Number of APO elements before image degrades?

I've been lucky enough to do a running comparison on a Takahashi FS-152 vs TOA-150. Both on the same mount using a Robin Casady Triad plate. A number of images taken with both scopes show the TOA needs a field flattener on a DSLR size chip while the FS-152 looks fine. I ordered the TOA field flattener. Looking at the flattener it looks as if it has two lens elements. With three glass elements already in the scope and two additional elements in the flattener, is there not some light loss going through all that glass? At least more susceptability to adjustment and QC problems? Both have excellent images with the TOA a slight edge visually on Mars. The color is (of course) better. The contrast is great in both with maybe a slight edge to the doublet. I have to make a decision eventually on one for imaging and worry about the extra elements.

TOA 150 Double Cluster image with edge magnification:

Attached Image:

Huh, surprised by that actually. I wouldn't have thought the corrected field would be any smaller in the TOA. Was that the same in all 4 corners?

Kevin