Hello.

A value like f/6 actually refers to the Focal Ratio of the telescope. You can calculate the Focal Length by multiplying the Focal Ratio by the Aperture (the diameter of the main mirror or lens). So, for the Liverpool Telescope with a focal ratio of f/10 and an aperture of 2m, the focal length is 2x10 = 20m (or 20,000 mm).

When designing a telescope, there are lots of things to consider. Some of the most important are the Field of View, the Resolution (related to the magnification) and how easy it is to see faint object (the "speed" of the telescope). It is impossible to design a telescope that is good for all the factors, and the Focal Ratio is one of the ways of working out the compromise decided upon for a particular telescope.

A telescope with a low ("fast") focal ratio offers lower powers and a wider field of view (for a particular camera or eyepiece) - smaller focal ratios mean shorter exposure times. Just the opposite is true for larger focal ratios ("slow" telescopes): higher magnifications result for a given eyepiece, but the field of view is more
limited and exposure times for pictures tend to be longer.

We hope that helps!

The SkyWatch Team