Reducer/corrector correction

A question to the optical experts:

When I tried to collimate my C8 with R/C on (bad idea!), I immediately noticed that a slightest shift of the (artificial) star from the field centre yields asymmetric diffraction pattern out of focus. I'd say it looked as out-of-focus coma; yet there was no noticeable coma in focus and the stars were sharp over the FOV - that's what R/C are for.

So how does it work that this asymmetry disappears in focus? (Or doesn't it?)

Dmitri

Two questions:
1) What was the distance between the C8 & the artificial star?
2) What other hardware was installed with the r/c (i.e. extension tube, visual back, diagonal, etc.)?

If the artificial source required a significant shift in focus (primary pushed forward), the primary baffle tube (or perhaps the r/c itself, or another item nearer the eyepiece) may be interfering with the image field to a minor degree. Any movement of the fresnel pattern off axis would cause some "flattening" of one sector the fresnel pattern. Both the center of the pattern and the opposite sector remain undisturbed. I have seen this effect in moving-primary scopes as well as long/slow refractors, even when focused at infinity.

Cheers & best wishes.
-Dan