spherical primary and collimation

Would a spherical primary mirror in a reflector have less issues with collimation than a paraboloid. Would it even be possible to make an off-axis scope without introducing more abberations than are already inherent in that design?

Brad

Less coma, yes. But if it's a long f/ratio scope it's tolerant to collimation errors anyway (and the difference between a paraboloidal and spherical and if it's a short f/ratio scope it's got so much spherical aberration the coma and collimation are the least of your worries.

And once you introduce correctors answers vary.

Talking about on-axis designs, if you have a spherical mirror in a Schmidt-Newt, collimation is less critical but you still want to collimate well. If you've just designed a Lurie-Houghton, then you have less issues. Grab a copy of Rutten and van Venrooij's "Telescope Optics" and start reading.

If you have an off-axis design with a spherical mirror, everything hinges on the corrector and/or mirrors you design behind the main mirror, obviously.

To answer your second question my latest 8 inch F/7 unobstructed scope on my web site uses correctors that fix the tilt aberrations without introducing any more aberrations. They can't fix coma or SA of the primary however. Collimation sensitivity is the same as a F/7 Newt.
Ed Jones