In astrophotography everything is in focus so one should never get rings.
When I'm rich I'll certainly buy a Minolta RF 250/5.6 such a nice portable addition to my walk-around bag.
At the moment they are a bit too expensive:
http://www.ebay.it/itm/Minolta-RF-5-...item4850e73dbe (this probably is exaggerately expensive).
For most subjects (those without shining out-of-focus points) the circular effect is noticeable but not really disturbing.