Eric wrote:
I'm trying to get my head around how the f number affects things



Background References
http://scienceworld.wolfram.com/physics/f-Stop.html
http://hyperphysics.phy-astr.gsu.edu...lenseq.html#c1

The effect of atmospheric seeing on telescopes of differing focal ratios
http://www.fpi-protostar.com/bgreer/...ges/seeing.htm

How Telescopes Work
http://science.howstuffworks.com/telescope19.htm

Focal Ratio (f/number)

"Focal ratio or f/number relates to the brightness of the image and the
width of the field of view. The focal ratio is the focal length of the
objective lens or primary mirror divided by the aperture. The focal
ratio concept comes from the camera world, where a small focal ratio
means a short exposure time for the film, and was said to be "fast."
Although the same is true for a telescope, if a "fast" and a "slow"
telescope are compared at the same magnification for visual rather than
photographic viewing, then both telescopes will have the same quality
image. Generally, the following information about focal ratios can be
helpful:

* f/10 or higher - good for observing the moon, planets and double stars (high power)
* f/8 - good for all-around viewing
* f/6 or lower - good for viewing deep-sky objects (low power) "