SCTs have quite a bit of field curvature. A common f/10 arrangement with ~f/2 primary and ~5 secondary magnification has secondary-to-primary r.o.c. of ~0.31.
With zero astigmatism, field curvature would follow so called Petzval's surface, given by 1/(2/R2-2/R1). For an
8" it gives field curvature ~180mm, and for an 11" ~250mm.
Actual field curvature is determined by the amount and sign of astigmatism, which is in turn determined by the amount of deviation in the power of the corrector from the design value. A dead-on corrector would result in a minimum amount of astigmatism, worsening field curvature by 10%-15%. Quite small deviations - something to expect in mass production - would result in significant increase of astigmatism (and coma), which would - depending on its sign - either further worsen, or lessen field curvature. It is possible that an 8" SCT has less field curvature than an 11" - everything is individual.
Astigmatism/field curvature of the eyepiece will determine the final visual astigmatism/field curvature (not to forget astigmatism/accomodation power of the eye).
I know I can see defocus close to 2" barrel in the C9.25, which should have less field curvature than C11 (~300mm Petzval). But how much of it any of us may see is - just like with our telescopes - highly individual.
With zero astigmatism, field curvature would follow so called Petzval's surface, given by 1/(2/R2-2/R1). For an
8" it gives field curvature ~180mm, and for an 11" ~250mm.
Actual field curvature is determined by the amount and sign of astigmatism, which is in turn determined by the amount of deviation in the power of the corrector from the design value. A dead-on corrector would result in a minimum amount of astigmatism, worsening field curvature by 10%-15%. Quite small deviations - something to expect in mass production - would result in significant increase of astigmatism (and coma), which would - depending on its sign - either further worsen, or lessen field curvature. It is possible that an 8" SCT has less field curvature than an 11" - everything is individual.
Astigmatism/field curvature of the eyepiece will determine the final visual astigmatism/field curvature (not to forget astigmatism/accomodation power of the eye).
I know I can see defocus close to 2" barrel in the C9.25, which should have less field curvature than C11 (~300mm Petzval). But how much of it any of us may see is - just like with our telescopes - highly individual.
