Author Topic: narrowband challenge NGC6188 (Ha, OIII, SII)  (Read 8125 times)

Offline troypiggo

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narrowband challenge NGC6188 (Ha, OIII, SII)
« on: 2012 May 11 22:24:40 »
I recently posted my Hubble Palette image of NGC6188 here.  Common consensus, me included, is that it's a bit too green.  In some ways I expect it to be heavily green, because it's predominantly Ha I think.  But I agree these look better when there's some of the yellows, magentas, blues etc as well.

My workflow for the image I created was:

- Batch preprocessing script for image calibration and alignment, image integration was linear fit
- LRGB combination process with L unchecked, R=SII, G=Ha, B=OIII
- STF auto with the channels unlinked. I find this gives the quickest and closest view before non-linear stretching of what the final will look like after stretching.
- DBE. Very simple as I'm no expert in it. Just generated the sample as default and applied subtraction correction.
- This is the part I think was pretty cool. I did an auto STF again with channels unlinked because of the DBE. Then I zoomed in to tweak the mid-tone slider of the R and B channels to get the colour balance I liked. Then you drag the little blue process icon from the STF to the HistogramTransformation's bottom bar. What this does is copy the STF setting to the HistoTransf to apply. Then just apply the HistoTransf to the image. You'll probably need to reset the STF now.
- Created a lightness mask and applied subtle noise reduction using GreyCStoration
- Very light morphtransform (erosion) to control the stars and found it also boosted the overall image.

So, have at it.  I've posted the files to the Endor file server.  Be really interested in how you get to your final result, too.  I've posted my initial attempt here too for simplicity.

Go to Endor, Shared Elements, Shared Repositories, "NGC6188 public".  Log in as "public" user password is "pixinsight" I believe.
« Last Edit: 2012 May 13 06:57:51 by troypiggo »

Offline Lex

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Re: narrowband challenge NGC6188 (Ha, OIII, SII)
« Reply #1 on: 2012 May 12 00:53:55 »
Troy,

Thanks for that, I will give it a try this evening using the magic force of PixelMath :D
Let the force be with us  >:D
Clear Skies!!

Lex

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Offline Euan

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Re: narrowband challenge NGC6188 (Ha, OIII, SII)
« Reply #2 on: 2012 May 12 03:28:39 »
Says "file not found"...
Atik 383L+ Mono // Borg 101EDII @ F4.4 // Modded EQ6

http://www.photonjunkie.com

Offline troypiggo

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Re: narrowband challenge NGC6188 (Ha, OIII, SII)
« Reply #3 on: 2012 May 12 03:42:35 »
Hmm.  Sorry.  Not sure how this Endor filesharing works.  Leave it with me for a bit.

Offline Lex

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Re: narrowband challenge NGC6188 (Ha, OIII, SII)
« Reply #4 on: 2012 May 12 06:04:01 »
Again,

All the times I wanted to share or download somethink the links do not work...
There is really a problem with filesharing on Endor  :-\
Clear Skies!!

Lex

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HADSO (Hagen Deep Sky Observatory)20 km W of Luxemburg City

Offline troypiggo

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Re: narrowband challenge NGC6188 (Ha, OIII, SII)
« Reply #5 on: 2012 May 13 06:53:18 »
Ok, think it's sorted now.  I've followed Juan's recommended method and updated original post above.

Offline Silvercup

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Re: narrowband challenge NGC6188 (Ha, OIII, SII)
« Reply #6 on: 2012 May 13 11:44:14 »
Hi:

I Have reached this:



I blended channels with pixelmath, and applied a SCNR green correction with minimum neutral. As I dislike narrowband stars I mixed channels Ha-SII-OIII as RGB and sustitute narrowband stars with "narowband RGB" stars.     

Hope this helps.

Silvercup.

Offline troypiggo

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Re: narrowband challenge NGC6188 (Ha, OIII, SII)
« Reply #7 on: 2012 May 13 13:43:23 »
Thanks Silvercup!  Could you please elaborate the actual maths you used for both the channel combination and pseudo-RGB stars?  Seems everyone uses slightly different factors.  And interested in the actual technique of substituting the "RGB" stars into the false colour NB image.

PS - your image seems to be mirrored from the actual target and FITS files I posted.  Is there a reason for this?

Offline Lex

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Re: narrowband challenge NGC6188 (Ha, OIII, SII)
« Reply #8 on: 2012 May 13 14:53:06 »
Hi folks,

Here is my rendition:

I also used PM to combine the different channels with certain percentages, then BN and CCB. SCNR to green. ACDNR. HDRTMT.
HT stretched with settings from STF. Curves and finally a little Dark structure enhancement.

One of my main goals were lifting out the shock waves of NGC6164+6165 @ the bottom right 

Hope you like it  >:D

Thanks for sharing and letting us fiddle around with your nice data; calming my mind in this cloudy period of the year  :sad:

BTW @ Silvercup: How did you to post a big picture? I am always just able to post tiny pictures cause the limitaion is at about 500kb, i prefer your larger "preview"!


« Last Edit: 2012 May 13 15:16:01 by Lex »
Clear Skies!!

Lex

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AZEQ6 GT, TS UNC 10" f5, ASI1600mm-c

HADSO (Hagen Deep Sky Observatory)20 km W of Luxemburg City

Offline troypiggo

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Re: narrowband challenge NGC6188 (Ha, OIII, SII)
« Reply #9 on: 2012 May 13 16:14:30 »
Nice work, Lex!  Thanks for showing me that it is possible to get the blues and yellows, not just overwhelming greens.  What I'm really interested in is how you got that colour balance.  I'm having trouble figuring out if it's my initial blending of the 3 filter images into the channels, or is it in the stretching afterwards.

My understanding of the Hubble palette is that it's 100% of each filter R=SII, G=Ha, B=OIII.  That's what I've been doing, then using the STF/HT stretch from there.  If I stretch the R and B channel midpoints enough to get more yellows etc in the nebulosity, the background becomes very magenta.

The trouble seems to be the strong Ha (obviously).  It's histogram is much "fatter" than the R and B.  So getting the peaks of each channel's histogram to line up is hard.  Unless I'm doing something wrong, which clearly I am or I'm missing something.

Could it be that it's a problem with my image capture philosophy?  I'm taking exactly the same length subs for each filter.  This image's were 8min subs with each channel.  Should the SII and OIII be longer?  Or does stretching take care of that?  I guess the cost of stretching vs longer subs is noise?

Offline troypiggo

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Re: narrowband challenge NGC6188 (Ha, OIII, SII)
« Reply #10 on: 2012 May 13 16:43:16 »
Aah, I hadn't been using SCNR on green. That certainly seems to take out the fatness of the Ha histogram. Maybe that's what I was missing.

Offline Josh Lake

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Re: narrowband challenge NGC6188 (Ha, OIII, SII)
« Reply #11 on: 2012 May 13 18:19:03 »
Fun data set, thanks Troy!

For my first attempt, I tried to split the difference between too green and *no* green. I found that SCNR can be used very delicately to strip away some of the green without losing it completely.

Unlike Lex, I pretty much ignored the shockwaves in the bottom right and focused on the bright/dark dust interaction in the center left.

My basic steps:
  • PixelMath with the green channel as: H*.8+S*.1+O*.1 (S for R and O for B as usual)
  • ABE, no real changes to defaults, Division
  • Histogram Trans
  • SCNR, amount = 0.55, Preserve Lightness = True
  • LRGB combination with H channel as Luminance -- the 'Vicent trick' as I call it. Saturation down to 0.310, forgot to check Chrominance Noise Reduction apparently, oops.
  • With new LRGB result, another subtle SCNR, 0.2 with preserve lightness = false
  • Several rounds of Curves (reducing green at one point with Curves) and Local Histogram Equalization to bring out the dust

I may have overcooked it a bit, I just kept pushing with the main glowing area in mind. I may revisit this later and back off a bit to keep it more subtle and soft.

In terms of composition, I might try a crop that cuts out 6164 and rotates 90 degrees to draw attention to the 'bridge'.


Offline troypiggo

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Re: narrowband challenge NGC6188 (Ha, OIII, SII)
« Reply #12 on: 2012 May 13 20:38:13 »
Thanks again for your input, guys.  I've had a bit of a play with it again myself, using SCNR to tame the green.  Seems to have gone overboard and now it's almost a bicolour image.  Not sure what to think of this.


Offline Silvercup

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Re: narrowband challenge NGC6188 (Ha, OIII, SII)
« Reply #13 on: 2012 May 14 00:23:26 »
Hi Troy:

Yes, you killed green channel. Try to apply SCNR with minimum neutral and a small amount, I did it with linear image and  with Auto STF unlinked channels, so I can see the result of SCNR.

Then I transfered STF to Histogram Transf. Usually after that you only have to Backg. Neutralization.

You must respect channels, so in a Hubble Palette Image obviously must be green for Ha, Blue for OIII and Red for SII, but there must be torquoise where Ha and OIII overlap, and yellow where Ha and SII overlap, and so on.

Another thing is that you want to change color balance to enhance blue and red.

This is my rendition of rosette nebula NB+RGB for example:



Lex, just type the URL of your image between [ img] and [ /img] (wihout spaces).

Silvercup
« Last Edit: 2012 May 14 02:45:11 by Silvercup »

Offline Lex

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Re: narrowband challenge NGC6188 (Ha, OIII, SII)
« Reply #14 on: 2012 May 14 03:31:21 »
Silvercup,

ok; that's just the case when you have the images stocked on a server but not if you upload them to the forum..
I guess the small ones are the only way to be represented with upload??
Clear Skies!!

Lex

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AZEQ6 GT, TS UNC 10" f5, ASI1600mm-c

HADSO (Hagen Deep Sky Observatory)20 km W of Luxemburg City