Scope "speed" is determined by its f/ratio. (Aperture/Focal length) She smaller the f/ratio the "faster" the scope. More light gathering power per given magnification (Scope Focal Length/Eyepiece Focal Length).
From a photography point of view the faster the better. Less exposure time require for a given shot. But there is a tradeoff. You will often get more coma near the outer portions of the mirror. And that is in mirror configuration. It is much more difficult to configure a fast mirror than a slower mirror. For photography you can either use a smaller chip to avoid the coma, or get a coma corrector. For visual work it may mean getting better eyepieces that will deal with the coma. (I'm sure the visual guys will jump in here if I get this too wrong
)
If you read through some of the Eyepiece forum threads you will notice many talking about Nagler eyepieces. Why? because they deal with mirror/lense issues better than many other eyepieces. Problem is you pay for them, Naglers are not cheap. Two or three naglers may cost more than your whole telescope setup. They are not the only eyepieces that do this but across the board probably better than most, especially at the longer focal length eyepieces.
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Don't you just love translated English?? Maybe we could start up the Fog Catalogue.
