NGC2359: Thor's Helmet Revealed

[Russ.Carpenter]

I’m not sure that anyone in astro-land will appreciate this, but I just posted a different approach to narrowband processing. Here it is: https://www.astrobin.com/vgosdd/

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NGC 2359: Thor’s Helmet Revealed

This unusual and highly detailed interpretation of Thor’s Helmet was inspired by the writing of Astronomer Travis Rector, author of Coloring the Universe and related scholarly articles. Professor Rector argues that processors of emission nebulas should try to accurately differentiate among a nebula’s ionized gases, while at the same time being mindful of color design theory.

In the past, I’ve been imitating other astro imagers who have been generally pursuing “pretty pictures” with the Hubble palette, or the realistic palette. But Rector’s writing convinced me that I should do some fresh thinking. I should think more carefully about the unique chemical composition of each narrowband object and choose a palette that highlights that object’s individual characteristics. In addition I should apply the basic principles of color design.

To my surprise, I discovered that the Hubble palette is often not the best choice. Most Hubble images are more or less a combination of orange and blue. This is known as a direct complementary color harmony and orange/blue is one of the most commonly used combinations in color design. For example, this is what the “golden hour” and “blue hour” in photography are all about.

In the context of astro processing there might be two drawbacks in using the orange/blue pairing. First, it is seen so often is our hobby that it might have become a stereotype. Second, it omits the hues centered around green, which means that it cannot convey a full range of information about the chemical composition of an emission nebula. It would be better to use triadic complementary colors, rather than a simple pair.

This is an on-going experiment, but at the moment I’m using these steps for three channel narrowband projects:

Step 1. Open the three narrowband masters in PixInsight and carefully study and compare the signals in all three channels.

Step 2. Generate images of all six of the narrowband palette options by using a process container in PixInsight that automatically creates six channel combinations, and then tile the images across the PixInsight window.

Step 3. Choose the channel combination that best shows the chemical composition of the object, while attempting to observe basic principles of color design.

Step 4. Process the image, using that palette, in StarTools. Do not modify the hue of any part of the object of interest.

Step 5. Use Affinity Photo to add the color key, copyright notice, final tweaks, and creation of various file formats. Do not modify the hue of any part of the object of interest.

In the case of NGC 2359, I choose the HOS palette. The resulting hues are close to a triadic complementary color harmony of yellow-green, blue-violet, and red-orange (using the terminology of my color wheel). The color key translates hues into Ha, OIII, SII and mixtures thereof.

Tech Notes for ASA 500/3.6:
ASA Newtonian, 500 mm aperture, 1900mm focal length, F3.6
FLI Proline 16803, 9 μm pixel, 4096 X 4096
ASA DDM85 equatorial mount
Processing with PixInsight, StarTools, and Affinity Photo

[Mike in Rancho]

Neat stuff, Russ!

There's a small handful right now, and I try to do so as well, who kind of push back against the pervasive anti-green sentiment in narrowband. And for me, same with magenta stars. So lately I've seen a few more SHO images leaving in areas of green to indicate the Ha, though when done in PI the stars usually still end up bleached out. I try to push back against that too lol.

From your description am I to understand you used no NB matrix in ST, or RGB bias adjustments? No Wipe either (since I think that can alter a color floor in the channels)? It sounds like you kept each channel to a particular base RGB color, and chose from among the possible permutations.

There's actually another thread going right now where I have an open question regarding how ST's NB matrices work. Or maybe more accurately, how I need to think about it to understand it properly.

Pending that, however, based on Ivo's earlier answer I'm moderately certain that the supplied mappings still generate a tricolor image with each emission filter assigned particular hues based on color wheel perception. I would need to look again, but I'm not sure if "full 100% RGB swaps" (for HSO, OHS, etc.) are included, such as you did here, and would thus need to be done (and remembered) in composition.

I really like your key though! And that somewhat dovetails with the question I have in the other thread. It probably isn't realistically possible to implement (?), but it would be pretty cool if the color module had a narrowband key of sorts -- little square color swatches or something that would show us exactly what hue now represents say SII, Ha, and OIII, as we choose from among the mapping schemes. And perhaps even adjust the bias sliders and saturation effects? That last part might be too far.

But that aside, I imagine keeping 100% RGB channels made it a lot easier for you to create the key you added to the image. All base colors and simple blends, kind of reminds me of computers from way back in the day.

[Russ.Carpenter]

Hi Mike in Rancho,

As always, your thoughts are really interesting.

I did not use the matrix. Instead I loaded the narrowband images into their appropriate channels in Compose, and did not modify the hues in the Color module. I did use Wipe--Hope that didn't introduce unanticipated hue shifts in the object of interest.

You are right about the color key. It's easy to create and understand if the RGB channels are kept "pure."

I'm glad you brought up the topic of green. After decades of RGB processing, I suspect that many astro-imagers have trained their brains to hate green. That might make sense in the RGB world, where there are very few "natural" sources of green, but it's irrational in the narrowband world. The Hubble palette made that prejudice much worse. By shifting greens hues toward red and gold, the Nasa employees needlessly reduced sampling of the color space and convinced thousands of astro imagers that green in narrowband images was a no-no.

Russ

[Mike in Rancho]

Hmmm. Did NASA set the trend, or just follow it? The original Pillars of Creation is quite strongly green, with very magenta stars.

I am trying to find where Wipe is further discussed in detail (nuts and bolts), but can't find it. I know from some discussions though that it is part of the reason that, as Ivo says, "Wipe is not optional." Even if one were to use the NB preset or otherwise zero out all the gradient removal effects. And likewise is the reason one must not white balance or neutralize the channels prior to opening in ST with center option.

It may have to do with sensing a color cast. The question is whether Wipe just takes note of and remembers that sort of thing for later use in Color, or actually lops off the bottom of a channel, kind of like "set black point to zero." And if so, how does that affect any channel exposure times that were intentionally set in compose for proper creation of luminance.

That being said, if such a thing wasn't done, I wonder if certain filter channels might be so overwhelmingly dominant, either in the target or even just the skyfog, that you'd never get a decent image.

Sorry for filling your thread up with pondering, Russ!

[Stefan B]

Hi Russ,

very interesting thoughts and image, thanks for sharing! Unfortunately, I can't find a copy of "Coloring the Universe" anywhere in stock (or at least to an affordable price...). Would love to have a look.

Step 3. Choose the channel combination that best shows the chemical composition of the object, while attempting to observe basic principles of color design.

Could you elaborate on this one? What are your criteria for showing the chemical composition? Should the dominant gas be presented in green since human eyes are most sensitive to green? And how to choose the remaining to channels then?

I do not have a mono camera so I am not able to use your advice. But I am interested in your thoughts on this!

By the way, my impression is that SHO images now tend to show more and more green again, although not as much as in the original pillars of creation image. I think this gives images a nice kind of glow.

Regards
Stefan

[Russ.Carpenter]

Hi Stefan,

Please excuse this tardy reply. This approach to narrowband processing is experimental, so all my thoughts are subject to revision!

I've attached an example of a palette comparison produced in PixInsight. These images are just simple channel combinations with temporary screen stretches--they are primitive, but useful. The comparison includes all possible channel combinations for a three channel narrowband image. I look for a palette that clearly differentiates among the three channels. Whether that palette also observes the rules of color design is actually a more difficult issue. I know the rules now, but feel that I'm only beginning to learn to apply them in practical situations.

Russ

[Stefan B]

Hi Russ,

thanks for your reply! I think I understand what you mean. So in your comparisons there is a good separation in OSH, HSO and HOS in my eyes. With the other three mappings the gases are a bit harder to discern, especially the green/teal ones. Is that what you mean?

It definitely makes sense to use such an approach instead of only going for the most common or appealing look. It makes it easier to recognize the nature of nebulae and additionally it's a nice change from everyday AP

Thanks!

Regards
Stefan