Yes, spherical abberation since the primary mirrors are fast and
spherical and suffer from lots of it.
The Schmidt corrector plate does the same for a Schmidt cassegrain which
also uses a spherical primary. Schmidt plates are much cheaper and
easier to make than the Maksutov's meniscus lens in sizes above 7".
Therefore you may never see a Questar larger than 7". The Maksutov's
principal problem is that that front meniscus lens has to be very thick.
Pouring a large diameter, bubble free lens blank is hard stuff.
Chuck Norcutt
Jeff Keller wrote:
> I think I've seen a lens whose front element's outside surface is flat and
> inner surface is concave. It might have been a Tamron.
>
> The Maksutov Cat lenses use a simple concave meniscus lens to correct
> spherical(?) aberations.
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