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Hello Astrobin friends, Perhaps someone could give me some advise on my problem below:  The image on the left is integration of 138 subframes, the only processing that I did before DBE was RGB Channels Split / Linear Fit to bring the B and G channels to the R level. The image on the right is the result of DBE process (in the middle) - it does not look that bad but still has some noise/gradient which I cannot get rid of by putting more background samples or playing with the Model Parameters. The remaining noise/gradient prevents me from further processing. Hopefully someone knows a solution. Many thanks in advance! P.S. I think the gradient ha something to do with my STC light pollution filter, this is the second time I have used it and had this issue again. Andriy. |
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A few comments : - it seems you have way too much samples : don’t forget to try to keep your gradient « simple », it shouldn’t be too convoluted. I usually use between 5 and 8 samples per row. And therefore you should also keep the « continuity order » parameter of the model as low as possible, like 2. Except if you image with lots of streetlamps right next to you and have the moon right above you, and you image for very long time with these sources rotating as your mount turns... otherwise, the gradient shouldn’t that complex... - all your samples are on the periphery of the image. Why ? You should avoid putting some on the nebula itself, but I see rooms where you can add some samples toward the middle of the frame as well. If they are « red » (ie. Pixinsight don’t take them into account to build the gradient that is subtracted), then increase again the tolerance until they are taken into account. - you should remove the gradient on each channel separately if you use a monochrome camera. Hope this helps ! |
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At first sight, quickly blurring the image (median should work better) your gradient should look like that |
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Hi Andriy, I have good experiences with applying ABE. I do apply it twice, first with division and secondly with subtraction. I suggest you give it a try. Cheers Gernot |
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Hi Andriy, It is a difficult image because you have a lot of nebulosity and you don't really know where you are placing the samples. I would at least start with fewer samples only in the areas that look very probably like being background and then apply an ABE too. But some RGB with lots of light pollution are very tough to model. |
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| Your DBE has too few sampling points. I normally end up with a much higher density and then I remove them when they overlap with object. Youmight have t repeat the process several times to get rid of severe gradients. |
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I can only reinforce what has been said above 1) DBE samples really don't sample the gradient. 2) In these circumstances, I have found multiple applications (subtraction + division) with ABE to work reasonably well. 3) But in the end, processing won't save you (particularly where there is extended and faint nebulosity). You need an optical system with a stable and reproducible flat field. Invest in that, rather than hours of image processing. Rule 1. Good data is everything Rule 2: See rule 1. |
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Many thanks everyone for the recommendations, very much appreciated. Will try every suggestion. Thanks again! Andriy. |
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Brian Boyle: Indeed. For this particular case I suspect the problem comes from bad field frames and not light pollution. My second guess is the use of Local Normalization. The author of PixInsight warns that LN is a double-edged sword. My advice to @Andriy Kozhanov is to use the simplest possible workflow in PI in order to find the source of his problem. How does a single calibrated light frame looks like? |
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Andriy, I think that there is something disturbing in the imaging train. Could it be that light is comming in somewhere? Or maybe dew. Did you change the filter position due to flats? What about the darks? Do you use a dew cap? I had the same issue, it was light coming through a gap in the filter wheel. By the way, if raw data is so much bad like you showed here, you can not expect that DBE can solve this problem. BR, Christoph |
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The image in the left doesn't look right to me. There is not just a gradient. There is something else. It may be correctable, or maybe not. If this is a normal image without any unusual troubles, the way you did DBE should work OK. It's not perfect, but it shouldn't be that bad. So there is something else in the image. For this particular image, I think you can first run a pass of ABE, with very large deviation parameter (>2) and function degree no larger than 2. This will suppress the global gradient. It will not remove the gradient (and whatever else in the image) cleanly, but this will give DBE a much better starting point. After this, the image background should look roughly flat. Then you can launch DBE. You can use DEB's automatic sample generation to evenly spread many sample points over the image, such as 40 or even 60 samples per row. Then you manually delete the sample points on nebulas, bright stars, or anything you believe to be real positive features on the sky. For this particular image, I think you should end up with deleting 10% to 20% of the sample points. If you delete much more than that, DBE won't be able to accurately model whatever imperfection in this image. Hope this helps. |
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Jérémie: Many thanks, @Jérémie. Have tried your advise and it seems to have helped a bit but the issue remains. Here is what I've got so far:  Also tried to put slightly more and slightly less samples across the field (except obvious nebulae areas) with no noticeable improvement. Will probably give it one more try and abandon. It was a great DBE exercise though, many thanks for your help. Most likely the root cause had been in the imaging process, not sure yet what it was, I suspect it was due to either my clip-in light pollution filter or probably it was not properly fixed to the camera. Will do some experiments with it too. There was nothing like streetlamps or the Moon nearby. Best regards, Andriy. |
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| Like I said, your sample points are too sparse. You will need 10x more sample points to get any hope of removing that very irregular background. |
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Hi Andriy, I can suggest you two additional procedures to deal with this kind of gradients. 1. Use DBE with fixed samples. You can fix the values of the samples in DBE and this will give full control over the correction. The procedure involves to place a matrix of samples (as suggested, not too many due to two reasons, one, to produce a smooth mode, and two, because is difficult and tedious to tweak every value of every sample) and fix their values to a constant number, I use to use 0.50000. Then choose subtraction as a correction method and make an iterative procedure in which you change the value of one sample, check the result, and refine, until you achive the desired result. 2. Use Multiscale Gradient Correction from PTeam This procedure has been developed by the PTeam and published in his website: https://pixinsight.com/tutorials/multiscale-gradient-correction/index.html The procedure needs the user to capture a wider FOV image in order to work. Maybe you have a wider capture from previous projects, but if not, you can use other sources from other users, the only restriction here would be that the wider FOV image shouldn't be too much processed. If you have any friend that has a wider image of this area and could share with you the master lights without postprocessing, would be ideal. In my opinion, this second method is the best gradient reduction technique currently avaliable. It is awesome how well it works. Hope it helps. |
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| You can try raising the tolerance to 4 and lowering the shadows relaxing to 2 also. And apply DBE first before any kind of linear fit or color calibration. |
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Gernot Schreider: Hi Gernot, many thanks, I've tried your advise too, did not help much, unfortunately. Do you recommend applying division and then subtraction with the same Model Parameters and samples size/distribution or do you change them before subtraction? I've tried both options though. Andriy. |
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Hi @Andriy Kozhanov Can you share your xisf file with us through Dropbox, WeTransfer or anything so that we can have a look ? Can’t do it right now (going to work), but I can have a look this afternoon and make some tests in PI. Have a nice day |
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Jérémie: Hi @Jérémie, yes, absolutely, here it is https://www.dropbox.com/s/02spmjlhudxjwae/IC405%20IC410%20integration-crop-colorslinearfit.xisf?dl=0 Really appreciate your help, but please don't spend much on this. Andriy. |
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Andriy Kozhanov: Don't worry : this won't be wasted time. I am definitely not a master as some others on the forum, so any opportunity to find a problem and try to solve it helps me progress as well  I 'll check that later today and let you know if I am luckyiest than you in removing this gradient. |
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This is what I can achieve in about 15 minutes. Left: your original middle: after a pass of ABE with function degree = 1, and a slight crop. right: after DBE. You can take a look at the parameters I used. You can achieve better by carefully removing the sample points and by adding fixed sample points. (But you need to first understand what all these mean.) As I said before, you really need to use very dense sampling to see any hope of removing the very irregular background. The background in your image does not just contain a simple gradient. There is something else that's quite wrong. Something isn't right in either your imaging, or calibration and stacking.  |
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Wei-Hao Wang: Indeed that's the way to do it. |
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Hi @Wei-Hao Wang, this is really great, many thanks for your help & advise! Will try it right now. Yes, you are absolutely right, the root cause was in the imaging process, it had to do something with my clip-in light pollution filter (or it was not properly fixed) - will do some more experiments later. Here is what I got with Optolong L-Extreme another day, the rest was exactly the same, except exposure 2 min with the LP filter vs. 5 min with L-Extreme: - equipment - pre-processing steps followed by colors linear fit (no ABE/DBE was applied to the image below):  This new image is plagued with other defects such as walking noise and a flats issue but that is a separate story. No such irregular gradient though. Many thanks for your help again. Andriy. |
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Hi Andriy, in this difficult case you have to use DBE as already suggested by @Wei-Hao Wang . I would nevertheless recommend to apply before a double pass of ABE. 1. dynamic crop - remove border from stacking process, important to eliminate artifacts, which would influence the process. 2. ABE w box size 15, box separation 20, correction=division 3. ABE w same but subtraction 4. DBE with tolerance 2.5 to ensure complete coverage of picture, sample radius=15, samples per row=7, correction=subtraction I do think you do not need as many samples as suggeted by @Wei-Hao Wang . In fact you should try to use the least number, but ensure that the whole picture is covered. This results in  Thanks for sharing this case. it is always good to have real life examples to learn from. Cheers Gernot |
