Re: [APML] RC Field Flattener

[Aplanatic@aol.com]

Hi Gil:

If that's the 12" f/9 RC, then, as I recall, the BFL is about 12" and the primary's f/ratio is about 3.5.  With that rough info here's the approx. design:

Primary:  ROC=84", SC=-1.19, D=12"
Secondary: Spacing=26.78", ROC=50", SC=-7.58, D=4.5"

The field is curved upward with an ROC of approx. 16.9".

With a good field flattener the spot size should be in the neighborhood of 50 microns RMS diameter at an off-axis distance of 1.5".  Without the FF the RMS spot size at the same off-axis distance is about 135 microns RMS diameter.

In other words, to do decent MF imaging with this scope, a FF is highly recommended.

Also, be aware that the baffling of the scope *may* cause a vignetting problem for medium format.  You need to check this.

Ideally, the usable diameter of the secondary should be about 4.5".  If smaller, then vignetting at the field corners may be a problem.  You should check this also.

If you want to incorporate a FF, how much BFL do you need for your application?  As the FF is moved farther away from the focal plane the problem gets harder and the aberrations induced by the FF increase.

Dave Rowe

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