Weird Star Shape Artifact

[MarkS]

I've forgotten who I was talking to at the weekend about stars with "black wedges".  There was a discussion about it on Cloudy Nights recently:
http://www.cloudynights.com/topic/547028-anyone-know-what-causes-this-artifact-on-bright-stars/

I've seen many "explanations" over the years including pinched optics.  Jerry Lodrigus came up with the correct answer: aperture vignetting.  I managed to prove it with the excellent Maskulator software:



It frequently happens with a large sensor (e.g. Atik 11000) on a refractor - I know Olly has posted a few images with this artifact.

Mark

[Carole]

I got a weird star shape (like the one top right) once and was scratching my head as to why.  It was the star I was trying to focus on.  After a bit I realised the filterwheel was sitting slightly off, and once I had moved it the star shape went back to circular. 

Sorry nothing to do with your theory, but it just reminded me of that occasion.

Carole