Re: F number

From: Roger Hamlett (rogerspamignored_at_ttelmah.demon.co.uk)
Date: 11/20/04 

Date: Sat, 20 Nov 2004 17:02:28 GMT

"Eric" <notValid@comcast.net> wrote in message
news:YLxnd.641546$8_6.228512@attbi_s04...
> I'm trying to get my head around how the f number affects things
>
> Can someone fill this in for me?
>
> Given all else remains the same...
>
> low F ---------- vs ----------- hi F
>
> Thanks
> Eric
Unfortunately, the F ratio, is an often overused concept in astronomy.
There are two main things that affect the preformance of a scope. The
focal length, and the aperture. The F ratio, is just the ratio between
these. In photography, when dealing with an 'extended' object, the F
ratio, provides a quick 'shortcut' to working out how 'fast' a lens is,
and calculating the exposure times. For most applications in astronomy,
the aperture and focal length are more useful, but inherently, if you have
the aperture (say), and the F ratio, you can calculate the focal length.
The F ratio, can be a useful 'guide' in some other areas. Firstly, a low F
ratio, when dealing with a Newtonian, implies coma will be a larger
problem. A high F ratio, implies that even a spherical mirror will perform
quite well. A low F ratio, implies the scope will be quite short for it's
aperture, and for a given illuminated field at the eyepiece, will need a
larger central obstruction.
Secondly, with refractors, a low F ratio, implies that colour correction
will be harder to achieve (and more money will be involved to get an
acceptable result), while a high F ratio, implies a system where colour
aberrations will be smaller.
On the SCT/Maksutov, a low F ratio, really implies a larger central
obstruction.
Then, since the F ratio, links the focal length and the aperture, for a
given aperture, a high ratio, implies a longer focal length instrument
(which may well be better suited to planetary observing), while a lower
ratio, then implies a shorter focal length, and a system that may be
better for 'wide field' observing. However this is true only when dealing
with a fixed aperture, which is why this must be treated with care.

Best Wishes

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