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Focal Ratio = Focal Length / Diameter of primary mirror or lens.
eg. Celestron C8 = 203.2mm (8") diameter, 2032mm focal length. F ratio = 2032/203.2 = f10. (usually expressed as 200mm dia and 2000mm f.l).
Newtonian 25cm (10"), f5 has a focal length of 25 x 5 = 125cm = 1250mm.
A refractor of 600mm focal length and f ratio of 7.5 has an aperture diameter of 600/7.5 = 80mm.
I have shown three examples with the unkown a different parameter in each case.
Fast is generally f5 and less.
Moderate is f6 to f8.
Slow is generally f10 and over.
The speed of the optical system will determine photographic exposure times but has little impact on visual use. However, a slow system allows the use of less corrected (simpler and less expensive) eyepieces than a faster system. Nagler eyepieces are corrected to f4 but work well in fast and slow systems.
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