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Hi everyone ! I had the chance to shoot for 4 hours with a dark bortle 4 sky this weekend. My target was the Pleiades. I already shot this target in october, but I had not an equatorial mount yet. That's why I gave it another shot. My main goal was to show more of the dark nebulosities surrounding the reflexion nebula. I am quite disapointed with how it turned out. There is definitely gaz around it, but it's not detailed at all. What do you think is wrong here ? Not enough integration time ? Bad processing ? Not adequate gear ?  M45 - The pleiades Specs : Camera : Canon EOS R6 stock Lense : Canon RF 70-200 F/2.8 at 200mm filter : CLS Astronomik Clip-Filter EOS R XL raw : 120 x 120s at 1600 ISO F/2.8 No DOF Processing : LinearFit on BG channel based on R channel to balance them Crop to remove stacking artifacts Photometric Color Calibration to get a good white balance Background Neutralization Dynamic Background Extraction to remove gradiant MultiScale Linear Transform to remove chromatic noise and luminance noise Stretch with ScreenTransferFunction LocalHistogramTransform to improve contrast on nebula Star de-emphasis using adam block technique to reduce star size Curves adjustement to increase saturation SCNR to remove greenish gradiant |
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I can't say it's a bad image given the equipment! So, a few things I'm noticing: - the image still a bit noisy. Have you been using any darks/bias/etc to deal with that? as you didn't list your darks and such here. Did you dither? Some of the nebula and dust is getting a bit lost in the noise and the star reduction. As most stars seem to have a bit of a dark ring or area around them if I start looking in really close, so perhaps look at the star reduction methods you have used. - speaking of the stars Star shapes could be a bit better, most are pointing left and up. Which could mean tracking issues. So perhaps you could look into your polar alignment and/or the exposure times. All the bright stars around Pleiades have been reduced and dimmed a lot, + all the bright yellow stars have lost all of their colour; making the star field seem a bit flat. No idea if that is a filter thing? or that the exposures at F2.8 were too long to get good colour in the stars. Because there is a lot of nice colour in the surrounding stars. so you could consider making separate short exposures for a star-layer and blending everything together. But; - What might be working against you too is the camera and lens that you're using. The camera has a pixel size of 6.56µm. Which is quite big.. which means that at 200mm you are significantly under sampling your image. Native Focal Length / F: 200mm / F2.8 Ideal Resolution : 0.67" - 2" arcseconds Resolution: 6.77" arcseconds per pixel Sampling: Significant Undersampling So all the little bits of detail in the stars and dust, are basically smaller than the pixels of the camera.. this limits how much detail and resolution you can get from an image because the pixels simply aren't small enough to capture those details.. You can "fix" that by using "Dithering" and "Drizzle". - And I believe you can use the app of the star adventurer to dither in there, IF you have your canon attached to the star adventurer with a shutter release cable. So if you're not doing that already, it's worth looking into. As it deals with noise too. It's definitely not a bad image though! |
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Thank you very much for you very detailed answer. As you guessed it, I did not use any dark, offset nor flats. I did not take time to shoot them as it was freezing cold and I was tired. I don't think I used dithering either. At least, I have set on my Star Adventurer app the dithering range to "0.5 arcmin". Not sure that is a correct value considering my focal length and pixel size. I indeed use my star adventurer to release the shutter using a cable and the app. Oh crap I never thought my 20 mpx would be an issue to get details, I thought that bigger sensors = bigger light buckets. Thanks for the information. How would dithering help with getting more thinner details ? I might have overdone star reduction, because they were really bloated in the first place. Mainly because of the aperture and tracking errors I guess. I don't know if the loss of colors on the stars are because of the stars reduction or the filter, as it's the first time I do star reduction, and use a filter. Thanks again for your feedback, really appreciate it ! |
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| I recollect that when I tried Adam Block’s Star reduction technique, it worked very well up to a certain point where I started getting dark rings around stars. I would monitor my progress with the technique so that I did not over do it. I would use it up to the specific point where the dark circles just began to appear so as to maximize its value. |
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I have also the 70-200L mkII and the 100-400L mkI and I have done some quick tests compared to using prime lenses and I think that using f/2.8 on the 70-200 @ 200mm doesn't help if you want details. The 100-400 mkI @ f/5.6 is basically unusable for astrophotography and while the 70-200 mkII seems quite a bit better, it is still not that good for astrophotography I think. If you look at the stars, they are not nicely round and show some coma: this blur is obviously also present in the nebulosities. I don't know if it is the main reason, but I would say that the 100-200L should be used at f/4 or even f/5.6 to get a decent result. |
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Didier Kobi: ... and the 100-200 mk I is catastrophic - at least my sample ... |
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As Didier already said, looking at star shape its evident some coma / distortion due to the lens at full aperture. it is also a zoom, so not the best for astro due to complex scheme and high count of lens elements. Try at first to reduce close the lens to f4 or f5.6. Evaluate it with short (30s) exposure, and you can evaluate the imrpoovements. For example my Zeiss Planar T 80mm is unusable @ f2.8 due to coma, but becomes excellent @ f4 |
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Thank you all for your replies. I will definitely try to close down the aperture to see if there's an improvement. But i'm afraid the stars will have this ugly diffraction I often see on dslr pictures. |
