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telescopes have fixed f/l, due to the fact the lens present inside them are fixed - also the aperture is fixed - there are no moving parts inside a telescope to control the aperture, we could say they are wide open all the time.
As for the desirability of the number, smaller f numbers like that on a camera will let in more light cf. longer f/stops- so for photography the lower the f stop the more desirable because the exposure time will be shorter, which is good as less you have to worry about the mount & tracking.
For viewing longer f/stops aren't a big problem, good planetary views can be had with a telescope of f10, or f15 like some of the SCT's. In fact some of the smaller f/stop like f5 reflectors, can suffer becuase the instrument isn't quite up to gathering the light & a good image across the field of view is difficult to obtain. You wn't experience coma with a f10 instrument.
hope this helps - but I think i'm waffling so will post & re-read
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