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Ok...
The point about a parabolic mirror is that will give an ostensibly perfect image limited only by diffraction for objects on axis. Off-axis, two things degrade the image - coma and field curvature (the focal plane isn't flat). These rapidly get worse - as the cube of the angle off-axis.
But... most scopes used for astrophotography are not operating at high magnification and hence are not attempting to resolve diffraction rings around stars. In addition, the atmospheric seeing limits resolution on most nights to 1-3 seconds of arc.
What this means is that truly diffraction-limited performance on-axis is not necessary for astrophotography, if you can find a solution that gives better images off-axis than a paraboloid does. For photography what matters is packing most of the light from a star into a small number of pixels on the sensor, and ideally maintaining roughly simlar-sized star images across the field.
As it happens, there is such a solution - a hyperboloidal mirror. On-axis it is overcorrected for spherical aberration, but off-axis the blurred image of a star is still tighter than the blur from a paraboloid. This principle is also the basis of the Ritchey-Chretien cassegrain. Hyperboloidal mirrors of the correct shape are not easily made and until recently were prohibitively costly.
Even better is to use a field-flattener and coma corrector, which is a lens placed in front of the focal plane, designed to compensate for the aberrations of your mirror.
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