focal ratio

The focal length divided by the aperture of a mirror or lens gives you the number known as the focal ratio.  It is usually written, for example, as f/6, where the focal length is six times longer than the aperture, and for this reason is sometimes referred to as the f-number.

This an important feature of a telescope, and is often mentioned in the specification of an instrument. In general, telescopes with small f-numbers are short and compact, have a wide field of view and a bright image, while those with large f-numbers are comparatively long, and have a narrower field of view but work better at higher magnification. Making a short focal-ratio lens or mirror is more difficult than making a longer one, so compromises have to be made or the cost goes up, and it may not work so well at high powers.

Typically, reflectors can be made as short as f/4 or even less, while refractors start at f/5. Catadioptric telescopes have longer focal ratios, such as f/10 for a Schmidt-Cassegrain telescope or f/14 for a Maksutov.

Two concave mirrors with the same aperture but different focal lengths (F). The f/4 mirror requires a shorter tube and gives a wider field of view but lower magnification with any chosen eyepiece than the f/8 mirror.