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Old 11-02-2019, 11:48 PM
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Stonius (Markus)
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Is this a colimation issue in my refractor?

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.

Cheers

Markus
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Old 12-02-2019, 01:54 AM
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Joshua Bunn (Joshua)
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Your stars look pretty good, I would say it's cause the star is off axis.


Josh
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Old 12-02-2019, 04:16 AM
glend (Glen)
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It could be caused by a slight image train weight shift, if there was any flexure in your setup. Which direction was down in your image?
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Old 12-02-2019, 06:06 AM
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Atmos (Colin)
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It’s not unusual to see reflections off-axis to look like this. The stars across the field look good so I’d say that it’s just because it isn’t in the centre.
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Old 12-02-2019, 07:00 AM
JA
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Quote:
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.

Cheers

Markus
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

Best
JA
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Old 12-02-2019, 02:56 PM
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Stonius (Markus)
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Quote:
Originally Posted by glend View Post
It could be caused by a slight image train weight shift, if there was any flexure in your setup. Which direction was down in your image?
That's a good point.

Down was in the direction of elongation.

It was all threaded connections though, so I'm trying to think of the flex point.

I guess I could test it by rotating the focuser 180 degrees and seeing if it happens again.

Markus
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Old 12-02-2019, 03:00 PM
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Quote:
Originally Posted by JA View Post
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

Best
JA

Thanks yes, microlensing is an issue in this series too (less so on the Lum channel here, but more pronounced on others). I expected to see it on this shot, so it was not a suprise. But I wanted this composition and I'm interested to try my hand at mitigating the effects of microlensing. If it all gets too hard I can always crop in, I guess.

Best,

Markus
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Old 12-02-2019, 05:32 PM
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Merlin66 (Ken)
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JA, et al,
The ASI 1600 is fitted with a BK7 AR coated window.
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