Radial star artefacts with refractor.

fredvanner

Well-known member
Apr 17, 2019
201
30
Wells, Somerset, UK
We are all familiar with the radial diffraction spikes from mirror supports. I have seen previous posts about radial diffraction patterns from refractors caused by lens pinching or intrusion of lens supports into the aperture. I am observing radial diffraction patterns that do not seem to fit this model - they are more or less uniformly distributed around the star. Does anyone have any suggestion regarding the possible cause? (Actually, I quite like the effect, but I'd like to understand it).
As an example, I attach a recent image of NGC7000 taken with my S/W Esprit 80 APO and ASI2600C camera (60 x 120s; ÷4 downsample). I also attach full resolution previews of the region round xi Cyg, in the final image and in a single frame.
 

Attachments

fredvanner

Well-known member
Apr 17, 2019
201
30
Wells, Somerset, UK
I believe that the far-field Fraunhofer diffraction pattern of a circular aperture is the familiar Airy ring pattern. Something needs to break the radial symmetry to form these radial rays. The only component that obviously breaks this symmetry is the imaging array, but I can't see how that could create this effect.
 

rakla

Member
Dec 17, 2018
21
1
Switzerland
ko-fi.com
Fred, you are correct of course. I was suggesting that we are subject to non-perfect conditions, which includes imperfect focus, atmospheric effects (light pollution, seeing, ...) and a non-perfect optical train (light scatter, obstructions, etc.). Furthermore we do not have monochromatic pin-point light sources, all of which could contribute to the observed diffraction. I might be totally of the beaten track of course... I can just confirm that I get the exact same effect with my S/W equinox 120ED.
/Ralf
 

fredvanner

Well-known member
Apr 17, 2019
201
30
Wells, Somerset, UK
It is reassuring that you get similar effects with similar equipment. The colour fringing in the radial banded patterns confirms your point about non-monochromatic sources. I can see no obvious scatterers or obstructions in the internal light path. I wonder about point scattering from more or less invisible imperfections on the inside surface of the lens hood.