focal ratio
f number
stop number
speed ***

The combined focal ratio is about f/3, but the image quality of the two mirrors alone would be poor.

The design is noted for allowing very fast focal ratios, while controlling coma and astigmatism.

They have long focal ratios, and are constructed with a blue aluminum tube.

They are available in short, medium, and long focal ratios.

They have long, medium, and short focal ratios respectively, in order of increasing aperture.

They have very long focal ratios and are sold in blue aluminum tubes.

The all spherical surfaces are much easier to manufacture, especially in short focal ratios.

A short focal ratio can be more easily obtained, leading to wider field of view.

Even though the principles of focal ratio are always the same, the application to which the principle is put can differ.

A high focal ratio also means the field of view will be narrow.