If the lens is ONLY for astro imaging, the 135mm F/2L is brilliant (as is clearly shown by Paul's images) the next option from that in my opinion is the 200mm F/2.8L, and Im sure the 200mm F/2L would be Awesome!!
of the 70-200mm family of lenses from canon all have varying quality. As far as I've seen (and I've owned them all at one stage or another)
In order of sharpness at their wide open apertures, F/4L IS, F/4L, F/2.8L, F/2.8L IS.
If its primarily for astro imaging, IS is really really not needed or used...
If I were in the market for a widefield lens at the moment, I'd be looking at the 135mm F/2L... I use my 70-200 F/4L @ 135mm all the time, as I love the incredible wide field.. you can just fit sooo much in..
My 15c.