Doesn't the feed shadow a significant amount of the reflector from the sub-reflector? The feed would appear to block 20%+ of the reflector surface.
Wouldn't it make more sense to design the sub-reflector to focus to the feed positioned below the reflector illumination angle?
That crossed my mind some years ago to , but I never tried it .
I think it be worse , just like bringing the lnb in a much lower position on a offset dish .
I once tried to receive C-band on a original Fibo gregorian dish with conical scalaring (TRF) ( 160 mm dia ) in front of that subreflector .
Got the same signal with or without the TRF but it workt .
Later I used a reduced ( 90 mm dia ) scalaring , same dia as the original Fibo Ku feedhorn , with a little signal progress .
I still use that C lnb at my 6 lnb revolver .