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Optical train issue? Odd Shadows - can you figure it out?


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Firstly a HUGE thank you to each and every one of you for your input. I was worried I was on my own on this one!


I've made a discovery in the recent cloudy nights (with more to come)


It looks as though I've been a bit of an onion and described the original picture as a "Sub", it was happylimpet's comment that triggered this... I believe the original included image is a stack of lights, darks, flats and bias as per my normal processing, then stretched in software.


This prompted me to look deeper into the single subs, prior to stack, now no matter what I do, I can't pull the hexagonal pattern out of the single sub which leads me to believe it could be an issue somewhere in my stacking process?


I will still be testing my entire optical train once the clouds go and pester someone else.


I have included an actual single light sub of the original capture at a reasonable quality should you wish to play, also a list of my DSS settings.


Register Settings:

Star detection threshold 10%, Reduce the noise by using a Median Filter is ticked


Stacking Settings:

Result:

Standard


Comet:

Standard


Light:

Kappa-Sigma clipping, Kappa 2.00, Number of iterations 5


Dark:

Median

Hot Pixels detection and removal

Dark Optimization


Flat:

Median


Bias/Offset:

Median Kappa-Sigma clipping, Kappa 2.00, Number of iterations 5


Alignment:

Automatic



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To add to that, I use absolutely no in camera processing, no noise reduction or any other push here dummy mode when using my camera for Astro, everything is manual and everything else in camera is off. So I can't see it being anything the camera itself is doing electronically, at least I don't think so.

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Haven't used DSS for a long time, but it will almost certainly be some interaction between the pixel rejection settings (Sigma-Kappa setting) and the diffraction spikes around the stars. The pattern of dark spots is octagonal, not hexagonal and you have four main spikes around the stars, plus another four smaller ones at 45 degrees to them, making for the octagon.

A DSS expert will pop up soon no doubt, but you most likely need to fiddle with the rejection settings (so try something other than sigma-kappa or change the numbers for example).  It might be that you have some subs that haven't been properly registered with the other ones, but still included in the stack for some reason so the ends of the spikes are getting pulled down towards the black by the rejection process.  Maybe eyeball all of the subs in a slideshow and see if any of them don't look good.

Not aliens after all.

I wouldn't sweat it over the optical train now, as the single sub looks pretty normal from what you have shown.

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