If field curvature is not a concern, you can make your own focal reducer using an objective lens from binoculars or a small telescope. The first focal reducers were made from 50-mm binocular objectives with F ^ 200 mm.
The field curvature of a Schmidt-Cassegrain, however, will be worsened by a focal reducer that is not specifically designed to counteract it. Meade and Celestron focal reducers do flatten the field, and, in my experience, the field of an f /10 Schmidt-Cassegrain is flatter with the f /6.3 reducer in place than without it. For relevant optical theory, see H. Rutten and M. van Venrooij, Telescope Optics (Willmann-Bell, 1988), pp. 152-154.
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