Star shape artifacts: black wedges

[sharkmelley]

It has been previously commented in this forum how some sets of equipment produce star shapes with "black wedges" or "inverse lighthouse beams" e.g. here:

The same question arose recently on Cloudy Nights: http://www.cloudynights.com/topic/547028-anyone-know-what-causes-this-artifact-on-bright-stars/

The cause was identified on CN as aperture vignetting.  This is where pixels away from the centre of the sensor are not illuminated by the full circular aperture of the scope or lens but some component of the optical train begins to obscure it.

I was pleased to be able demonstrate this effect using Niels Noordhoek's excellent Maskulator software:

In some cases the offending optical component might be identifiable (e.g. the filter wheel) and replaceable to reduce the amount of obscuring it causes.  In other cases it might be part of inherent design of the scope e.g. the internal baffles of an SCT.

Mark

 

[StuartJPP]

I know this is an old topic, but it does keep cropping up every now and then on here and other astronomy forums.

On the Canon 500mm f/4 L IS lens that I have got the rear part of the barrel, just before the camera mount, is square not round, probably about a 3 or 4cm tube. Initially I thought that this was the cause of the vignetting and although it does play a tiny role at the extremities it doesn't appear to be part of this particular issue.

I couldn't mentally picture where this "almond" shape of vignetting was coming from, so I put my lens to my eye and looked down the tube and moved it back and forth, to and fro. It was plain to see the almond shape that Mark and others identified as the source of the issue.

The video below tries to capture the issue and as far as I can tell there would be no way to eliminate this issue other than by cropping.  Head-on, i.e. in the centre of the sensor the image is circular but as the the light travels further away from the centre of the image circle the more they become distorted.

 

 

[Hals]

Nice one Stuart. Funny thing is I've noticed this effect but only during one imaging session. I've had many since with no changes to the equipment barring slight focus changes and my stars have been all ok, displaying no artifacts.