I believe it is an internal reflection of your imaging train Geoff caused by the bright star Alnitak in the field of view. The center of the "doughnut" is the secondary mirror. You will probably find that it will happen on any really bright star without changes to your imaging train to reduce the reflection. One of the largest internal reflections I have ever seen though. I got a reflection as well when I imaged this area of sky:
https://www.facebook.com/CapturingTh...type=3&theater
Were you imaging an area devoid of bright stars the night previous you mentioned?