[ATM] additional lurie-houghton degrees of freedom

[vladimir sacek]

Ukyo Chen wrote:

>Actually a totally coma free lurie-houghton is not the best solution of the highest MTF and smallest spot size/ wavefront error. There are other factors like spherical that impacting the image quality...<

"Aplantic" is, by definition, corrected for both, 
spherical aberration and coma. But I just noticed that 
allowing for a small but still negligible amount of spherical 
aberration can reduce astigmatism and further lessen the 
field curvature.

Using "aplantic" radii, and all else equal, for Guy's system,
only with the mirror radius changed to R=-80" (-2000mm), 
results in spherical aberration of less than 1/20 wave PV. 
But the astigmatism, even only slightly lower, results in flatter field: 
~-4500mm vs. ~-2000mm the aplantic version.
Coma is, similar to spherical aberration, entirely negligible.

That is what I see with the R/V raytrace. Wouldn't hurt to double
check it with some more advanced software.

Assuming similar levels of chromatism, I would probably opt for 
the flatter field, over the 100% aplantic version.

Vlad