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Originally Posted by ausastronomer
AN, there can be a difference between an F5 and an F7 newtonian scope given equal optical quality, particularly if the F5 scope is a tube scope and not a truss scope.
With a tubed scope you need a larger secondary mirror due to the necessity of the tube to have a larger diameter than the Upper Cage Assembly on a truss scope.
An F7 scope will have a much larger diffraction limited field of view. This means off axis aberrations will be reduced and images better quality off axis. Notably coma and field curvature are reduced in the F7 scope.
CS-John B
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John - excellent point. I did mean on-axis images, but didn't make that clear. Off-axis, things do get hairy. And as a telescope maker, we have a limited choice of semi-major axis diameter diagonals to choose from (unless you make your own, which is unusual). So the diagonal size used for a f5 is probably (but not necessarily) the same as for a f7.
Impressed with your 10inch GSO, I am green. I think 10inch f5 is the ideal scope, considering light gathering, conpactness and portability.