Hello - relative rookie here.
Even though the weather was cold last night, it was the very first clear night of 2017 in southwest Ohio, so I figured I'd give it a go.
I wanted to try imaging the Horsehead Nebula, using my Celestron C6 CST, with my D5100 and the Celestron 0.63 focal reducer. Unfortunately, I noticed some strange arcs of light in my images (even showed up in the camera viewfinder), and I'm not sure what the cause is. I've seen this before and never figured out the cause, although I suspect the focal reducer. It seems to be caused by very bright objects. The attached image shows an arc coming from Alnitak, and another faint arc coming from an object off screen to the right.