A 400mm lens with a speed of f5.6 would have an objective diameter of 71 mm, which is certainly large enough to provide the resolution needed to see the rings of Saturn. At a magnification of 8x the planet would not be large enough within the viewfinder to maximize the resolving power of the lens and therefore a great deal of cropping in post is needed. The limit to effective cropping depends upon the ISO setting selected at the time of exposure. A planet is brighter than a comet so an ISO of 3200 is probably not necessary, nor is it necessary to have an exposure of several seconds. At that length of exposure the rings would be smeared out a little. I really cannot recall what exposure would be needed at f5.6. If you are considering coupling a 1.4x TC then your exposure would need to be longer, but the slight degradation in image quality might counter the slight boost in magnification.
When Saturn is viewed in a telescope there is satisfactory detail at 60x (3,000 mm lens). Even then the planet is very small. A good viewing size is achieved at 200x (10,000 mm lens). Obviously using the straight focal length of a telephoto lens will not deliver the magnification needed to create a captivating image of Saturn.
The viewing angle of a 400mm lens with a FX sensor is about 5 degrees (FOV). That is equivalent to 18,000 seconds of arc in the sky. Saturn is about 20” of arc. The rings maybe 50” or more. Do the math and you will appreciate how small Saturn will look on a jpg without cropping and how much cropping would be needed.
The only way to get enough magnification is with an ocular lens element between the 400mm lens and the camera’s sensor. Sort of like a TC, but with a much shorter focal length. This would boost the magnification of your image but it would also greatly reduce the amount of light falling on your sensor, therefore requiring a much longer exposure. You could experiment with digiscoping, coupling a small bridge camera to a spotting scope, or to a telescope. Good luck with that.
Saturn at about 20x (1,000 mm lens)
About 100x
About 200z with 10" telescope
