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Firstly, is it going to be for astronomy only? or will it be used for other photography as well? If astro only, the 200 F/2.8 prime is probably where you want to be looking... If it will be a multi-purpose lens, then the 70-200 2.8 is a great lens indeed.. as is the F/4
Having 2.8 will not have less noise due to less exposure time, More exposure time raises the signal to noise ratio, hence, more exposure time = less noise.. Clarity? Well.. the 4 different 70-200L lenses are of all very clear, however, the F/4L IS is know to be the sharpest. followed by the F/2.8L IS, then the F/4L, then the F/2.8L. These differences in sharpness will go unnoticed to most people, however in side by side tests, you can usually tell the differences.
Also, the 200 F/2.8L prime lens is sharper than them all again, at the cost of loss of some versatility, but if the lens is for astro work only, you'd be hard pressed to find better in this price range... Whats more, if you were to buy the 200 F/2.8L prime instead of the 70-200 F/2.8L, you'd have enough spare change to buy the 85mm F/1.8 which would give you extremely sharp images and give you some versatility....
I've owned the 70-200 F/4L (non IS) and the still have the 70-200 F/2.8L IS, More often than not, I use the 200 F/2.8 prime...
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