I was experimenting with the use of the Optolong L-eXtreme filter in the heavily light polluted area of San Diego with the California Nebula as my target. After focusing and getting the auto-guiding fine-tuned to less than 1 arc-sec, I took some test shots. There's a star near the nebula called Menkib and it is showing up with a short comet-like trail in my images. Nearly all the other stars are all crisp and round but if you zoom in on some of the brighter stars in the upper-right corner, you can see very faint trails. All the trails appear to point toward the center of the image. What could be causing this?
SkyWatcher Esprit 100
Starizona APEX focal reducer (0.65)
Optolong L-eXtreme dual band filter
ZWO ASI2600MC-Pro imaging camera
ZWO ASI290MM guide camera on an OAG
Pegasus FocusCube 2 auto-focuser
The back focus for the Starizona focal reducer is 58 mm and the user manual conveniently has instructions for my exact setup, which I followed precisely. The only difference is that I have the filter installed which is only 1.85 mm thick which should only add 1.85/3 = 0.6mm to the back focus. So it doesn't seem like back focus is the problem, but maybe I'm missing something.
The attached image is a single 60-sec exposure - no calibration, no stacking, just debayered.