Schmidt camera is famous for a extremely wide field camera and less abberation, but it has a field curvature, which is not suitable to CCD camera. I designed a flat Schmidt camera with Meade SN-10, Schmidt newtonian. Schmidt newtonian has huge coma like Newtonian, but it would be a perfect optics potentially.
Coma of the Schmidt newtonian will be zero if the C.P. is located at twice of its focal length from the primary mirror. Fathermore, the field curvature will be reduced if a positive lens is located just in front of CCD. I traced optical rays of the Schmidt newtonian with a lens, and I found a solution.
To remove the field curvature, Petzval condition should be zero. Mirror's power is 1/((-1)*(focal length of the mirror)), and a positive lens' power is 1/(1.5*(focal length of the lens)). In this case, 1/((-1)*(1000mm)) + 1/(1.5( f )) = 0, so f=666mm. The lens is placed at 47mm away from CCD plane, and the correcting plate is done at 1.3 times of the mirror's focal length away, 1300mm. The corrector lens redeces half of coma, so the C.P. should be located 1300mm away from the primary mirror. It also makes the original focal length shorter, and the effective focal length of this optics is 920mm, whose Fno is 3.6.
I used a Bord 50ED objective lens, f.l.=500mm, for this, and it is located at 50mm away from CCD.
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