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For a full LRGB session, what percentage of the total session time are you giving to each filter? A recent video by Sky Story suggested 25% distributed evenly between L,R and G with the balance of the time dedicated to L. As an example, if you had a 5 hour session to work with, each of the R,G and B filters would be given 25min, L would be 225min. Of course repeated for the umber of nights needed. Are these numbers reasonable or are you doing something very different?
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1:1:1:1 - The Golden Ratio. Anything else is NONONO. Besides, aren't you in B8? If it is then no LRGB for you.
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3 or 4 L to 1 RGB is what I usually do.
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I've read 1:1:1:1 as you start into mono processing because it makes things easier, as well I see experienced people keep to this ratio in their work. I've learned by trying and failing that LRGB is not any more effective than OSC in bortle 7/8. |
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andrea tasselli: Now you're just making me sad... Actually though, I've had some success shooting OSC under bortel 8 so why would LRGB be any different as long as I keep the sub exposures short? |
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Keep them short won't save you and why should it. LRGB is an exercise in frustration and disappointment in my B7 skies. I can't imagine doing it in B8. At least get a strong LP filter.
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Generally, I take 20 subs of R, G, B each, and that's enough for most of the targets. Then, L as much as I need. This is under Bortle 1. I do not think there is a recipe. Every target is different and after collecting some data, I stack and do quick processing to see what I'd need more. For reference: https://app.astrobin.com/u/OgetayKayali?i=zcwwdj#gallery https://app.astrobin.com/u/OgetayKayali?i=oumc67#gallery |
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People tend to use large ratios of L to RGB (meaning predominantly L) because they think this is an efficient way to go deep. And it is, if you don't really care about color. The best images I have seen here use ratios a lot closer to 1:1:1:1. Remember that L contains NO color information, and the LRGB combination process is simply converting from RGB to L*, a*, b*, then replacement of the L* with a version of captured luminance, and then reconversion to RGB. In effect, this is taking the L and splitting it in some way between the R, G, and B channels using a mathematical algorithm. If the original RGB channels are short integrations and noisy, the resulting image will not have good color information because the a* and b* are not changed meaningfully in the conversion. |
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Arun H: I think this is the critical part here Arun, and that's why there will be no recipe. Because in darker skies, this allows higher L to RGB ratios, while for less dark you need more RGB. Also LRGB ratio is very target-specific. For some targets, color is well defined and exposing more will give you so little. I'm quite obsessed with the real color of the targets, and I often check other top images before publishing. Some happened to show very different colors, and when I exposed even more, I only realized this is due to processing. Because people wanted to emphasize different colors by creating color contrast, some did masking, which totally overruled the impact from the LRGB ratio. I saw some IOTD with high RGB amounts with wrong colors on some parts of the nebulosity (where there was absolutely no such color at all anyway). Unless one is going after true colors, very careful about processing, and don't have access to dark skies, I do not think one needs 1:1:1:1. Any empirical evidence can change my mind. So far, my tests and comparing to others have not proven to me that 1:1:1:1 makes a difference for me. But again this is for Bortle 1, I can get Ha from R. |
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Kay Ogetay: I generally agree. I'm under the darkest sky in the world so your results may vary. I take a LOT of RGB and about twice as much (per channel) of Lum data. Remember that RGB drives color noise and Lum drives spatial noise. Color noise tends to be less objectionable than spatial noise so it's easy to deal with in processing. Lum data also has a higher SNR than any of the RGB channels so 1:1:1:1 will also work just fine. As Kay says, "I don't think that there is a recipe". Do what works best under your sky conditions. John |
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Kay Ogetay:Arun H: Hello Kay, While I agree there is no magic formula, here is one example of an image with a L:R:G:B ratio close to 1 for each filter, and taken from a dark site: https://astrob.in/t3jq6j/ At least to my eyes, the colors are rich in all parts of the image. On the question of target specificity - I would say it also depends on what part of a target. There can be huge ranges in brightness in a single target. So, a bright target might, on the surface, allow a ratio like 3:1:1:1, but inspection of the fainter parts might show problems with color fidelity that are not obvious at first glance. Also - the absolute value of the integration time in the RGB channels probably matters more than some ratio. Point being - once you get to a certain threshold in RGB time, a 1:1:1:1 may not give meaningfully different results than 3:1:1:1. |
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@Arun H , thank you for the kind pointer to my image above.  As @John Hayes called out above, the right thing to do is whatever is ideal for your case. What I have found for my specific case, imaging from a remote observatory with deep field and moderately fast systems (e.g. CDK14, CDK24, f/4.8 to f/6.5), is that the ratio of LRGB subs depends on the following factors: 1. Sky darkness 2. The target 3. The system, particularly focal ratio 4. The camera sensor type 5. Filter types Expanding on the above: 1. No use shooting tons of luminance from a Bortle 6+ site. Maximize RGB and create a synthetic lum instead. Even from a dark site, 2:1:1:1 is as far as I would go. Anything more, particularly with the synthetic super lum route has yielded no value in my case for the majority of targets. 2. Where L appears to matter the most is galaxies. For most other types of broadband targets, anything more than 1.5:1:1:1 hasn’t added much value. The brighter the target, the lesser the value of L. 3. Faster focal ratios appear to make capturing more amounts of lum unnecessary in my experience (e.g. CDK14 with a reducer compared to native), given the above criteria #2. RGB captures so much spatial information in addition to chromatic information at faster focal ratios that 1:1:1:1 or 1.5:1:1:1 seemed plenty for many targets. Again a synthetic super lum is the way to go. 4. Believe it or not, a lot of the “collect gobs of luminance” is still a hangover from the CCD days. And that is coming from someone who is actually setting up a good ol’ PL 16803 with his new CDK20 at Obstech. For any modern IMX4xx CMOS sensor, a synthetic super lum with anything more than 2:1:1:1 has seemed to be wasteful in my experience.5. This is a hypothesis on which I don’t have data, but hope to soon with my CDK20 setup referenced above, is that our current RGB filters are holding us back by losing a bunch of data. My hope is that creating synthetic lum from a flat-top Johnson-Cousins BVR filter set (like the one Chroma sells), and spending the time one would with a traditional L filter instead on the B, V, R filters will make L irrelevant with non-galactic targets. Probably not with a CCD sensor, but likely so with CMOS sensors. |
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For me (10" newtonian, Bortle 6/7 skies) RGB is as much as needed, L is as much as I can. I should add that I shoot almost exclusively galactic fields. Typically it amounts to 60 to 80 min for each RGB filter and 8 to 12 hours of luminance data. I need these deep luminance data sets to capture features like faint tidal arms and to bring out as much details as possible, given that deconvolution needs good SNR to be efficient. |
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Ani Shastry: Hi Ani, All this is in line with my thinking as well. In the end, it is about getting sufficient signal to get good enough color information. It is quicker to do this both with fast focal ratio scopes and from a dark site. That is why, in those cases, higher ratios of L to RGB work. I am going to venture to say that people that are imaging from dark sites and using these high ratios of L to RGB may be able to get away with smaller time devoted to L than they currently do. From my B6 site, I simply gave up on L. My reason for this has nothing to do with the discussion above but simply that gradients were much easier to manage with RGB and flat correction was much more reliable. This is much less an issue from dark sites. Anyhow, just focusing on RGB and forgetting about L, I am actually able to get decent RGB images from my backyard as long as I put in sufficient integration time. |
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Lots of interesting input here so thanks everyone for the help. It just looks like a lot of testing is the order of the day. Funny enough that fits with the weather Oklahoma has been under for the last few months. It's very humid and clouds are persistent. It's rare to get a full night that is already very short. Crappy for seriously imaging but perfect for testing.
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If you shoot 1:1:1:1, you can create a super lum (by integrating all 4 channels) that has more signal than the RGB, but on it's own, isn't a 1:1:1:1 shot Lum equivalent signal to the RGB? Maybe even a little less if you consider RGB filter overlap….. Seems like spending that Lum time getting more RGB would be better color and equivalent to the 1:1:1:1 super lum. Is there something else the Lum provides? That said, I shoot Lum not to increase signal, but to capitalize on good (well….better) seeing without compromising signal. Cheers, Scott |
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I think the best thing to do is to do some tests. The worst results I got were with the ratio 1:1:1:1 but I admit that if I shot from a desert it would probably be the best combination. However from a polluted sky the experience suggested me to give more importance to the luminance and in the end my final ratio is almost always 3:1:1:1. I also read things like that the color can be done in BIN2 (and the luminance in BIN1) and even in this case I just wasted time. The only thing I play with is the length of the exposures, often my luminance in the single shot exceeds in length that of the RGB, for example luminance 300s and color 180s. But in the end the integrated time is always in the ratio 3:1:1:1. |
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I know all of this is target and conditions dependent but I wanted to float the idea of doing the L frame with no filter in place. That's how I got decent depth when shooting galaxies this year. Because I use a 585 sensor which has extended IR sensitivity the difference between that and using a uv/ir cut was very noticeable, about a stop. I'm wondering how that would effect overall color balance? The L frame clearly would go deeper so would that result in the very faintest areas being rendered as just grey, no color?
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If you are using a refractor, I'd assume you'd run into significant issues with halos on stars and such, depending on how well corrected the scope is.
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Oh yes, forgot to mention that I'm using a reflector with no coma corrector so the color correction isn't a problem. I don't think the technique would be applicable to most refractors. If you can do it, it's also helpful if you don't have perfectly dark skies as a lot of the IR pass area is outside of the usual pollution frequencies which really drop off at 700nm and longer.
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Tony Gondola:andrea tasselli: I've had some minor success with RGB L in Bortle 8+ using my Edge 14 (note that the "L" is an Optolong "L Pro) - I try for three or four times L to RGB, which although not suggested by leading Astrobin members (nor is RGB L in Bortle 8 in any case!), I think the L Pro has some positive affect when applied to the RGB. My M51 is probably the one that I'm most satisfied with. But in general, here in Bortle 8+, the RGB data is frustrating to process; lots of garbage to deal with which is why I usually lean heavily towards NB these days. |
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When it comes to single exposure times, you should of course have identical ones. I didnt read through, since im only having a coffee and then to work, but seeing all that 1:1:1:1 stuff when it comes to total integration time confuses me. Personally i do deviate strongly from that ratio. Depending on the brightness and color of what i'm imaging, and on the total integration time over all filters. I usually start with 1:1:1:1 or 2:1:1:1 (the first one being L), and after i captured enough RGB i might increase this ratio even beyond 5:1:1:1 |
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Tony Gondola:andrea tasselli: I shoot from a bortle 4/5, better than yours but not great. I use an IDAS P3 and keep it fixed in my optical train after the filter wheel. At first I used it only for luminance but then I tried it on RGB and I was thrilled with the result and left it. In pixinsight with the SPCC I have my calibration curve (IDAS P3 + Antilia LRGB) and everything works as it should. Since it is fixed it is also in use with Ha O3 and S2 but obviously it does not create any problems. |
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Michele Campini:Tony Gondola:andrea tasselli: Hi Michele Interesting idea to put two filters in series. Did you notice much lower signals (i.e. having much more time on data collection) after you added second one? I am also on bortle 5 and I am struggling with RGB acquisitions. Are any others here also using this solution to fight pollution? |
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Tony, I have the following saved in my Zettelkasten notes: - For Limited Imaging Time (2-4 hours total): Use traditional LRGB ratios like 2:1:1:1 or 3:1:1:1. The efficiency advantage is real and significant when noise dominates the images. I hope this to be useful. |
