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Old 28-08-2020, 09:04 AM
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New MultiLevel Processing Technique for Narrowband Astro

Hey folks,

I've spent some of this enforced Victorian stage 4 lockdown period learning a new narrowband processing technique from fellow astrophotographer, Eric Coles in the USA.

It involves adjusting the histogram in Photoshop to balance colours in selective areas. The process yields a much more pleasing balance of colours on hydrogen alpha dominant emission nebulae such as these pictured.

https://www.astrobin.com/full/3fsrjr/0/

https://www.astrobin.com/full/ac8fpp/0/

As Ha no longer dominates the field, there is more variation in colour. Probably not as accurate from a scientific viewpoint, but much more pleasing aesthetically!

For those interested in learning more about Eric's method Here is the Link

Let the games commence!
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Old 28-08-2020, 09:50 AM
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Very lovely, Andy.

I'll check out the link. I'm doing a repro of a narrow band mosaic so it is very timely.
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Old 28-08-2020, 01:25 PM
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always good to explore more processing technique options. thanks for sharing andy.

cheers
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Old 28-08-2020, 05:26 PM
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Sorry, not a fan of selective processing.

The end result is a total distortion/fabrication of the physical processes going on in the object.

I am very much an advocate of "respecting the light".

Non-linear transforms are absolutely fine, so long as they are applied globally and preserve the relativities of the object's intrinsic flux

With that level of "interpretation" you might as well get out the airbrush tool and draw the scene or use crayons...or whatever

No law against it....but this crosses too many lines for my liking.
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Old 28-08-2020, 06:05 PM
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Very interesting. I watched the video, its certainly in the "Art" realm, and he admits to that Peter. Not exactly groundbreaking PS processing, but this selective colour balance certainly goes further than most of us would try to get away with on AP pics. Interestingly, a question after his demonstration relating to NB filter colour allocation seemed to be misunderstood and it looked like he wasnt aware of channel colourising, he just allocates NB filters to RGB channels which is a bit basic given trends lately you would think he would be aware of. I like the result and has got me thinking. I have tried this sort of thing before with bad results but he uses a selection feather of 200 which seems obvious now and several different selection layers based on brightness I hadn't thought of. Ill be trying all this, looks interesting.I have trouble with colour balance in bright areas all the time, this seems to be the solution.
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Old 28-08-2020, 06:53 PM
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Originally Posted by Bassnut View Post
Very interesting. I watched the video, its certainly in the "Art" realm, and he admits to that Peter..
As indeed he does. Just not a course this horse wants to run around
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Old 29-08-2020, 07:08 AM
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Quote:
Originally Posted by Peter Ward View Post
Sorry, not a fan of selective processing.

The end result is a total distortion/fabrication of the physical processes going on in the object.

I am very much an advocate of "respecting the light".

Non-linear transforms are absolutely fine, so long as they are applied globally and preserve the relativities of the object's intrinsic flux

With that level of "interpretation" you might as well get out the airbrush tool and draw the scene or use crayons...or whatever

No law against it....but this crosses too many lines for my liking.

Have to agree there on all counts.


My personal taste is to show what is really there as accurately as is possible. One of the many reasons I instantly reject a starless image or a grossly vivid distortion of ionised gas/plasma colours. Whilst the eye can't detect narrow band particularly well without filtering and processing, I truly believe we should be showing it as correctly as we can, without taking artistic license.



Hockney vs Ansel Adams - both distorted reality to one extent or the other, but I for sure appreciate Adams' artistic license (contrast embellishment etc) over a Hockney techni-colour expression
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Old 29-08-2020, 07:24 AM
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Thank you Andy for sharing that.

I personally don't mind creative impulses to drive the colour palette in backyard amateur narrowband hopefully-pretty images. I do however dislike any harm done to luminance channel that results in false structural detail.
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Old 29-08-2020, 08:58 AM
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That's quite interesting. My first reaction was close to Peter's, though I'm perhaps a bit more liberal here...

After thinking about it a bit more, it might not be as "bad" as I initially thought.

Who here has a problem with "balancing the histogram" of an SHO image as a whole? Most of us do that, looks pretty **** if you don't (imo).

So now, if I took a photograph of a smaller area, say just the tarantula itself, what would I do? I would probably try to balance that histogram, or at least that would probably be my starting point. So in that sense, whether I selectively balanced the histogram of that region in a larger image, or whether I took an image of just that region and balanced the histogram, the result in that area is basically the same.

The more I think on it, the more I feel a bit ambivalent about this one... I'm less opposed to it than I originally was, but still...

Maybe it'll become part of my standard workflow :p
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Old 30-08-2020, 01:07 PM
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Hi folks, good to have a robust discussion about this technique - having been in touch with Eric himself, with his permission, I thought I'd share Eric's own views on this...


"Let's say you are imaging a target like The Rosette Nebula in NB, one of my favorites. And you start with an AP 130 GT scope and an SBIG STXL 11002 CCD camera. You will get the entire nebula in the FOV. You accumulate the data and after some processing finally adjust/balance the final histogram "globally." That give you one result. Now you image the Rosette with an AP Honders with a 16200 camera, with a smaller FOV. You don't get the entire nebula. Now in the end you adjust the final histogram globally and you get a slightly different color scheme. It's similar, but now the same. Finally you image the nebula with an RCOS 16", you get a much smaller FOV, you adjust the final image globally and you get a noticeably different color scheme. So which one is "correct."

You night say that by using different scopes and cameras you have selected different area of the nebula and have violated some rule of imaging. You should have gotten the same result, but different because balancing the histogram of different areas gives you different results. This is not a theoretically example. Over the years I have done exactly what is described in the previous paragraph with the three scopes. And I kind of wondered why the results were so different, until I realized that they should be different because I was balancing different regions of the nebula with different contributions of HOS emissions. Of course they should be different.

And that is the point of revised processes. In effect you are selecting different areas of the nebula with different contributions of the three emissions. And you absolutely should expect the adjustment process to yield varied results. If they didn't it would be shocking.

One more point. In most cases if we just mapped the HOS filters to SHO=RGB, they would be green. Then isn't that the most "natural" way to view the final mapped image? I see people do that and their image look fine, but they miss some of the variation in the structure of the nebula. So what most people do is try to "balance" the histograms to get the pale green to deep orange images we all enjoy. You have taken a big step away from the natural image into a representation of the nebula and a piece of art. What law says you should not take a more subtle approach like I suggest?

Of course the answer is none. In the end all our images are pieces of art and should please our eyes. And if we can add in a little science along the way, that's just great. But that's just my opinion, I could be wrong.

Stay safe.

Eric Coles, Ph.D."

So there you have it!

Cheers

Andy
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Old 30-08-2020, 03:39 PM
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I disagree with suggesting you could look at just part of an object with a narrow field instrument and balance the colours in an arbitrary fashion at that location, but in a wider field you would use a different histogram and that's all OK

Doing so distorts the intrinsic colour spectrum and amount of flux/light emitted from parts of the object.

It may make for a pretty picture, which is fine, but tells us nothing about the relative physical processes going on. The fact we arbitrarily assign SHO colours is license enough...but locally vary the intensity? Nup.

I was at Wangi Falls in NT a few years back...and took some snaps of the falls. There was too much green, so I re-balanced the histogram....

As I said, no law against it.
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Old 30-08-2020, 04:29 PM
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Definitely the one on the left Peter



It must be all the red bull-dust in the NT air...
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Old 30-08-2020, 06:11 PM
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Got a bit of a field curvature there Peter

Wangi falls - a very pretty part of this planet - and whata great illustration / analogy of the ever dominating green in SHO palette
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Old 31-08-2020, 09:17 AM
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Quote:
Originally Posted by Peter Ward View Post
Sorry, not a fan of selective processing.

The end result is a total distortion/fabrication of the physical processes going on in the object.

I am very much an advocate of "respecting the light".

Non-linear transforms are absolutely fine, so long as they are applied globally and preserve the relativities of the object's intrinsic flux

With that level of "interpretation" you might as well get out the airbrush tool and draw the scene or use crayons...or whatever

No law against it....but this crosses too many lines for my liking.
Agree, almost completely. Enhancing the faint (low signal) areas in your data can require selective luminosity processing to compress the dynamic range. However, these are not arbitrary selections.

The video describes completely arbitrary selections within all parts of an image and then modifying the colour balances and luminosities within each selection independently. Aaaaargh! You may as well get out the oil paints!!

I completely reject the comparison to shooting at different focal lengths. Assuming same exposure details, if you visually compare the raw channels of narrow and cropped wide field data, they will appear identical! Moreover, histograms should never be your ultimate frame of reference for processing. The relativities and tones within the raw channel data should be the source of your processing strategy.

Finally, aligning histograms in extended objects, i.e. covering most of a frame, is wrong headed. By all means adjust black and white points to remove "no data" parts of the frame and remove colour casts, but try not to mess with the histograms after that.

Last edited by marc4darkskies; 31-08-2020 at 09:56 AM.
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