F number

On Sat, 20 Nov 2004 02:14:16 GMT, Eric wrote:

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

A capital F usually denotes focal length. It seems to me that you're
really asking about focal ratio, as in f/4.5, f/10, etc.

I will assume that's what you mean.

By all else being equal, I mean that the aperture of the telescope and the
quality of the objective are the same. Also the same is the field of full
illumination when talking about an obstructed instrument.

The lower the focal ratio:

1) The higher the magnitude of image aberrations, including spherical
aberration, coma, astigmatism, and chromatic aberration (the latter only
with refractors, of course).

2) The wider the field of view for a given eyepiece, and the lower the
magnification.

3) The less time taken to expose an extended object (planet, nebula,
galaxy, etc.) on film or CCD. Star exposure time depends on aperture
only, and is not affected by focal ratio.

4) The shorter the tube in a refractor or Newtonian (or any other
single-reflection focal system).

5) The larger the central obstruction in a Newtonian. The same is true of
compound systems (SCT, MCT, Cassegrain, etc.) in practice, though it's not
a given (secondary power/position can be changed).

Take the opposite of each item, and that's what you get with a higher
focal ratio.


--
- Mike

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