significance of F number vs Focal length and Aperture

[AussieBill]

Given that

F = Focal Length / Aperture

Of what use is this number and why is it important to me ?

I would prefer to know the Aperture and the focal length of an instrument and work out the F number if I ever need to know it.

What do we use the F number for ? i.e.

High F number telescopes are good for ..............

Low F number telescopes are good for ..............

Did anybody ever wake up in the morning and suddenly say "Aha - I need an F4.6 that will do the the job ! Its obvious, why didnt I think of that yesterday " ?

Bill

[Merlin66]

Quick answer:
High focal ratio's good for stars, gives highest resolution and smallest star image size.
Low focal ratio's for extended objects like nebulae and faint extended gaseous clouds.

[AussieBill]

Merlin,

Thanks very much, that was particularly frustrating for me and I couldn't remember reading such a clear explanation anywhere.

Bill

[Merlin66]

Not original...
It came from a book I read many years ago.....
"Tools of the Astronomer", by Miczaika and Sinton, Harvard Books, 1961.

[torana68]

and yes its a good reason to have at least two telescopes :)

[N1]

High f numbers are also less demanding on the eyepieces, the collimation (newtonian reflectors) and the objective lens (refractors) - all other things being equal, they give a cleaner image than shorter f ratios, and/or do so more easily. The main trade offs are their narrow field of view, the longer exposures needed in imaging, the bigger mounts required, and the fact that they generally get pretty unwieldy from medium size apertures up. Note the above doesn't necessarily apply to folded designs like Cassegrains, which also have high f numbers. They have other challenges though.