Telescope Principles

Obviously, the most basic tool for astronomy is the telescope. I will discuss some of the very basic principles that are most important to telescope performance and design.

First of all, there are two principle kinds of telescopes: refractors and reflectors. The difference between these is in what kind of primary the telescope employs. The primary is the main focusing element in the telescope. In the case of a refractor, the primary element is a glass lens (also called the objective lens); in the case of a reflector, the primary is a mirror. Glass lenses suffer from a basic limitation, that gravity distorts them over time. The upper limit on the size of a glass lens is about 1 meter. Hence, almost all modern professional instruments are reflectors, although many amateur instruments available still rely on refracting lenses.

Refractor or reflector, telescopes are usually categorized by the diameter of their primary. A larger diameter enhances two fundamental quantities desirable to astronomers: light gathering ability and resolution.

First, let's consider light gathering. This is quite straightforward: the bigger the telescope, the more light it collects, so the fainter objects one can see. Because most telescopes have circular mirrors, the light gathered is simply proportional to the area of a circle, which is pi times the square of the radius (half the diameter). Hence, the telescope's light-gathering ability is proportional to the square of the diameter. In photographic terms, this means that a 10-second exposure on a 10-meter telescope would require 1000 seconds on a 1-meter telescope to detect the same faint object. For this reason, when it comes to professional telescopes, diameter is the name of the game. It's a "size does matter" business.

Next, let's look at resolution. Consider approaching car headlights on a long flat road. At first, the lights seem merged into one bright speck, but, as the car approaches, one can slowly start to resolve the headlights into two. With a telescope, one could resolve those headlights much sooner than one can with the naked eye.

Similarly, when astronomers want to separate two very close stars in the sky better resolution is critical. The same principle applies to resolving fine details of any kind (such as reading a license plate from half a mile away, or detecting fine structure in the cores of galaxies).

Resolution can be quantified in terms of angles. The smallest angular separation between two points that an observer can distinguish is the resolution of the telescope. This angle can be expressed approximately as A = 1.22l/D, where A is the smallest resolvable angle, l is the wavelength of light being observed, and D is the

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