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Fake startrails, problems with flats?

Luc Coiffier DeepSkyStacker (DSS) Processing Calibration Deep Sky Vignetting
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Leonardo Henning avatar
Leonardo Henning avatar
Hello everyone,

I use DSS to stack my frames, but with or without flat frames distortions appears in the corners. This distortions ruins my image, because this made me to cut the edges and reduce the field. I'd like to know the reason this happens and how to fix it.

In this post I read that flat only fix up to 30% vignetting, maybe that's the reason? (I dont know how to measure vignetting level)

I'm using a Xiaomi Redmi Note 10s to take these photos

Here's my images with (first image) and without (second image) flats.




Thanks in advance
Die Launische Diva avatar
Die Launische Diva avatar
Hello Leonardo, and welcome!

This is not due to flats but due to the inability of DSS to register (align) very wide field images like yours. I had the same problems several years ago when I began the hobby. That was the main reason (among others) for me to move to another, more complete software, PixInsight. I would suggest you to try Siril, a free and very capable software. Give it a try!

This is a nice result for a mobile phone image.  Given your second image, if Siril succeeds aligning your images, you should have a nice image to begin with!

Clear Skies!
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Chris White- Overcast Observatory avatar
Chris White- Overcast Observatory avatar
As mentioned this looks like a registration issue.  There might be other causes though, not just the algorithm being used.  It looks like you have some field rotation.  Make sure your polar alignment is excellent.  This is especially true for very wide fields of view.  Additionally, you have astigmatism evident in the lower left and upper left, but I dont see it on the right side.  This could be due to some image train tilt.  As you get further off axis your stars are not as good (size and shape) so registration becomes more of a challenge.  Fixing the optical issues in your field may improve registration.  That said, I dont use DSS… so give another software a try and see if it does a better job.  You have the data so it would be a simple test.
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Leonardo Henning avatar
Leonardo Henning avatar
As mentioned this looks like a registration issue.  There might be other causes though, not just the algorithm being used.  It looks like you have some field rotation.  Make sure your polar alignment is excellent.  This is especially true for very wide fields of view.  Additionally, you have astigmatism evident in the lower left and upper left, but I dont see it on the right side.  This could be due to some image train tilt.  As you get further off axis your stars are not as good (size and shape) so registration becomes more of a challenge.  Fixing the optical issues in your field may improve registration.  That said, I dont use DSS... so give another software a try and see if it does a better job.  You have the data so it would be a simple test.

For this image I capture 25 minutes of exposure (50 frames with 30 seconds of exposure), but my tripod is untracked. Within 25 minutes of exposure do you think a realignment is necessary to solve these issues?​​
Leonardo Henning avatar
Leonardo Henning avatar
Die Launische Diva:
Hello Leonardo, and welcome!

This is not due to flats but due to the inability of DSS to register (align) very wide field images like yours. I had the same problems several years ago when I began the hobby. That was the main reason (among others) for me to move to another, more complete software, PixInsight. I would suggest you to try Siril, a free and very capable software. Give it a try!

This is a nice result for a mobile phone image.  Given your second image, if Siril succeeds aligning your images, you should have a nice image to begin with!

Clear Skies!

I will try this new software tonight and then I post here my results, thanks for the suggestion. I had already tried Sequator but this problem was worse on him.
Chris White- Overcast Observatory avatar
Chris White- Overcast Observatory avatar
Leonardo Henning:
As mentioned this looks like a registration issue.  There might be other causes though, not just the algorithm being used.  It looks like you have some field rotation.  Make sure your polar alignment is excellent.  This is especially true for very wide fields of view.  Additionally, you have astigmatism evident in the lower left and upper left, but I dont see it on the right side.  This could be due to some image train tilt.  As you get further off axis your stars are not as good (size and shape) so registration becomes more of a challenge.  Fixing the optical issues in your field may improve registration.  That said, I dont use DSS... so give another software a try and see if it does a better job.  You have the data so it would be a simple test.

For this image I capture 25 minutes of exposure (50 frames with 30 seconds of exposure), but my tripod is untracked. Within 25 minutes of exposure do you think a realignment is necessary to solve these issues?​​

If you are not well polar aligned it could definitely cause problems.  Its not about lining up on your target, field rotation causes the corner stars in an image to rotate around the center.   With very wide lenses this problem gets worse. It's not your only problem, but it might be contributing.  How do you polar align your mount?  Are you polar aligning your mount?
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Die Launische Diva avatar
Die Launische Diva avatar
@Chris White I think Leonardo uses just a mobile phone and a tripod 🙂
Chris White- Overcast Observatory avatar
Chris White- Overcast Observatory avatar
Die Launische Diva:
@Chris White I think Leonardo uses just a mobile phone and a tripod 🙂

Ah, ok.  I didnt realize that.  So yeah, there is a lot going on here that would complicate registration.  Your suggestion of trying a different program is probably the best.  Maybe a more robust algorithm could better correct the geometry challenges.
Leonardo Henning avatar
Leonardo Henning avatar
Die Launische Diva:
Hello Leonardo, and welcome!

This is not due to flats but due to the inability of DSS to register (align) very wide field images like yours. I had the same problems several years ago when I began the hobby. That was the main reason (among others) for me to move to another, more complete software, PixInsight. I would suggest you to try Siril, a free and very capable software. Give it a try!

This is a nice result for a mobile phone image.  Given your second image, if Siril succeeds aligning your images, you should have a nice image to begin with!

Clear Skies

I tried and I love it! Many thanks for the recommendation. Under light pollution conditions he gets worse than DSS, but the overall image is a way better. Some distortions remains in the corners, but with some cut I can remove it. Maybe with a better realignment I can remove it without cut my images.
Here's my image on Siril
Die Launische Diva avatar
Die Launische Diva avatar
Leonardo Henning:
I tried and I love it! Many thanks for the recommendation. Under light pollution conditions he gets worse than DSS, but the overall image is a way better. Some distortions remains in the corners, but with some cut I can remove it. Maybe with a better realignment I can remove it without cut my images.

Great image! I think you are on a good track!

Siril has a toolfor removing light pollution (gradients). As for realignment, it wouldn't hurt to try it, even for such a wide field camera like yours.
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