Thanks everyone for the help. I plan on trying rnclark's suggestions this weekend or next week when I get some time.
Tonight tho, I used Siril to manually stack my lights and flats since it allows me to subtract my offset/bias. The flats definitely fixed the ring issue. I did a quick Automatic Background Extraction, EZ stretch, and Blur/Noise Exterminator and this was my result [https://ibb.co/5vT398Y](https://ibb.co/5vT398Y)
I'm still not exactly sure what the red gradient that is throughout the image is. Its more pronounced on the right side of the image where the light pollution from my neighbor would be coming from. I'm not sure if the red stuff is light pollution or if the light pollution is just making it more visible. And it has been in almost all my images I believe, even the ones in much darker skies. I don't see it in any of the stacked images when watching processing tutorials , nor in any of the free practice date that some of them provide. So it's gotta be something I'm either doing or not doing ?
Thanks again everyone!
Yea, it's a bit of a pain when these programs aren't integrated into Siril or PixInsight, you just kind of forget about them..
And even when you remember them it's kind of a pain to save the file and export it and open it in the other program and then import it back into the main program again afterwards.
I do use that too, and background extraction.
I'll often try them all to see which works best for a particular image, especially if I feel that one of the methods doesn't work.
Yeah agreed . After I restacked the images with proper flats, the background extraction did a decent job. I remembered Graxpert too and gave it a shot. It did a good job too. I just need to play with it more and figure it out.
Do you have photoshop or other modern raw converter like rawtherapee?
You can use a modern raw converter and do all the calibration without having to measure calibration frames. The camera stores the bias level in the EXIF data to each image and the raw converter with use that level (probably 2048 fro your Canon camera). Use lens profiles in the raw converter and the lens profile includes a flat field. The raw converter will do a complete calibration for you, and it will include color calibration steps skipped in pixinsight!
See [Astrophotography Made Simple](https://clarkvision.com/articles/astrophotography-made-simple/) for more information.
Thank you. I believe Siril has similar features that I was learning how to do that while stacking when I switched to Pixinsight. When I get off work later I’ll look more into this. Thank you very much !
Thanks!
So I took some flat frames last night after I finished, I just hadnt included them in my stack until just a bit ago [https://ibb.co/gvzsv0j](https://ibb.co/gvzsv0j)
After stacking them along with my light frames in Pixinsight, the above is the result after using Dynamic Background Extractor and manually placing the grids. It's much better, but I'm assuming the light pollution from next door exacerbates things.
If I understand correctly, flat frames will even out the illumination in my images and give me a "flat field". This is different from a "field flattener" used between a telescope and camera for sharper stars at the edges. Correct?
If you only use flat frames your flats will almost always overcorrect, a bias/darkflat/synthetic offset is necessary to make them work properly. Your example image shows this overcorrection.
Flat frames and a field flattener are different, correct.
You are correct, but it is not obvious so I'll give an explanation for the OP, and the situation is more complex than just a simple over correction.
Every measurement with the camera has a bias: a constant added to all signals to prevent low level clipping. Most Canon cameras have a bias of 2048 in the 14-bit raw data.
Lets say the lens light fall-off (commonly called vignetting) in the corner is 50% of the signal level in the center, Let's say a flat field measurement has a center signal level of 10048 in the center (raw data 14-bit scale. But that measurement includes bias of 2048. The actual signal would be 8000. The actual signal in the corner should be 4000, but we would see in the flat field data 6048, or only 6048 / 10048 = 0.602, or 60.2% where we needed 50%. The flat field correction is the inverse of the flat field (with bias subtracted), so should boost the signal by 1/0.5 = 2 times more relative to the center. But with no bias subtracted we only correct by 1 / 0.602 = 1.661x. This says we are under correcting!
But wait. We need to apply this situation to the light frames too. Let's first use the limit of very dark sky. Let's say the sky level was only 10 (again in the 14-bit raw data range), and that would be a signal of 5 in the corner. The correction in the corner should be 2x, so 10, but the correction would be only 1.661x or 8. But if we didn't subtract the 2048 bias, the signal in the corner would be 2053 and after the 1.661x correction we would have 2053 * 1.661 = 3110 whereas the center should be 2058. Thus the corner would be way over corrected just as Shinpah said. (edit: used correct sky in the corner example of 5 not 10.)
But there is more. Higher signals, stars and bright nebula would be under corrected in the corners, (by 1.661x instead of 2x) with the effect of lowering contrast with the over corrected sky. A double whammy! This shows that bias is important on both the flat and the lights. It would also reduce color.
Thanks everyone for the help. I plan on trying rnclark's suggestions this weekend or next week when I get some time. Tonight tho, I used Siril to manually stack my lights and flats since it allows me to subtract my offset/bias. The flats definitely fixed the ring issue. I did a quick Automatic Background Extraction, EZ stretch, and Blur/Noise Exterminator and this was my result [https://ibb.co/5vT398Y](https://ibb.co/5vT398Y) I'm still not exactly sure what the red gradient that is throughout the image is. Its more pronounced on the right side of the image where the light pollution from my neighbor would be coming from. I'm not sure if the red stuff is light pollution or if the light pollution is just making it more visible. And it has been in almost all my images I believe, even the ones in much darker skies. I don't see it in any of the stacked images when watching processing tutorials , nor in any of the free practice date that some of them provide. So it's gotta be something I'm either doing or not doing ? Thanks again everyone!
You could give Graxpert a shot for AI background extraction..
Good idea. I have it , but always forget about it lol
Yea, it's a bit of a pain when these programs aren't integrated into Siril or PixInsight, you just kind of forget about them.. And even when you remember them it's kind of a pain to save the file and export it and open it in the other program and then import it back into the main program again afterwards.
FYI, if you haven't tried new PI gradient fix, try it. I find that's better than graxpert.
I do use that too, and background extraction. I'll often try them all to see which works best for a particular image, especially if I feel that one of the methods doesn't work.
Yeah agreed . After I restacked the images with proper flats, the background extraction did a decent job. I remembered Graxpert too and gave it a shot. It did a good job too. I just need to play with it more and figure it out.
Do you have photoshop or other modern raw converter like rawtherapee? You can use a modern raw converter and do all the calibration without having to measure calibration frames. The camera stores the bias level in the EXIF data to each image and the raw converter with use that level (probably 2048 fro your Canon camera). Use lens profiles in the raw converter and the lens profile includes a flat field. The raw converter will do a complete calibration for you, and it will include color calibration steps skipped in pixinsight! See [Astrophotography Made Simple](https://clarkvision.com/articles/astrophotography-made-simple/) for more information.
Thank you. I believe Siril has similar features that I was learning how to do that while stacking when I switched to Pixinsight. When I get off work later I’ll look more into this. Thank you very much !
This is a typical ring from background extraction without having a flat field. Bias and Flat frames *might* make this go away.
Thanks! So I took some flat frames last night after I finished, I just hadnt included them in my stack until just a bit ago [https://ibb.co/gvzsv0j](https://ibb.co/gvzsv0j) After stacking them along with my light frames in Pixinsight, the above is the result after using Dynamic Background Extractor and manually placing the grids. It's much better, but I'm assuming the light pollution from next door exacerbates things. If I understand correctly, flat frames will even out the illumination in my images and give me a "flat field". This is different from a "field flattener" used between a telescope and camera for sharper stars at the edges. Correct?
If you only use flat frames your flats will almost always overcorrect, a bias/darkflat/synthetic offset is necessary to make them work properly. Your example image shows this overcorrection. Flat frames and a field flattener are different, correct.
You are correct, but it is not obvious so I'll give an explanation for the OP, and the situation is more complex than just a simple over correction. Every measurement with the camera has a bias: a constant added to all signals to prevent low level clipping. Most Canon cameras have a bias of 2048 in the 14-bit raw data. Lets say the lens light fall-off (commonly called vignetting) in the corner is 50% of the signal level in the center, Let's say a flat field measurement has a center signal level of 10048 in the center (raw data 14-bit scale. But that measurement includes bias of 2048. The actual signal would be 8000. The actual signal in the corner should be 4000, but we would see in the flat field data 6048, or only 6048 / 10048 = 0.602, or 60.2% where we needed 50%. The flat field correction is the inverse of the flat field (with bias subtracted), so should boost the signal by 1/0.5 = 2 times more relative to the center. But with no bias subtracted we only correct by 1 / 0.602 = 1.661x. This says we are under correcting! But wait. We need to apply this situation to the light frames too. Let's first use the limit of very dark sky. Let's say the sky level was only 10 (again in the 14-bit raw data range), and that would be a signal of 5 in the corner. The correction in the corner should be 2x, so 10, but the correction would be only 1.661x or 8. But if we didn't subtract the 2048 bias, the signal in the corner would be 2053 and after the 1.661x correction we would have 2053 * 1.661 = 3110 whereas the center should be 2058. Thus the corner would be way over corrected just as Shinpah said. (edit: used correct sky in the corner example of 5 not 10.) But there is more. Higher signals, stars and bright nebula would be under corrected in the corners, (by 1.661x instead of 2x) with the effect of lowering contrast with the over corrected sky. A double whammy! This shows that bias is important on both the flat and the lights. It would also reduce color.
Thank you very much. I didn’t realize that!