[sjastro]

F-ratio = Focal Length/Aperture.

If you vary the aperture and keep focal length constant, then S/N ratio is a function of the f/ratio.

However the point to Stan Moore's article is what happens when you vary the focal length and keep the aperture constant. In this case S/N ratio is not a function of f/ratio.

S/N ratio is a function of the aperture or area of the primary mirror or lens. For example a 10" mirror at f/5 and a 10" mirror at f/10 will both collect the same number of photons, the only difference being the photons reach focus at different distances from the mirror.

Note that this does not apply when imaging using eyepiece projection. The S/N ratio is a function of the diameter of exit pupil.

The diameter of the exit pupil = focal length of eyepiece/ F-ratio.

Steven