I just purchased an MD 50mm f2 (no sign of Rokkor or X), and I have a Minolta 45mm pancake. I had, and used (manually) a Nikkor 50mm F1.8 D (first gen autofocus) for astro. I also purchased the slightly different formula 50mm F2 ais. Upon testing, I found the Nikkors were the only ones that I would use in future. The 45mm pancake was still wanky and soft with coma at borders at F5.6. The Minolta 50mm F2 was borderline acceptable at F5.6, and the two Nikkors were almost imperceptibly different at f5.6, and could be used at F4.5, but still better at f5.6. Of course this means you must use a star tracker because 4 seconds is the limit before orbital smearing starts. The 50mm F2 Nikkor has simpler stars because it has 6 blades while the f1.8 D has 7, which means more star points but slightly more diffusion. I'll be using the F2.0 Nikkor.
