By Bill Pellerin
Houston Astronomical Society
GuideStar Editor
When amateur astronomers get together and talk about their telescopes, they usually discuss the aperture of the telescope first. Why? Because it is easy to determine, in fact, it is probably the parameter that is of most interest to knowledgeable telescope buyers.
Professional observatories make great efforts, and spend a lot of money, to increase the effective aperture of their telescopes. The Keck telescopes in Hawaii combine the light from two mirrors (two telescopes, really) providing an aperture of 280 feet and a resolution of 5 milli-arcseconds. At Mount Wilson, there is the Infrared Spatial Interferometer – three 65” telescopes, spaced up to 279 feet apart to get high-resolution infrared images.
The next most discussed parameter is focal ratio, which is stated as f/10 or f/5, whatever it is. This number defines the focal ratio, the aperture = the focal length (f) divided by the focal ratio. Or, the focal length = the aperture * the focal ratio. The focal ratio is of great importance to those who image the sky.
For example, a f/10 SCT (a common telescope in use by amateurs) of 8” diameter has a focal length of 10 * 8 = 80 inches. It is more common that the focal length is talked about in millimeters, so applying the correct conversion (1” = 25.4 millimeters) we get the focal length = 2032 mm. It’s peculiar that astronomers talk about diameter in inches and focal length in millimeters, but that is often the case.
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