In optimal world I would buy a lens that is good in every way. But I'm on a budget and have to find a balance in quality and price. I'm looking for a lens for star photography, so in my case it will ...
There's a whole host of inexpensive mirror lenses available in the 500 mm - 800 mm range (this for example). They have slow max apertures (f/5.6-8) and a donut shaped bokeh (not a problem ...