Thank you. I'm not up to date on telescope design (by at least 40 years), but the Cassegrain design used a spherical mirror, I think. It was used for wide-fiel

Thank you. I'm not up to date on telescope design (by at least 40 years), but the Cassegrain design used a spherical mirror, I think. It was used for wide-field astrophotography, and (again, I think) it has a curved focal field, so it needs a curved sensor. I'm certain that Cassegrain designs have been obsoleted. I'm just bragging about what an anachronism I am. [Edit] The Cassegrain design also requires a refracting lens in front of the mirror; and the lens is a peculiar shape. [Edit again] I may be thinking of a Schmitt. I've never played with anything more interesting than a Newtonian; I was just an amateur stargazer. All these fancy designs were made for professional astronomers. For me, they were strictly theoretical.