What f stop for a 'Nifty-Fifty'?

[michaelmorris]

I've started experimenting with using my Canon 50mm f1.8 II (the so-called 'Nifty-Fifty') with my astro-modified Canon 1100D camera.  I've take two sets of subs of the same target and notice a marked difference in image quality at the edges between the ones taken at f5.6 (lovely round stars pretty much all the way to the edges) and f2.8 (a lot a radially distorted stars at the peripheries of the image). 

What f stop do others us with a nifty-fifty and how much radial distortion of stars do you get?

[JB80]

 f4.0 is what I generally use, I have seen otherq use  or recommend more  like f4.5. I haven't totally experimented but f4 is good for me.

I do end up cropping the edges but if the target is central then it's fine. Most of the distortion at that point is probably field rotation in the stacked images.

[sharkmelley]

I have a Nikon equivalent and need it stopped down to f5.6 to prevent star distortion at the edges.

Mark