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Old 12-02-2019, 07:00 AM

JA is offline
Join Date: Oct 2016
Location: Melbourne, Australia
Posts: 1,279
Originally Posted by Stonius View Post
This is a sub from my Luminance Data.

Just looking at the non-symmetrical halo around Alnitak.

Is its asymmetry caused by being positioned in the top left of the image? Or should I look to my collimation?

I did check it on the night using the tube reflections method and it seemed spot on.


It could be lens flare/reflection caused by an off axis light source or a refection from filters or even from the sensor itself. There appear to be multiple faint haloes around the star, displaced ever so slightly along a line towards the image centre, which is typical of lens/optical system flare. When these ever so slightly displaced haloes appear on top of one another they appear as an ovalish shape as a whole and I can certainly see how they could blend together to form the IMPRESSION of a collimation issue (whilst not actually being one). See attached (exaggerated) diagram.

I recall reading of an issue with reflection artifacts with the ASI1600 caused by a lack of/type of anti-reflective coating on the sensor window. I also recall the image had faint diffraction type spikes in a 4/8 spoke pattern, which are also present and similar to those in your image, but I'm not sure. The spikes may have been caused by a diffraction/reflection from the prism of an off-axis guider. Did your image also use an OAG?

To try to verify the cause, perhaps a different type of filter could be trialled, with a different antireflective multicoating. Perhaps a photographic filter

Attached Thumbnails
Click for full-size image (Flare Reflection.jpg)
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Last edited by JA; 12-02-2019 at 08:04 AM. Reason: + diagram
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