I was reading over at Cloudy nights...before the site went bung...a few weeks back about an
astrotech field flattener for the likes of the ED80.
It is not a reducer but from the images I saw it flattened the field very well indeed.
Probably google for some
comparison images. (I think these are the images...not sure cannot get into cloudynights).
This site shows an opposing view on the astrotech flattener! hard to tell, maybe the distances between focal plane & camera were not right.