Spotted this while pixel peeping around my image of the state of liberty nebula.

Looks odd, like a rectangle that sort of resembles a Tie Fighter coming straight on.
Most dominate in the Ha channel but still visible in S2 and O3.
Have not seen it before on any other images taken with the ZWO 2600mm.
Drizzling the image improves the sharpness of the artefact.
My question is, what is it? what has caused it?
Any ideas would be much appreciated.

Cheers Greg