[bratislav (Bratislav)]

Originally Posted by Don Pensack

Technically, a coma corrector is engineered to best correct a particular f/ratio, but there is some flexibilty, with longer f/ratios being better corrected than shorter ones.

This is incorrect.
Coma correctors are designed around mirror's focal length, NOT f/ratio. Once design has been set, coma will be corrected for all f-ratios (up to the point - high degree aberrations aren't corrected by a simple corrector so once they become visible, that limits the usefulness. In case of Paracorr this seems to be around f/4 or thereabouts). For people who are familiar with elementary optics this is self evident - every f/4 paraboloid contains f/5, f/7, f/8, f/9 ... ad-infinitum beams already. So if corrector works with well with say f/4, it works with anything slower just as well (in fact better).

Which brings up the issue of field width. Coma increases linearly with distance away from the center point of the focal plane. If the eyepiece has a narrow field that does not extend very far from center, then much less coma will be visible than if the eyepiece has a wide field extending quite a ways out.
In particular, eyepieces with large field stops, like a 41 Panoptic, 55mm Plossl, 31 Nagler, 35 Panoptic, etc. will show more coma at the edge of the field than a 10mm eyepiece with a small field stop. True, the comatic image will be magnified in the higher power eyepiece, but ultimately it is the field stop of the eyepiece that counts.

This is also incorrect. Again, this is elemetary optics 101.
Let's compare two identical hypotetical eyepieces - 20mm f.l with 20mm field stop, and 40mm one with a 40mm field stop.
Coma is a linear function of off-axis distance (coma blur will be twice as big at 20mm off axis compared with 10mm off axis). But magnification is a linear function too - 20mm eyepiece magnifies twice that of 40mm. In the end, what eye sees right at the field stop is the same angular blur.
The only parameter that counts is apparent field of view - wider the eyepiece is, more coma is shown. 11mm Nagler will show a lot more coma than 55mm Plossl, for example.

Bratislav

PS the example above is only valid if eyepiece's astigmatism does not mask coma; 55mm Plossl will in fact have so much astigmatism in say f/4 mirror that coma will be practically invisible.