In a single concave mirror coma, the sagittal coma blur (b) varies with the square of the aperture ratio (f-number), or N², and the the distance off-axis, h:
b = h/16N²
In other words, coma of a concave mirror is independent of the conic constant (and waves of correction).
The coma contribution for an 8“ f/4 paraboloid is the same as that of an 8” f/4 sphere or 8" f/4 hyperboloid. The reason MPCC MK111 doesn't work so well with an 8" f/4 paraboloid is because it was probably designed for a paraboloid of a specific f-ratio other than f/4. Clearly, a single set of corrector optics cannot be optimal for a whole spectrum of focal ratios. But it will work partially for a certain limited spectrum, within reason.
Thanks for the info on coma.
I was referring to spherical aberration. Afaik, a two element coma corrector such as the MPCC MK111 can't correct both the S.A and coma in any paraboloid, regardless of the F ratio. Hence the need to hyperbolise the primary or add an extra (or more) corrector element.
It would be interesting to compare coma correction in MK111 / optimised hyperboloids in various focal ratios. My feeling is that coma is well suppressed or removed over a range. The Paracorr solves sufficiently for both SA and coma over a spectrum of F ratios.
If a coma corrector induces coma at the same rate as the primary but of opposite sign and the primary conic influences SA but not coma then it's easy to see there is a solution. Possible false colour and I assume field curvature need addressing too. Astigmatism?
Edited by davidc135, 13 December 2021 - 04:54 AM.