I'm using a WO Zenithstar 73 APO, which is a doublet with 430mm F/L and a ZWO ASI294MC Pro for imaging. I was imaging the Iris nebula last night. I took 50x3-minute light frames, followed by flats and dark flats. I also used darks when stacking the subs.
There is a bright star on top of the image, and it has a large halo around it. There are also thick lines around it, which remind me of diffraction spikes. I think refractors should not cause diffraction spikes, so I am wondering what could be the source of these lines. Is it normal to have them for bright stars? Or could it be caused by something in my optical train?
With regard to the halo, can I do something (other than post-processing) to reduce that?
There is also a weird artifact at the bottom of the image, which I think should be removed by flats. However, I did not see the same artifact in my flats. If you know what that could be, and why it wasn't present in the flat frames, please share your thoughts.