Something to note: a 600mm lens on a full 35mm format gives roughly the same Field of View as a 400mm lens on an APS-sized DSLR. Most galaxies are far, far away, and consequently difficult to fill the frame with one. Cropping can come in useful, e.g. the spiral nature of M88, bottom left corner can be easily seen in the inset 1:1 crop. The closer galaxies like M31, M33 can be framed quite nicely using a 600mm camera lens on full 35mm format, or even a 400mm lens on a 1.6x crop DSLR.