Not sure about astrophotography, but for photographic uses Canon's 300 / f2.8 from least to most expensive (several hundred to several thousand, various types 1980s FD, 90s older L series, post 2000 L series) should be right up there with their best lenses (even their 400mm and 600mm) and are very sharp/contrasty. The 70-200 2.8 is also excellent and useful for other work: sports, portrait, etc... A good buy used also due to their solid mechanical construction. The Nikon 300/2.8 is also fantastic.
Good hunting and
Best
JA
FAMOUS Canon 300 2.8L shot of Henry Kissinger reading Top-Secret Document at UN in the 1970s from Newspaper photographer in gallery:
http://img1.imagilive.com/0516/Kissinger.PNG