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DeepSkyStacker => black dots in the final stacked image?! :-(

Luc Coiffier DeepSkyStacker (DSS) Processing Stacking Deep Sky NGC7635
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Jan Klíma avatar
Jan Klíma avatar
Hi guys!

2 last nights I spend with shooting Bubble nebula => when I stack images in DSS from both nights separately, everything seems to be OK after stretching in Photoshop...

But when I try to stack images from both night sessions, I see ugly black dots in the final stacked image (after minor stretching in PS).

I had the same setup both nights.

Please - any advice? What could be wrong? (I have attached a printscreen BlackDots.jpg)

I dont use calibration frames in general.

Regards
Jan
David Moore avatar
David Moore avatar
Sorry I can't see any dots in your image.
wsg avatar
wsg avatar
Your image is almost 600% zoomed in and no black dots are visible in the image.  Can you show a full frame of the image?
Ok in a darkroom with full screen I can see the tiny dot pattern.  Is this pattern visible in a non zoomed image with normal processing? The point being why worry about it if you can only see it at 600% zoom in a dark room.
That being said it does not look like dark noise or a sensor matrix pattern and It does not look like it would be related to calibration in any way that I have seen. Hopefully someone with this experience will chime in.
jewzaam avatar
jewzaam avatar
I see the dots and it's interesting the pattern being consistent like that. Any processing done to this before posting?  This is something I _might_ expect with using some star extraction tools, especially Starnet++.  Also you mention not using calibration frames and I just want to note you probably should as there's a good reason for darks and flats even with cooled astronomy cameras.  I don't believe they'd be a reason for this pattern though.
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Rodrigo Roesch avatar
Rodrigo Roesch avatar
I have seen this type of artifact when using non cooled cameras in a warm night and lack of dithering.
dkamen avatar
dkamen avatar
How are you stacking? Drizzle with kapa-sigma clipping?
kuechlew avatar
kuechlew avatar
The dots are quite obvious but we need more information:
- What types of calibration frames do you use?
- How do you take your flat frames? (e.g. through a t-shirt which could have some structure in its fabric ?)
- Are you using the very same Raw compression mode (which one?) and Raw bit depth (which one?) for all your images and calibration frames?
- Did you "blink" through all your images - calibrated and uncalibrated - to check if any of them exhibits this pattern?

In the worst case providing your data and calibration frames may be helpful.

Clear skies
Wolfgang
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Jacob Heppell avatar
Jacob Heppell avatar
Looks like a classic case of walking noise from not dithering.
Try a few different stacking algorithms and compare. Most of the time I use auto adaptive weighted average (AAWA).
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dkamen avatar
dkamen avatar
The dots are quite obvious but we need more information:
- What types of calibration frames do you use?
- How do you take your flat frames? (e.g. through a t-shirt which could have some structure in its fabric ?)
- Are you using the very same Raw compression mode (which one?) and Raw bit depth (which one?) for all your images and calibration frames?
- Did you "blink" through all your images - calibrated and uncalibrated - to check if any of them exhibits this pattern?

In the worst case providing your data and calibration frames may be helpful.

Clear skies
Wolfgang

He did mention using no calibration. This makes me suspect the stacking algorithm rejects warm pixels in half the subs (they would be placed slightly differently each night) leading to an underexposed stack in those places.
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kuechlew avatar
kuechlew avatar
The dots are quite obvious but we need more information:
- What types of calibration frames do you use?
- How do you take your flat frames? (e.g. through a t-shirt which could have some structure in its fabric ?)
- Are you using the very same Raw compression mode (which one?) and Raw bit depth (which one?) for all your images and calibration frames?
- Did you "blink" through all your images - calibrated and uncalibrated - to check if any of them exhibits this pattern?

In the worst case providing your data and calibration frames may be helpful.

Clear skies
Wolfgang

He did mention using no calibration. This makes me suspect the stacking algorithm rejects warm pixels in half the subs (they would be placed slightly differently each night) leading to an underexposed stack in those places.

Sorry, I had overlooked this sentence. In this case the cure is straightforward. There is a reason everyone is using calibration frames and all sorts of issues can occur if you don't use them.

Clear skies
Wolfgang
Jan Klíma avatar
Jan Klíma avatar
Good morning Guys! Solved the problem!

It was some stacking (registration) bug in DSS! I stack always the same way and never had problem like this, but this time this fail occured.

I tried to stack these images in Astro Pixel Procesor and this "carpet pattern" is completely gone!

Thx a lot to all of you!! 

Best RegardsJan
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