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Statue of Liberty Nebula - NGC 3576 / 3603 with plenty of SII

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#1 AaronH

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Posted 18 April 2024 - 09:28 PM

It's April already, yet I've managed to produce only my second image for the year (and my second SHO image overall). It's been incessantly cloudy here this year, with very few breaks in the clouds for imaging, so the Antlia 4.5nm SHO filters I received for Christmas haven't seen much use. I managed to give the filters their first light on the Carina nebula a month ago to test them out (and to realise just how tricky SHO processing can be), but that's been the extent of my imaging for the year.

 

Finally the clouds broke, and as is so often the case, it happened right as full moon rolled around. While I usually avoid imaging under moonlight, I had some mods to test on my 8" Newt, so I figured I could give it a go, and at worst have some useful "test data".

 

My usual process when imaging is to run the subframes through PixInsight's SubframeSelector in batches as they roll in, to assess how the data is shaping up. If it's looking dire, I can just pack up before bed and avoid getting up early to tear down the imaging gear. As the subframes rolled in, I was surprised by how good this partly-cloudy, moonlit night was shaping up. My new Antlia SHO filters were doing a decent job of cutting through the moonglow, and the seeing was exceptional. I'm usually pleased if I can capture a half-dozen subframes around the zenith with a FWHM under 1.8". However, PixInsight's SubframeSelector reported a consistent FWHM (Moffat4) below that figure, and in the end my data set had a median FWHM of 1.7-1.9" for the various SHO channels. My best subframes were around 1.3".

 

I was excited to process this data, as SHO processing is still something of a novelty, so I decided to throw every tool I could find at the task. NBColourMapper, NarrowbandNormalization, GHS stretch, Arcsinh stretch, the works. The result was terrible. I compared this over-processed mess to the simple STF stretches in PixInsight, and greatly preferred the simplicity of a basic MTF stretch. So I decided to see what could be achieved using MTF stretching alone.

 

I applied BlurXTerminator to each channel, performed a masked noise reduction to get rid of excessive grain in the darker regions, applied GraXpert to get rid of the gradients (particularly in the moonlit OIII data), then simply applied a HistogramTransformation to each channel. I matched the darkest regions, avoiding clipping, then matched the midtones around the brighter area in the middle of the "Statue". Then it was a case of simply doing a standard SHO combination with a very small amount of blending between channels to soften the colour transitions. I could immediately see the result was what I was after. A few more iterations to fine-tune the MTF stretch for each channel to get a good colour balance, and the image was shaping up.

 

This did, however, highlight some issues. The filters had performed beyond expectations, but my OIII had still been impacted by the moonlight, had a residual gradient, and the SNR wasn't the greatest. Meanwhile the SII data was quite weak. So I went back and collected more subframes, with a focus on SII and OIII. The stretching and combination process was repeated, and I was satisfied with the results.

 

To finish off the image, I applied HDRMultiscaleTransform to tame the highlights (while not going overboard - I still wanted the bright areas to be bright, just not clipped). Some noise reduction with NoiseXTerminator, a gentle UnsharpMask to the highlights, some final stretching and curves adjustments to add a bit of drama, then final star recombination (using RGB data for colour).

 

Of note, no SCNR was needed, because the green had been balanced via the stretching process. Instead of "taming the green", I just brought up the other levels to match it. This is possible due to the prevalence of all three SHO bands in this target, and the resulting image highlights the interesting SII regions across the frame. I'm happy how the subtle swirls of SII in the background "popped" in the final image, particularly the starless revision.

 

GSO 8" f/5 Imaging Newtonian, ASI2600MM Pro, Antlia Edge 4.5nm SHO filters.

23h integration. 89.5% average moon phase.

 

SHO_RGBStars_1200px.jpg

[Astrobin]

 

I've also uploaded the 2x Drizzled version of the data. I think it holds up decently, even with the ambitious 0.39" pixel scale.

 

I went to the trouble of also preparing a "Traditional Version" of the image that doesn't use BlurXTerminator or NoiseXTerminator. This was partly to prove to myself that I hadn't pushed the processing too far with the AI tools, but in reality also because I've noticed occasional scepticism and cries of fakery at the very mention of AI tools. If the thought of NoiseXTerminator gives you a migraine, and the mention of BlurXTerminator makes your eye twitch uncontrollably, this version is for you.

 

The traditional version uses regularlized Richardson-Lucy deconvolution, along with a first-pass of Mure denoise, followed by EZ Denoise (TGV and MMT). Mure was particularly impressive with this data, the Ha channel was basically clean after being run through it, and the other two channels only needed the slightest applications of EZ Denoise. I'd like to call this version an AI-free version, but I can't. I still used StarXTerminator for generating ringing supports and for isolating the stars for stretching and applying RGB colours. Life is too short for manually crafting star masks. I started down that road, remembered how tedious it was, and decided to just use the best tool for the job instead.


Edited by AaronH, 18 April 2024 - 09:59 PM.

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#2 Look at the sky 101

Look at the sky 101

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Posted 18 April 2024 - 09:39 PM

wow


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#3 DanMiller

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Posted 18 April 2024 - 10:10 PM

wow

I think that says it all.  Just wow.  

 

Dan


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#4 DanMiller

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Posted 18 April 2024 - 10:13 PM

Sigh, I just went through Stellarium to see if that nebula ever came up in my area of the world.  Nope.

 

Beautiful data and great processing.  Very nicely done.  You should print and hang that.  Seriously.

 

Dan



#5 AaronH

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Posted 18 April 2024 - 10:38 PM

Sigh, I just went through Stellarium to see if that nebula ever came up in my area of the world.  Nope.

 

Beautiful data and great processing.  Very nicely done.  You should print and hang that.  Seriously.

Thanks Dan!

 

I didn't realise how well it is positioned for my latitude until I spotted it while panning around in Aladin. I'd seen images of this nebula before, but my subconscious instinctively determined: "Statue of Liberty, must be a Northern Hemisphere target". Instead, it's positioned right between the Carina nebula and the Running Chicken.

 

That region of the sky is full of interesting targets. It almost makes up for us missing out on M31.


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#6 DanMiller

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Posted 18 April 2024 - 11:02 PM

Thanks Dan!

 

I didn't realise how well it is positioned for my latitude until I spotted it while panning around in Aladin. I'd seen images of this nebula before, but my subconscious instinctively determined: "Statue of Liberty, must be a Northern Hemisphere target". Instead, it's positioned right between the Carina nebula and the Running Chicken.

 

That region of the sky is full of interesting targets. It almost makes up for us missing out on M31.

Lol, at least I get to image the North American Nebula even if I cannot image the statue of liberty.  By the way.  The statue of liberty is much nicer than the North American Nebula.

 

Dan


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