Wednesday, March 31, 2010

Astro101: Telescopes and Observatories

Today I return to my (very) occasional series Astro101, in which I talk about some of the most basic concepts in the science of astronomy.  Since today is the deadline for proposals to use the telescopes available through Kitt Peak and other national observatories, I thought that a natural topic would be telescopes and observatories. Just as a caveat, here I will be defining the two words as they are most often used in present-day astronomy.  As is often the case, there are exceptions, and the definitions have changed over time.  But those are discussions for another day.

Telescopes: At their most basic, telescopes are scientific instruments used to collect and analyze light.  When most people think of telescopes, they think of something like this, the 36-inch refracting telescope at Lick Observatory:

Sketch of the Lick Refractor
Image Credit: Library of Congress Prints and Photographs Division

But telescopes have a variety of sizes and designs, and they can look at all types of light, not just visible light.  Here are some examples of different types of telescopes: the Robert Byrd Green Bank Telescope, which looks at radio wavelengths of light with a dish 360 feet across; the MMT, which looks at visible light with a mirror 21 feet across; the Hubble Space Telescope, which looks at infrared, visible, and  ultraviolet light with a mirror 8 feet across; and the Fermi Gamma Ray Telescope, a satellite that looks more like a box than a telescope but detects gamma rays, the most energetic type of light.

The goal of telescopes is to collect as much light as possible, focus it as sharply as possible, and transmit that light to scientific instruments (cameras) for analysis.   The light-gathering power of a traditional telescope depends on the area (size) of its main mirror.  If you want to collect more light, you need a larger mirror.    This is why the most powerful telescopes in the world are so big.  The sharpness of an image depends on the diameter of the telescope (bigger telescopes give sharper pictures), the wavelength of light (longer wavelengths give blurrier images), and how much Earth's atmosphere blurs the light.  Earth's atmosphere doesn't blur radio waves, but radio waves have very long wavelengths, so the giant Green Bank Telescope cannot produce images sharper than your own eyes.  Optical wavelengths of light are very short, but Earth's atmosphere blurs optical light, so the largest optical telescopes in the world don't get much sharper images than a high-quality backyard telescope, at least not without help.  The Hubble Space Telescope is above Earth's blurring atmosphere, so it can make images 10 to 20 times sharper than the typical ground-based telescope, which is why astronomers like to use it even though its mirror is small compared to the typical ground-based telescope.

Another reason for putting telescopes in space is that Earth's atmosphere blocks many types of light.  Radio waves and optical light reach the ground without problems, but Earth's atmosphere blocks some infrared light and all ultraviolet, X-ray, and gamma ray light.  So, if you want an X-ray telescope, it has to go in space.

More and more telescopes also have a new technology called adaptive optics.  This technology is capable of correcting infrared and visible light for the atmosphere's blurring, allowing a telescope on the ground to make images as sharp as the Hubble Space Telescope at a fraction of the cost!  This technology is still changing and is very complicated to operate, so astronomers only use it if they really have to.

I could go on and on about many details on telescopes, but that is beyond the scope (ha!) of this post.  Let's instead contrast telescopes with...

Observatories: An observatory is different from a telescope, in a sense that the word "observatory" implies all of the buildings and support staff taking care of a telescope.  An observatory often has more than one telescope.  For example, the Kitt Peak National Observatory includes the 19 visible-light telescopes and two radio telescopes located on Kitt Peak in Arizona, plus the dining hall, dormitory and offices on the mountain, plus offices in Tucson, Arizona, and all of the staff working at each of those places. But an observatory can consist of just a single telescope: the SOFIA Observatory consists of a single telescope mounted in an airplane that looks at infrared light from above a lot of Earth's atmosphere, and the MMT Observatory consists of a single telescope on Mt. Hopkins south of Tucson, Arizona.

So, in short, a telescope is a part of an observatory.  A telescope is, at its most basic, a single piece of equipment.  An observatory is a larger entity, a collection of one or more telescopes and all of the associated infrastructure. 

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