I've recently started experimenting with astrophotography. I'm using Celestron 8" SCT (not EdgeHD) on Advanced VX mount with Canon 600D attached via T-adapter and Celestron F/6.3 Reducer-Corrector #94175. For guiding I'm using Celestron GuideScope with NexGuide.
I've run into following troubles with reducer-corrector:
1. A bright stars near the edge of the frame creates big oval-shaped artifacts (crop #1). I don't see such artifacts without reducer-corrector. Is it normal? Is there any way to get rid of them?
2. While stars at the center of the frame are reasonably round (crop #2), near the edge of the frame have a strange, non-round shape (crops #1 and #3). I thought a corrector supposed to fix those things, while in fact it make it much worse. What's wrong with my setup? Do I need some kind of spacers as "Getting started: long exposure astrophotography" recommends or my corrector is broken?
crop #1: top left corner
crop #2: center
crop #3: bottom right corner
all crops are parts of the same single 60s autoguided image