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#1 dansawyer

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Posted 25 September 2020 - 08:02 PM

This is cropped from the center of a siril stacked image of NA nebular. The image has distinct red bands running from the lower left to the upper right. The camera is a canon M1. The images used were 40 images of 50 seconds each. 

The lighter area on the left is part on NA, the darker area is the gap between NA and Deneb (cropped up and right). 

I have 30 darks and 30 biases that were not included in the processing. Are these artifacts due to not including darks and biases.

 

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  • r_na_stacked_s_red.jpg


#2 17.5Dob

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Posted 25 September 2020 - 08:16 PM

That's "Correlated Noise" aka "Walking Noise" caused by not dithering...and not including darks....

Darks can help a little, but it can't solve the slow drift in your mount that aligns all of your noise into streaks...

"Dither or Die"


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#3 Jim Waters

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Posted 25 September 2020 - 08:57 PM

Simple solution.  Dither or die..

 

I Dither ~5 star diameters.

 

https://astrobackyar...trophotography/


Edited by Jim Waters, 25 September 2020 - 08:58 PM.

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#4 dansawyer

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Posted 25 September 2020 - 09:24 PM

Walking Noise and Dithering are totally new. The photos were taken roughly toward Zenith, but the overall sky is a Bortle ~7, a fair amount of skylight. When I examine the converted images there is no evidence of a background pattern. The photos were taken with a Skytracker Pro and there is no significant evidence of either poor polar alignment or tracking. 

Siril is supposed to have a dithering component. I will reprocess with darks and bias and try to find dithering.



#5 dansawyer

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Posted 25 September 2020 - 09:36 PM

Let me ask some followup questions:

Is this affected by the camera? By the sensor? 

Is this affected by the exposure duration?

Is this affected by the ISO? 

Is this affected by the stacking algorithm? 

Is this affected by sky back light? 



#6 dansawyer

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Posted 25 September 2020 - 09:46 PM

I found a good link:

 

https://www.dpreview.../thread/4159947

 

To quote Roger:

 

It is pattern noise.  The T2i is an old generation sensor without on-sensor dark current suppression, and with high pattern noise.  You need to do dark frame subtraction to reduce this problem, or better get a newer camera.

The directional nature is due to drift from frame to frame from less than ideal polar alignment, so the pattern drifts in one direction.  Dithering would help too: offset the frame a few pixels in random directions every few frames.

Roger

 

Am I reading this correctly that this is an artifact of the camera. I am using a Canon M1 with MagicLantern, a relatively old camera now. 



#7 17.5Dob

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Posted 25 September 2020 - 09:51 PM

Walking Noise and Dithering are totally new. The photos were taken roughly toward Zenith, but the overall sky is a Bortle ~7, a fair amount of skylight. When I examine the converted images there is no evidence of a background pattern. The photos were taken with a Skytracker Pro and there is no significant evidence of either poor polar alignment or tracking. 

Siril is supposed to have a dithering component. I will reprocess with darks and bias and try to find dithering.

 

 

Let me ask some followup questions:

Is this affected by the camera? By the sensor? 

Is this affected by the exposure duration?

Is this affected by the ISO? 

Is this affected by the stacking algorithm? 

Is this affected by sky back light? 

The SkyTracker has the absolute worst tracking capabilities of any camera tracker there is...If you blink to your first exposure and your last exposure, how much field shift do you see...

It has nothing to with your camera..or sensor or ISO ....it's your mount continuously drifting over time

Adding in darks and using Kappa Sigma stacking can help "a little", but without being able to dither, which physically takes place between frames while you are shooting,  you will always run into this problem. Older Canon cameras are notorious for this...


Edited by 17.5Dob, 25 September 2020 - 09:53 PM.


#8 SilverLitz

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Posted 25 September 2020 - 10:29 PM

Dither and EXACTLY match the temp and ISO and exposure time of darks and lights.



#9 dansawyer

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Posted 25 September 2020 - 10:49 PM

Comment 1 to 17.5 Dob: The tracking period was 40 minutes and there was no appreciable offset between the first and last images. I was surprised at the quality of the tracking. Not only were there no observable star trails on individual frames; there was only minimal, on the order of 1% total drift across the entire set. 



#10 17.5Dob

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Posted 25 September 2020 - 11:18 PM

Comment 1 to 17.5 Dob: The tracking period was 40 minutes and there was no appreciable offset between the first and last images. I was surprised at the quality of the tracking. Not only were there no observable star trails on individual frames; there was only minimal, on the order of 1% total drift across the entire set. 

Blink your first image and compare it to your last..are they EXACTLY  the same.........no image shift.....


Edited by 17.5Dob, 25 September 2020 - 11:23 PM.



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