ATM Focal Ratio question

[alphasig@mail.usa.com]

I understand that shorter, faster focal ratios are better for photographing star clusters 
and nebula and that slower ratios are better for observing planets and the luner s
urface.

What I do not understand is why?

All the F/ratio really is is the ratio of the apature to the aparent focal length. The 
aparent focal length is really nothing other than an expression of the angle at which the 
light converges. If I have an otherwise fast system and the last thing I do is run the 
light through a concave lense before it exits so that the folcal point is extended back 
farther, I suddenly have a slower system. It seems we are almost talking about degrees of 
magnification here, which I understand to be a variable function of the chosen ey
epiece.

Can somebody help me understand why different focal ratios are better for 
viewing/photographing different objects, and how focal ratio differs from magnification 
power?

Dave

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