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Frustrating times trying to process NGC6888 LRGB image files  

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(@vincent-mod)
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August 12, 2019 11:56  

Yes, it's clear something causes the data to be mixed in a wrong way. The extra filter you have in front wouldn't cause dramatic differences, but that's just guessing on my part as I don't know the specifics around that. Maybe it does cause Cygni to be green after calibration, or it's because not all frames are switched and you get some cross contamination in the signal.

The give-away in the data that something is wrong with either the filters or maybe even the assigning of the R,G,B flags to the filenames is the fact that the blue channel is so bright, it shouldn't be on this object. And when I switch it around it looks more normal, so the simplest explanation is that the data has the wrong assigned labels.

This post was modified 1 month ago by Vincent Groenewold - Moderator

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(@mestutters)
Red Giant Customer
Joined: 2 years ago
Posts: 59
August 12, 2019 15:36  

Hi Vincent,

I've taking the liberty of uploading all my Crescent Lights to your server. Sadly some of them are not of the highest quality owing to light clouds and as mentioned earlier I was caught out by the recent heatwave which raised overnight temperatures to >20°C, causing the camera cooler to struggle and leaving water-silk like patterns on some of the subs.   Credit to APP in that the integrations all look clean to my eye despite these problems

As you will notice the subs were captured over several nights but when I look at the subs taken with each filter they do all seem to be reasonably consistent in appearance.   This makes me think this is not some intermittent fault.

If the subs are being consistently labelled wrongly then I guess we should be able to reassign the channels and get a decent result. Sadly I have nothing else to use as a comparison to decide which are which.

It being almost full moon I won't be able to get further data for at least a while

I'm pleased you think the Neodymium filter is not the issue but I think I will take it out just to satisfy myself it is really not the culprit here.

Hoping we can pin this issue down soon

Best Regards

Mike


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(@vincent-mod)
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August 12, 2019 15:59  

Well I'm not sure it isn't causing issues, I would simply be surprised it would have this specific effect. As far as I understand a skyglow filter, it doesn't filter out R, G, B completely and when red comes through, having a red filter after that would work I think. It probably will diminish the signal a bit, but I wouldn't expect it to completely change the color, that makes no sense in my mind. I'm not 100% sure though if I'm missing something obvious here, but taking it out is always a good idea as having extra glass in between diminishes an already relatively dim signal. You will have less contrast if light pollution is high, but it depends then if you still can get more signal out of that soup and correcting the pollution with the LPC tool in APP or not.

Anyway, thanks for the subs, I'll have a look at them and see. If I simply do my conversion like posted above I'm guessing the result will be similar.


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(@mestutters)
Red Giant Customer
Joined: 2 years ago
Posts: 59
August 13, 2019 19:19  

Hi,

Just to let you know that by using the integrations produced with mode Equal and by using L(L+Ha), R(Ha), G and B in RGB Combine, I have managed to produce an image that is I think a considerable improvement.  I think there is perhaps a little residual LP that could be removed and I would like to improve the colour of the larger blue stars. Something to work on.

combine RGB image v11.1 lpc cbg csc St

I don't know if you or Mabula have had any further thoughts?

Regards

Mike


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(@vincent-mod)
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August 14, 2019 11:10  

No other thoughts than we already gave at the moment. Your new processing looks better, but is still more purple/pink than red. Also the stars lack color and are pink, this still points to switched labels. I'll have a go at your lights again, with my switching around scheme. 🙂


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(@vincent-mod)
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August 14, 2019 12:31  

So I did an entire integration with all your lights (only 1x1 bin to keep it simple), just using bias and with my scheme I mentioned a few posts back. After the combination, the yellow stars indeed look green so that to me indicates contamination of certain channels with others. However, it still gives the best starting point and after color calibration I get them to turn orange like expected and blue-ish for the blue stars. Tweaking the colors a tiny bit, adding some saturation, contrast and sharpness (all in APP) and it looks as good as I can get it with this data. Honestly, to me this proves the labels are somehow mixed/switched as using the normal labels I can't even get close to this result.

ps. I also decided not to use the luminance. It has almost no signal for the nebula. I have no experience in taking luminance though so maybe I'm mistaken here, but I still decided to leave it out as I didn't see it would add a lot.

combine RGB image SC mod St

This post was modified 1 month ago 5 times by Vincent Groenewold - Moderator

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(@vincent-mod)
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August 14, 2019 13:56  

Decided to stretch it a bit more carefully, did a double saturation on it (maybe a bit overdone) and saved again. Does pop out the blue better and the more careful stretching got the surrounding dust out more while trying not to clip the bright crescent.

combine RGB image SC mod St St

 


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(@mestutters)
Red Giant Customer
Joined: 2 years ago
Posts: 59
August 15, 2019 15:25  

Hi Vincent,

I took a good look at my previous image in natural light and see what you mean about the stars being pinkish. I've had another shot and the result is below.  Possibly a little too pink for your liking but as I was after catching the faint nebulosity in region around the Crescent I am now reasonably happy with the result.   Looking at Astrobin there are numerous interpretations of this object so I'm not sure which colour balance is definitively the most correct: nearly all the pictures awarded Image/Pick of the Day are made with Ha/O3 which yields a very different result anyway. Either way, having this dialogue with you has made me look more closely at what I was achieving.

I've just spotted Diego's video of Wolf Nebula so think I'm not alone in having some difficulties in colour balancing images with significant Ha backround content!

I'm still not convinced there is anything wrong with my filter wheel configuration but I'm thinking of setting up a long sequence in SGP and imaging a known colour test target so it should be very obvious if an error is occuring.  If I include a large number of filter changes then hopefully this will also show if I have an intermittent fault.

Regards

Mike

combine RGB image v14.1 lpc cbg csc St

 


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(@vincent-mod)
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August 15, 2019 15:43  

Well, in the end it's ofcourse what you like best yourself. 🙂 And it's true that in "reality" hydrogen has a more pinkish glow to it. But the stars should be orange/white/blue if you look at what their spectra look like visually and that's not the case in the processing done with the labels as they are. However, let's not keep on processing as that could take up a few pages more. In the end it's about your taste and what you're happy with and I'm happy you got some more insight from it either way!

This post was modified 1 month ago by Vincent Groenewold - Moderator

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