Need futher proof of the difference with a spherical v parabolic:
I've taken delivery of a 76mm Celestron Firstscope, with a 300mm focal length. Nice image in low power, just. With the central field in focus, using a lowly kellner EP, the outer 1/3 is
just out of focus. Worse right on the edge. That is because the light from the primary is not arriving at focus at the same place, which it does with a parabolic shape. Tweeking the collimation screws on the secondary holder really helped improve the image quality. A better EP would help a little more, again.
I'll be using it as an overgrown finder, for which its spherical shape is neither here nor there as it is not the main scope for viewing. It is by no means grotesque. It suits my needs as a kick-ass finder. I just would not want it as my main scope for viewing.
Mind you, at f/4, it is a very fast telescope, which would make the problem of a spherical mirror even worse. But it was a concious decision,
. Really.