Hi JA,
Using a full-frame 6D without a field-flattener... well, it's not pretty.
The 1100D I use is full-spectrum and cooled, modified by me. Not going to do that to my 6D
But to the point, a simple doublet has a field curvature and that is need to be corrected even for a APS-C size sensor... let alone a 6D full frame.
So the FF/FR has dual function. An Astrograph corrected for full-frame is beyond my $$$
Cheers,
Mick