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Andrew Rambaut: There has been a useful discussion here in AB about this and similar effects a while ago. The basis is that the marginal rays get clipped by internal baffles (or any similar obstacles) so that off-axis you don't receive the full bundle of rays but a clipped one, resembling cat's eye when the iris isn't fully open. |
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| It’s called “aperture vignetting”, a good discussion here: https://www.cloudynights.com/topic/854842-black-wedges-pinching-or-aperture-vignetting/ |
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OK - interesting and some nice simulations of the effect here:https://www.cloudynights.com/topic/547028-anyone-know-what-causes-this-artifact-on-bright-stars/#entry7388127 Does look very similar to my first image. I don't know why the aperture would be almond shaped - it seems a classic reason is the filter wheel is slightly misaligned, but I am not using a filter wheel. Possibly the dew shield is askew - I will try with it retracted next time. I will look at the set up more closely. |
