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Field Flattener for 11" F10 SCT

Started by jsuro, 07/20/2003 01:49PM
Posted 07/20/2003 01:49PM Opening Post

Is it theoretically possible to build a 48mm format field flattener for an 11-inch, F10 SCT that would not change the F-ratio of the scope? Would it have other side effects like vignetting? Would it work with eyepiece projection as well as prime focus?

Lastly, could anyone point me in the right direction as far as researching this issue?

Take Care,

Jose Suro
Tierra Verde, Florida
Posted 07/20/2003 07:51PM #1
Jose, the simplest case of such a field flattener is a thin plano-concave lens placed close to the focal plane. The closer it is, the more negligible aberrations it induces.
Assuming low astigmatism, as it should be in an SCT, needed radius of curvature of the lens surface can be approximated as 1/3 of the SCT's Petzval curvature. Assuming an f/2 primary and secondary magnification 5, Petzval curvature would be 254mm, giving the lens surface radius as ~85mm.

Since SCT's astigmatism is not zero, there would be some very mild field curvature remaining. In order to get zero curvature, you'd need to know optical parameters of your system, determining its astigmatism and field curvature.