Angular Resolution and "Seeing"

Telescopes are designed to focus light into an image, or picture. The clearer the image, the more information can be learned from it. In general, three things control the clarity of a telescope's images: aperture size, quality of construction, and the observing site.

1) Aperture size refers to the diameter of the mirror or lens used to focus the light. Any lens or mirror distorts the light from a distant star (which appears as a point of light because it is so distant) into a series of concentric rings called a diffraction pattern. The larger the telescope aperture, the smaller the pattern. The picture at right shows the diffraction patterns produced by telescopes with (A) a10-cm (4-inch) diameter mirror, (B) a 1.0-meter (40-inch) mirror, and (C) a 10-meter (33-foot) mirror. Smaller patterns mean a star's light is concentrated into a smaller area on a photograph, resulting in a clearer picture. This is one of the reasons astronomers usually prefer to use large telescopes.

2) Quality of construction usually means how carefully the mirrors and/or lenses in the telescope were made. A mirror has an ideal shape (generally a "paraboloid" or 3-D parabola) to which the opticians try to grind the lens or mirror. The closer the optician comes to achieving this perfect shape, the better the images the telescope will produce. Usually, the optician has to invest more time and effort in making a better quality lens or mirror, so it costs more. Thus, there is a tradeoff to be reached between mirror quality and mirror cost. In recent years, telescope opticians have developed some strategies that help them make large, high quality mirrors at lower cost (for example, spin-casting and active optics).

3) The observing site -- where the telescope is located -- is perhaps the most critical thing controlling a telescope's image quality. Imagine a beam of light coming from a distant star. It may have traveled for millions of years, but in the last fraction of a second before this light reaches your telescope, it encounters the earth's atmosphere. Even if the sky is free of clouds, the light encounters pockets of warmer or cooler air, and pockets of drier or moister air. These pockets act like lenses and bend (or "refract") the light very slightly, changing the angle at which the beam of starlight hits your telescope mirror. As the wind blows the pockets of air along, the angle at which the beam hits your telescope changes, causing the image of the star to flicker and move around. If you are taking a photograph of the star, the moving image builds up into a large, fuzzy image of the star, as shown at right.

At sites near sea-level, like Bowling Green, Ohio, the light must pass through the full depth of earth's atmosphere. It is likely to encounter many pockets of warm/cool and dry/moist air, and thus be severely blurred. Click on the image at right to see a picture of the inner part of the globular cluster M5. The image shows a region 80 arcseconds on a side. The stars appear blurry and very large in diameter (about 3 arcsec across, typical of what we see in B.G.). Many stars appear merged together. It is difficult to count how many stars are present, and to measure the amount of light associated with each star (its apparent brightness). An astronomer might not be able to obtain much useful information about the cluster from this picture.

About a century ago, astronomers learned that mountain tops afforded a clearer view of the stars -- not because the telescope is closer to the stars, but because it is above more of the earth's atmosphere. There are fewer warm/cool or dry/moist air pockets to look through, so the beam of starlight is bent less. The image through your telescope moves less, resulting in clearer photographs. The image at left shows the same region of M5 as seen from a top-quality mountain site like Mauna Kea, Hawaii. The star images appear crisper and smaller in diameter (only about 0.5 arcsec across). An astronomer can count the number of stars more accurately and make more accurate measurements of the brightness of each individual star.

The higher up in the atmosphere you are, the fewer air pockets there are above you to blur your view of the stars. Astronomers have hung telescopes from balloons and flown them in rockets, but the best long-term solution is to place a telescope on a satellite in earth orbit. Currently, the Hubble Space Telescope is the "diva" of space telescopes. Its 2.4-meter diameter aperture provides a small diffraction pattern (see Item 1), and though its main mirror was originally ground incorrectly, it has been fitted with corrective optics (essentially fancy "eyeglasses") which make it one of the highest quality optical telescopes available (see Item 2). Because it is in space, the earth's atmosphere does not blur or distort its images at all. The image at right shows the same region of M5 as seen from the HST. Now the star images are super-crisp and very small (0.1 arcsec in diameter).

The looping movie below enables you to compare the images obtained at the three sites directly. If you were an astronomer, which would you rather use? It is not hard to understand why the HST is one of the most sought-after astronomical telescopes in the world today. Nor is it hard to understand why astronomers are looking forward to the launch of an even larger Next Generation Space Telescope in 2009.

Other advantages of a space telescope include, (1) it is never cloudy, (2) it is not affected by "light pollution" -- the glow of city lights that brightens the sky and "washes out" many faint astronomical objects, and (3) it can see ultraviolet and infrared light that are blocked by the earth's atmosphere. Disadvantages include the much higher cost compared to ground-based telescopes and the difficulty in fixing the telescope if something breaks.

Andrew Layden & Neil Beery
Bowling Green State Univ., Nov. 2002.