This webpage
http://www.astrosurf.com/~buil/
is a good start for lots of very useful information about lenses and their
suitability for astro work.
Have a look here:
http://www.astrosurf.com/~buil/testopt/50mm.htm
This is another one with lots of relevant information:
http://www.samirkharusi.net/
The number that describes the quality of the lens system is MTF (Modulation Transfer Function). Unfortunately, this parameter is not available for every lens (Canon is the only company that published it for their lenses, but should we trust them? )
http://software.canon-europe.com/fil...Book_10_EN.pdf
BTW, MTF is in principle equivalent to star test, results of which I have presented in my previous post (However, my comparison test was very crude because it is not standardised.. but it does reveal everything what is really important).