Star shapes?

[tony_cameraguru]

Hi all

I have been imaging M45 and have come up with this form of issue with the stars. It almost looks like I have left (and I did think this) the Bahtinov mask on, however I didn't. This is not the first time I have noticed this star issue. Any ideas? This is my equipment: Explore Scientific 80ED Apo Triplet; Explore Scientific Field Flattener; Canon 250D mod; 2in UHC filter; 60mm Guidescope; ZWO ASI120MC-S.

[steppenwolf]

I think this is a diffraction effect caused by 'aperture vignetting'. This is not the same as normal vignetting but appears because the edge of the image field doesn't receive a full circle from the cone of light through the aperture,  rather, it 'sees' an elliptical shaped aperture.