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I notice that some nebula I shoot display multiple colors, including red, orange, blue and green, like these: https://www.jerrygerber.com/photography/IC1396.jpg https://www.jerrygerber.com/photography/SH-132.jpg https://www.jerrygerber.com/photography/SH2-296.jpg Other nebula only display red and orange: https://www.jerrygerber.com/photography/NGC7380.jpg https://www.jerrygerber.com/photography/IC5070.jpg It doesn't seem to be dependent upon which filters I use as both the Optolong L-Enhance and the L-Ultimate display different colors for different nebula. When I shot this nebula a few weeks ago, I didn't use any filter and got these colors: https://www.jerrygerber.com/photography/IC1848.jpg My question is why do some nebula seem to have a greater variety of color and some do not? I obviously do not shoot in SHO as I am using an OSC. Jerry |
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Jerry, Hopefully someone who has experience using the two filters that you use can chime in on your question. But it is my understanding that these two filters are not just light pollution filters, but rather filters that allow one to create images that resemble narrow band images, but by using OSC cameras with increased efficiency vs. using OSC camera using single band pass narrow band filters. I think what colors you assign to the signals is really up to you. Find some instruction as to how to process these. Except for the first image in the list of links you provided, these look quite nice, with good emission light signals and definition of details and shadows. My guess is they will process well. The first image appears to have a lot of large-scale noise, making the image look pock-marked in areas. I would guess that these filters should not be generating greens as much as I see in your examples, but I have no idea how your are processing the images and how you are doing background correction, etc. I am guessing that these two filters are not really designed to generate a "natural" color image, given their reduced bands and bandwiths. I will assume that in that sense, they will really ruin the natural color balance of the stars, which are basically black body broadband emitters of light. However, for some emission nebulae, they might serve well, since the emitted light of the nebula probably falls within the sweet spot of the given two bands. Again, someone with experience will probably resolve that question. If what you desire is a natural color image, then I believe using a IR/UV cut filter (if you are at a dark sky area), or a broader band light pollution filter, if in a higher Bortle setting. I use an L-Pro Optolong filter as my IR/UV cut in all cases with my OSC camera, since it generally yields images that look natural and when doing SPCC on the post stacked image, it is generally so close to natural that I can hardly tell that SPCC actually made any changes in the color balance. (Yes it does, but relatively slight!) Best, Alan |
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Jerry Gerber: Probably because something is wrong. |
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andrea tasselli:Jerry Gerber: Care to elaborate? |
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andrea tasselli:Jerry Gerber: Well that's incredibly insightful and helpful. Well done! |
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Jerry Gerber: The variety of color does not exist in those nebulae themselves therefore it must come from processing. How you do it, I can't guess. |
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Hi Jerry, Emission nebula do vary in the amount of oxygen III emission (blue colour) they have relative to the dominant hydrogen alpha emission (red colour). The Sh 2-132 (Lion Nebula) does have a big patch of OIII that you have picked up (and you can see it in other people's photos). Because it is much dimmer than the Ha you will only bring it up with unlinked stretches (stretching the blue/green independently of the red). There are lots of ways of doing this and you can also combine the blue and green channels to create an OIII specific channel - Cuiv the Lazy Geek and many others have tutorials on this. The two filters you mention both pass light at these two frequencies and just differ in how narrow the bands they pass is (i.e., how much they remove other frequencies which will be light pollution). But essentially it is down to how much OIII there is in the nebula and how much you try to bring it up relative to the dominant red in processing. If you keep them with their same relative intensity you will only ever see red for most emission nebula (but some don't have much OIII so they may be red anyway). Very nice images by the way - going to give the Lion Nebula a go next time I get a clear night, inspired by yours. |
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andrea tasselli:Jerry Gerber: Andrea, this is a community where people share and learn from each other. I personally find very sad when people like you only (and often) answer to show disdain to say the least. My advice in cases like these is simple: close your socials, it will make more sense and you’ll be happier. As for the question originally asked I think it has been wonderfully answered by all, so I can only add that, from my experience, L-enhance produces a very notorious green shift (you see it mainly in the linked autostretches). I tend to process in unlinked mode and correct the green cast to my liking using SCNR to different degrees in the nebula and to full extent on the stars. But I don’t eliminate it completely in the nebula because green, for me, is important in order to play with the green curve which gives me the yellow nuances and also contributes to teal instead of pure blue. One thing I often do, too, as images are not well balanced with these filters, is I use a reference image for editing, which represents the colour goal I’m aiming for. I do background neutralizatiom. I open Curves and select the green curve and place three pips in equal distance from each other… and play with the midtone and highlight pips to equalize. That is how I balanced the color for my Helix nebula right off the L-enhance filter to kind of achieve the colors I saw on the Hubble image. |
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Álvaro Méndez:andrea tasselli:Jerry Gerber: *Well said! I wish I had chosen to answer with your eloquence rather than my sarcasm! And I belive you and the others are giving the correct explanation. |
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Jerry Gerber: Jerry, Have you imaged the same nebulas with the two different filters and do you use Pixinsight to calibrate the color? What is being said above is true in that different nebulas have different mixes of elements and thus, different emission colors. However, using PI's photometric color calibration, with the proper pull-down for either the L-enhance or L-ultimate should show approximately the same colors for the same nebulas. Also, by using the Optolong multi-band filters you could consider that you are effectively imaging in SHO. Regards, Charlie |
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Jerry I think I can add something to this discussion, I do use the narrow band filters L-eNhance and L-eXtreme)for nebula, so I do have some experience. As mentioned already, with these filters you are for the most part getting just OIII (L-eNhance contaminates this with a little Hb, which will mimic the Ha signal, but generally relatively faint). So areas rich in OIII only should appear teal (a balance of green and blue) and Ha dominant areas red. In the nebula that you show with no obvious teal color, but white or orange etc there is OIII but in the same location it is not as strong as the Ha in that area, so you get a shade that is not quite red. If there is no OIII the whole nebula will be red. Changing filters changes the bandwidths and also changes the efficiency of the filters. For example, OIII signal is right where red and green filters overlap in the OSC camera. But generally green is picking the signal up with more efficiency than blue pixels, and there are two green to one blue, so green tends to be stronger. To get a nice teal color for strong OII regions green generally needs to be de-emphasized. But when you change filters you will change the balance of color between green and blue in OIII regions and between the green-blue with the Ha, since you change how much light each filter is taking in. These filters vary a lot, so trying to use the same color adjustments to get the same color with different filters just is not going to work. You will need to change your color calibration when you change your filter to get the same result--if that is what you want. More about color choice below. But remember, if you don't know how much red from Ha is in an area of the image that also has OIII you don't a priori know what color the nebula should be. This makes it difficult to calibrate color. In fact, one can argue that correct color in NB is ill-defined. Since our eyes don't see with NB filters, what color is right?--our eyes are very sensitive to OIII actually, but not very sensitive at all to Ha. In either case the NB filter in no way really mimics what we would see. Calibration works for stars, because they are of defined color, as long as we use RGB filters only. Even for stars, with NB calibration can only get close to true color with some channel mixing, but even then it won't be exactly right. So for NB nebula, and especially in SHO where there is even more color choice, you do have a lot of choice on how you want it to look. In the Wizard for example you can bring out blue and green with the right stretches, or you can leave it with no obvious blue or green as you have. One might argue that the latter is more natural, but with a different filter than you used that blue/green might show up naturally. One final thing, NB filters do let in continuum light in the blue/green and red, the narrow the filter the less of this of course. So you may be picking up more than just OIII and Ha from the nebula. And you will be picking up some light pollution with these filters too. This can change the color. To get rid of the continuum contribution, and just get the OIII/Ha, you can subtract the continuum signal by subracting some fraction of a comparable RGB image. Not necessarily easy to do, and beyond this thread to explain (and I have not done it), so I will not say more about it. Hope this helps Rick |
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| It is a good idea to do a color calibration first but if you want to take the blues more visible you are best removing the stars after calibration (so they stay the right colour) and then add them back in after stretching the starless nebula (and doing the sharpening). |
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Jerry, it very much depends on how you process the OSC image. It looks like you are processing in the same way as you would a wideband image, so you will get what you get and colours will be dependent on the relative proportions of Ha and Oiii in the nebula. An alternative approach that you might want to experiment with is to create an Ha image and an Oiii image from your data and process them as a narrowband image. You can split the integrated image into its R, G and B constituents, optionally after extracting the stars. With the filters you have, Ha = Red and Oiii can be constructed as (0.7 * Green + 0.3 * Blue) (for example). You can linear fit Oiii to Ha. You can then run your noise processing on the Ha an Oiii and optionally on a luminance created from the original before splitting (or use Ha as luminance). You can then stretch Ha, Oiii and L (if using) to similar levels and combine back to a colour image using one of various techniques. My preference is HOO using a Power of Inverted Pixels equation in PixelMath which accentuates difference in relative concentrations of the gases:// R:Ha// G  (Oiii*Ha)^~(Oiii*Ha))*Ha + ~((Oiii*Ha)^~(Oiii*Ha))*Oiii// B iiiAn LRGB combination puts it back together again (the L image gets extra attention to bringing out detail and sharpness)You can then adjust the colour mix to your preference with a combination of Curves, ColorSaturation, ColorMask, etc. (if using PixInsight) Hope this helps... Roger |
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Andrew Rambaut: Thanks Andrew for explaining that so well. That makes sense. I definitely like bringing out the OIII if it's there in the first place, but not too much. Color in DSO, for aesthetic rather than scientific purposes is really interesting, too little and the object looks faded and a bit dreary, but too much and it looks unnatural and contrived. It's like salt in food! I'm of course still learning how to make color work so that the star colors are varied and natural and the DSO has a good balance of hydrogen, sulfur and oxygen. |
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Just to defend Andrea Tasseli (this is not about comments to this particular image): I find Andrea's comments very usefull and insightfull. I hope he is not discuraged to continue commenting. It happens to everyone who is contributing that comments dont suite someone. |
