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drizzle problem  

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(@jeroenm)
Main Sequence Star Customer
Joined: 4 years ago
Posts: 31
August 26, 2019 08:12  

Hi Mabula (@mabula-admin),

 

last week I started reprocessing the data of a 3-pane mosaic of the Horsehead Nebula and M42 I made last winter. Each pane is made up from RGB and Ha with a total integration time of ±16 hours per pane. Because the frames are undersampled and dithered I thought it was a good idea to use 2x drizzle during the integration: the stars were originally square, with the drizzle function they get circulair, which makes processing the stacks a lot easier. 

In just one of the panes (the one with M42; the other panes are fine) I got some strange artifacts near the border of the stack in all 4 channels when I use drizzle. This doesn't happen when I don't use drizzle. At first sight it looks like the original frames that make up the stack are dithered too well, but that's not the case.

 

In the screenshots you'll find a stack without using drizzle and a stack with 2x drizzle. Is there any setting causing this problem?

drizzle
normal

 


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(@minusman)
Neutron Star Customer
Joined: 4 years ago
Posts: 195

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(@jeroenm)
Main Sequence Star Customer
Joined: 4 years ago
Posts: 31
August 27, 2019 12:54  

@minusman

Thanks for your reply. I tried different settings in the integration tab and now I got a good result. Looks like the LNC settings and/or MBB setting are causing the problem. LNC level 1 with 3 iterations and a MBB setting of 8% was causing the "borders".  I decreased the MBB setting to 5% and didn't use LNC at all. Now the stacks are looking fine. In the screenshot the stack of the red data (top) compared to the stack with the artifacts (bottom).

Schermafbeelding 2019 08 27 om 12.49.38
drizzle

 

This post was modified 2 years ago by Jeroen

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(@minusman)
Neutron Star Customer
Joined: 4 years ago
Posts: 195
August 27, 2019 14:20  

Actually, it's not artifacts at all, it's dithering. This means that the edges of the pixels are not always served with imformation. That's why it looks so holey.
You need a lot of frames and it has to be dithered randomly to counteract the effect. My experience is at least 50-100 or more frames necessary to have an advantage over pixel interpolation.


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(@jeroenm)
Main Sequence Star Customer
Joined: 4 years ago
Posts: 31
August 27, 2019 14:29  

@minusman

Thanks for the answer. I certainly hoped to get a lot more data, but as usual the weather last winter was very bad here. I started this project (a 3 pane mosaic of the Horse Head Nebula up to M42 in R, G, B and Ha) early November last year. At the end of February when Orion was disappearing in the evening sky I almost used every single clear night to make this mosaic. I ended up with a total of 533 (good) light frames. Not nearly enough to use drizzle at my benefit. 


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