[RCC] NGC6888 - The Crescent Nebula - Forums

Topic starter

Jul 4, 2024 · Edited: Jul 4, 2024

NGC6888 - The Crescent Nebula in HOO (Amos B.) - AstroBin

I’d like some general feedback on this image (and to an extent, any other steps in the processing steps...) Typically, I follow very similar processing techniques across most images for most narrowband data I use. Usually, it goes something like this… And I hope I’m not missing much, but I probably am.

Blur X (correct only); Star align all images; Dynamic Crop; Blur X; Star X on all images; Script to combine narrowband stars to turn them into RGB stars, which works well enough; Stretch each channel and try to keep the histogram fairly balanced across each so one doesn’t completely wash out some of the other data. This is where I struggled a bit early on and it’s less of an issue now but there has been a time or two, I’d get some really weird crazy colors if one signal was far more prominent than the others.

Following that, it would normally move to channel combination & narrowband normalization, find out what works well for what I like, and then after that, masking/minor adjustments before recombining that and the star image. That’s the general workflow with a couple of variations if I feel like I need to. Rarely I run into weird gradients but on the rare occasion if I’m shooting something too close to the horizon, that’ll be included early in the post processing.

Where I would like feedback, specifically is how much image “stretching” do most consider too much? And in the same hand, when is an image, or even color channel, pushed too far? This example of NGC6888, the Crescent Nebula – I tried to ensure the Oiii was prominent, but feel I might have overdone it? On the other side of the coin, I enjoy seeing the Oiii data pop, and really let the fainter whisps shine through the powerful Ha.

It's all subjective at the end, and I’m extremely aware of that, but I’d like constructive criticism across any/all aspects. I’ve been delving into this hobby for a little over a year and would like to try to improve where I can. Thank you for taking the time to read the wall of text.

Jon Rista

Jul 5, 2024

I don't know what others think, but for me, you stretch the data as much as it can "handle" and not more. A good, deep, clean signal can handle a LOT of stretching, and often this can reveal fainter structures that most images do not expose. Not every integration can handle such a stretch though. If you stretch more than the data can handle, it'll be noisy, and depending on the quality of the data, that could look pretty bad (interpolated data will often show more mottling and worse mottling in the background than bayer drizzled or mono data.)

What do I mean by "handle" here? There is intrinsic noise, and the intrinsic SNR of your integration, before stretching. Then there is the APPARENT NOISE. This is the noise as it presents once stretched. Consider this:

Same data, different stretches. The second only appears to have more noise, because the faint background details were stretched more than the data could really handle.

The amount of noise an image has, and its final SNR, often has little to do with the SNR of the original signal, and a lot to do with just how you stretch. There are definite techniques to stretching, which is the conversion of a linear image into a non-linear state. You can stretch in many ways. Purely algorithmically with a variety of algorithms, or purely manually with curves, with a histogram tool, etc. Or some combination of both. You can finesse your skill with stretching, and MANAGE the various ranges of the signal, so that you can stretch those that can handle it, perhaps more than areas that cannot. Further, there are ways of stretching that can brighten data, without increasing contrast beyond the point the data can handle. Sometimes its enough to brighten, without increasing contrast (FWIW, most of the time, the basic tenant of a stretch is that you are VASTLY increasing the contrast of the signal, which initially is very low contrast, and very dark. By applying a curve, you pull up parts of that signal, increasing contrast first and foremost (making some parts brighter than others, i.e. ratio if bright vs. dark increases.)) Sometimes you may only need to increase the contrast of some parts of your image a little, while also increasing brightness (which, in comparison to increasing contrast, is more of just a shift in all the tones, vs. a change in the ratio of bright to dark.)

There is a lot of technique in stretching, and it can leverage many different stretching tools to get the job done. I usually use a histogram stretch for my basic initial stretching, then a combination of histogram, curves, and maybe some other tools such as local contrast enhancement, to finesse my stretched image.

If your data can handle it, then there is nothing wrong with a stronger stretch. Conversely, if stretching just makes parts of the image really noisy, then those parts probably shouldn't be stretched as much, and that may mean stretching less overall, or just getting more creative and "localized" in your stretching process. There is also nothing really wrong with making an image brighter, without necessarily increasing contrast much. In the case of the image you linked, it appears "brighter" than a lot of images of the same region. It also has improved contrast, but the region overall is brighter in general. I don't think there is anything wrong with that. The image does seem to lose a bit of the color contrasts in crescent itself, and the oxygen signal dominates the hydrogen, which is a little unusual. CERTAIN PARTS of the image might benefit from some increases in contrast, but the image overall looks good to my eye.

FWIW, I am viewing on a monitor that has a very wide gamut, and in its native color space mode, it has a tendency to greatly enhance the colors and sometimes contrasts of images of deep space. So, I may well be seeing a more contrasty image than you, and that is something to take into account. The screen I use, an ASUS ProArt very wide gamut screen, allows me to switch to AdobeRGB and sRGB screens, which helps me see what most people are likely seeing. The image definitely has less contrast in sRGB, the brightest areas of red are not as bright or vibrant, and the blue (which overpowers the red/Ha in the crescent nebula itself) is not quite as overpowering. It helps when processing to have a screen that allows you to switch between gamuts, and has a wide native gamut itself. You can see how a wider range of potential viewers will see your images.

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rhedden

Jul 6, 2024 · Edited: Jul 6, 2024

I'll add a few general comments, since I'm presently processing an image of the same subject, and I've spent way too much time studying past images of it.

Most people who examine your images critically are going to look for common technical flaws first.

1) Are the stars round in the corners (and throughout the image)? (Looks good to me.)

2) Are the stars processed well? I think some of the brighter stars in this image appear overexposed or over-stretched, so this image might get criticized a bit there. Star colors look attractive, though.

3) Overall color scheme / histogram - I think the colors are pleasing in this image, but be aware that some people hate intensely saturated reds. In the Crescent itself, the blue hues are overwhelming the ragged H-alpha structures that are buried in there. Looking at past IOTD and Top Picks for NGC 6888, most of them did a good job bringing out the H-alpha details in the Crescent.

4) Stretch - Without going into detail about my two monitors, I think the brightest points of the Crescent do give the impression of being too bright or overexposed. (I'm facing the same problem with my Crescent project; seems to be a processing challenge.)

5) Framing - Not the biggest deal to me, but some people religiously follow the "rule of thirds" or expect novel framing when the subject is a popular target. Just the fact that you centered the image on the Crescent will bother them and potentially lead to the Dismiss button being used (if submitted for Top Picks). These people will think,"Who needs another Crescent Nebula image centered right on the Crescent?" (I also centered my recent image right on it - so you know my opinion! :happy-1

6) Processing of secondary targets. Specific to NGC 6888: some people are going to judge this image by how good of a job you did capturing the "Soap Bubble" planetary nebula. The Crescent is bright and easy to capture, and there are many fine images of it. The Soap Bubble is not so easy if the goal is capture a deep impression of it. I had a hard time locating it in your image at first.

7) Noise reduction and sharpening: I think the image looks reasonable in both regards. No objections here.

Hope these comments are helpful in one way or another!

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Amos B.

Topic starter

Jul 6, 2024

Thank you both for offering extremely valuable & constructive feedback. It is one thing I did struggle with on the processing side of this image - the 'over' saturation of the Oiii, and to an extent the Ha as well.

When I was working with this imaging set in PI, I was surprised at how well things were coming along, but perhaps a less extreme stretch of the data will make the ha/oiii balance better. I will go through and give it another go, and take both posts of feedback and see if there's much improvement.

Thank you kindly.

However in regards to framing, it's something I will admit I struggle with. When it's an object such as this one, part of me of course, wants to mostly frame the crescent as the center. Though, to that end, if I can get the soap bubble to shine through a little better, I could see changing the frame of view with that in mind. Either way, it's good feedback and much appreciated.

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Jon Rista

Jul 7, 2024

Amos B.:
Thank you both for offering extremely valuable & constructive feedback. It is one thing I did struggle with on the processing side of this image - the 'over' saturation of the Oiii, and to an extent the Ha as well.

When I was working with this imaging set in PI, I was surprised at how well things were coming along, but perhaps a less extreme stretch of the data will make the ha/oiii balance better. I will go through and give it another go, and take both posts of feedback and see if there's much improvement.

Thank you kindly.

However in regards to framing, it's something I will admit I struggle with. When it's an object such as this one, part of me of course, wants to mostly frame the crescent as the center. Though, to that end, if I can get the soap bubble to shine through a little better, I could see changing the frame of view with that in mind. Either way, it's good feedback and much appreciated.

I think you could leave the OIII/Ha balance alone. You've brought out a number of other structures around the field that have notable OIII structure, which don't often show up in fields like this. Your stronger blue channel is bringing those things out.

Regarding framing...this might be a good field to do some mosaicing on. There are a number of interesting regions that are partly in the field, but not fully. Re-centering will push one or more of them out of the frame. Instead, maybe just acquire some additional areas that slightly overlap this field (by say 20% or so), to fill out a larger field, and show off those additional structures better. You have the curtain of Ha along the top edge, a notable hydrogen structure surrounded by oxygen gas in the lower right, and some notable filamentary OIII and Ha structure to the lower left. Shifting the field anywhere will crop out something. Expanding the field, however, could show off these other structures, which are not often shown off in images of this region (your field is actually quite large, which is probably who so many are captured here). Most of the time, just that Ha curtain to the top of Crescent is shown, but I'm honestly more interested in the other stuff, as its so seldom imaged!

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