focal ratio

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focal ratio

The ratio f /d of the focal length, f , of a reflecting surface or refracting medium to its effective diameter, d , i.e. to its aperture. The numerical value of the ratio (often called the f/number) is usually written f/4 or f:4 for a ratio of 4, say. The reciprocal of the focal ratio (d /f) is the aperture ratio. The limiting magnitude – i.e., the apparent brightness of the faintest detectable star – depends on the focal ratio of a telescope: for telescopes used under the same observing conditions, the larger the ratio the fainter the limit; if photographs are being taken, however, the larger the ratio the longer the necessary exposure time.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006

focal ratio

[¦fō·kəl ′rā·shō]
(optics)
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
Figure 2 shows how the limiting magnitude changes with focal ratio for a given focal length.
The tuning procedure involves adjusting the lens focal ratio (g) that will minimize the error reported by the phase error.
Shoot at shorter focal lengths, higher ISO speeds, or faster focal ratios, all to reduce exposure times and image scale.
Amateurs with access to a machine shop (or a friend with one) can also purchase inexpensive aluminum lens caps and mill them out to the specific aperture sizes needed to achieve more precise focal ratios. Hole-saw drill bits come in a limited range of sizes, so you'll need to calculate the f/ratio produced by a particular bit simply by assuming the new hole is your clear aperture.
Dr Arditti explained that the HyperStar system was a means of reconfiguring widely available Schmidt-Cassegrain telescopes (SCTs), which typically had focal ratios of around f/10, into Schmidt camera configurations with much faster focal ratios of around f/2.
Focal ratio is much less important for visual observing, because there's an additional element in the optical train: the eyepiece.
There is no all-purpose telescope for deep-sky photography, but if I had to pick one that can do a lot, I'd choose something with about 6 to 8 inches of aperture and a focal ratio of f/4 to f/8.
In asking why the diameters of the original and replacement mirrors were different, I speculate that if the original was a 26 1/4-inch mirror operating at f6, and the manufacturers of the replacement conveniently had a blank available of 23 1/4-inches, then producing the replacement to operate at f6.8 (the focal ratio of the existing mirror) would give approximately the same focal length, enabling it to fit into the original tube without need for focal modifications.
Our test telescope on loan from the manufacturer came with an optional (\$150) field flattener that retains the telescope's f/7 focal ratio and 714-mm focal length.
A scope's focal ratio is the focal length divided by the aperture; for instance a 100-millimeter refractor with a focal length of 600 millimeters has a focal ratio of f/6.
Combined with its low-noise Sony detector, modest power requirements, and the scope's reasonably fast f/5 focal ratio, I figured this "bare-bones" setup should do well for me if I took relatively short exposures under exceptionally dark skies.

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