Re: ATM Change of Plans, F/2 -> F/3.2

[Aplanatic@aol.com]

Hi James,

A good change in plans in my opinion.  Parboloidal mirrors get harder to figure something like the third power of the numerical aperture, depending on what one means by "harder."

Are you interested in making the focal reducer or buying it?  If you design/make your own then a custom design can significantly reduce coma and field curvature in the final system.  Also, the aberrations can be reduced further in a custom design by making the primary hyperbolic, but that eliminates the possibility of using the primary without the reducer/corrector.

Third-order coma goes as the field angle to the first power times the aperture radius to the second power.  So, as long as the reducer does not introduce significant aberrations of its own, the coma should be the same for the native f/2 and the reduced f/3.2 at the same off-axis distance at the focal surface.

Dave Rowe