Hi,
I stopped down the Photar to f/5.6 for this image. At f/3.5 it still shows a lot of coma. With extension the f/5.6 is probably more like F/16, so diffraction comes into play. I guess that depth is around 6 mm, and 40 slices at f/5.6 may be to few to cover depth.
many variables in that equation…
Best regards
Erik
That lens doesn't look particularly sharp. Is it better at longer distances expensive?