Expert: Paul Wagner - 3/20/2011

Question
Could you help me. When a telescope has on it : 10" f/10  or  10" f/6.8
what does the f/ mean. I no the f stop on a leans. Like F1 is larger then a f4. but is a 10" Celestron telescope with a f/10, better then a 10" Celestron telescope with a f/6.8. what is the difference between the too. Witch one would give a better view & why . I hope I"m saying this right. I will appreciate any help. sincerely, John

Answer
The f/ratio actually refers to the size (or aperture) of the lens in relationship with the focal length.  In cameras, this ratio is adjusted by opening or closing the aperture--the focal length (or distance from the lens to the "film" is always the same.

But in telescopes the lens (or mirror) is always wide open.  But  different mirrors have different focal lengths--and that's why the f/ratio is different.

A shorter focal length scope (f/6.8) will have a larger field of view and the objects will appear smaller and brighter.  A longer scope (6/10) will have a smaller field of view, but objects will appear larger and slightly dimmer.  So it all depends on what you expect to observe.

Deep sky objects like nebulae are generally easier to see in the f/6.8.  Planets look better in the 6/10.

And, of course, the f/10 will have a longer tube because the focal length is 100 inches.  The f/6.8 will be 68 inches.

Make sense? Hope that helps!