Sometimes referred to as f-ratio, f/stop, or f-number. The focal ratio of a telescope tells us how fast or slow a telescope is. The higher the focal ratio, the slower the telescope speed. What this means is that a lower focal ratio telescope is able image dim objects much quicker than a higher focal ratio telescope. High focal ratio telescopes (f/11 – f/15) are more suited for high power viewing/photography such as lunar, planets, and bright stars. Lower focal ratio telescopes (f/4 – f/5) are best for low power viewing/photography such nebulae, galaxies, star clusters, and interstellar clouds. Also, higher focal ratio telescopes tend to be more forgiving on eyepiece quality, due to the longer focal length that the light travels (having less curvature, and producing less coma).
There are telescopes that have focal ratios in the middle range between f/6 – f/10. They are not suited to do one task very well, but they will provide a decent mix.
Focal ratio, is given by the following equation…
f-ratio = ( ƒ_scope ) ÷ ( D_scope )
My telescope has a focal length of 650 mm and a diameter of 130 mm. The focal ratio will therefore be…
f-ratio = ( 650 mm ) ÷ ( 130 mm ) = 5 (usually represented as f/5)