As you say Mike it depends on the setup but some of your wider field images taken with the FLI have halos. Two examples are below. Those halo types are what made me change from Astronomiks to Astrodons. I don't get those at all now. I was getting them on two separate systems with different camera.
http://www.pbase.com/strongmanmike2002/image/158972601
http://www.pbase.com/strongmanmike2002/image/156946951