Airy disk on a Mak

Last night's observing was the best I've ever experienced. High power airy disk was just about rock steady and picture perfect - hardly any shimmering or turbulence. Burt I noticed the first ring on the airy disk while symetric, was not continuous. There was a slight but distinct gap in the ring at about 11 o'clock. I have an Orion Starmax 127. Could that gap be caused by some kind of pinching of the primary (I've had to colimate the scope), or something else I could adjust, or is just an artifact of imperfect optics? The resolution I get on planets, moon and doubles seems very, very good.

Sounds to me like you've got a trace of coma from
slight mis-collimation like Mitch says. I was able
to fix this in my ETX-90 my tweaking the collimation
screws behind the primary mirror. But I have no idea
how you'd make the same adjustment on a Starmax.

You also may want to check optical surfaces. If there is something on them that causes partial transmission loss, like a narrow segment towards the perimeter, it could result in a (visual) loss of a segment of the diffraction ring.

You see a spike when an optic is cooling and it has rising warm air inside the telescope tube. My 12" SCT shows this when I first set it up. A broken airy ring in only one place might be a mild form of this phenominon.