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.

focal ratio

[¦fō·kəl ′rā·shō]
(optics)
References in periodicals archive ?
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.
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.
This also applies to the techniques for grinding, polishing and figuring of telescope mirrors of various sizes and focal ratios.