, we can observe all wavelengths — the statement that any given telescope could be equipped to observe any wavelength is absolutely misleading.

X-ray telescopes work quite differently from visible telescopes, since X-rays tend to penetrate mirrors rather than reflect. In fact, X-rays only reflect at “grazing incidence”, so X-ray telescopes must be constructed with mirror surfaces nearly parallel to incoming light, rather than perpendicular as in optical telescopes. (Check out this page for more on X-ray telescopes.)

In addition, and perhaps more importantly, gamma-rays cannot be focused with current technology. Gamma-ray telescopes do not use reflection-optics for this reason, and generally use some other kind of technology to determine direction-of-origin, including optical masks and silicon-strip detectors. (Check out this page for more on gamma-ray telescopes.)

So, while it would absolutely be possible to observe all wavelengths from the Moon’s surface, due to its lack of atmosphere, it is not possible to use the same telescope to observe all of those wavelengths.

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