For Photography, the Canon 500mm lens is very similar to the Televue Genesis. I've tested the two side by side. However the Canon wins out in terms of size, weight and convenience (as well as having a variable aperture, autofocus and image stabilization!). I didn't test field flatness, just resolution in the central region of the image.
However in an absolute resolution contest (e.g. splitting close double stars) I suspect that the Televue would win. A lens really doesn't need to be (and rarely is) diffraction limited wide open. The limitations of film and digital sensors mean they don't need 1/10th wave optics.
I do have a way of putting an eyepiece on a FD Canon 500/4.5L converted to an EF mount (see http://www.bobatkins.com/photography/technical/Lens-scope-adapter.html
) that I have and it makes an excellent telescope. However it's limited to 0.965" eyepieces because there's not enough back focus distance to get a 1.25" diagonal and eyepiece in there. I don't have any high quality short focal length 0.965" eyepieces to do resolution tests with the lens.
Field flatteners are for photographic use, often when you want to shoot with a full frame 35mm or Medium format camera. They are often used with telescopes which just have a doublet or triplet objective. I think the Genesis is pretty flat field without a flattener, because the second doublet flattens the field as well as shortens the objective focal length (i.e. act as a reducer for the ~1000mm fl of the objective doublet) and compensates for chromatic and seidel aberrations. I'm not sure if there's a field stop between the front doublet and the rear doublet in the Genesis, but I haven't ever looked at it closely.