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  #1  
Old 15-09-2018, 02:15 PM
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Slawomir (Suavi)
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NGC 253 with RGB

Hi all,

Over the past few nights I managed to collect some RGB data and thought of putting the image together.

https://www.astrobin.com/full/366760/0/

I quite like the appearance of the galaxy, but I am not convinced with the stars. I think Luminance subs were a bit too long for my camera/scope combo (15 minutes) and I collected too much data too low above the horizon (I would normally start each session when the galaxy was around 30 degrees up).

All up got about 12 hours of Lum and about 2.5 hours per RGB channel.

Thank you for looking
Suavi
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  #2  
Old 15-09-2018, 02:31 PM
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vlazg (George)
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The galaxy is fantastic Suavi but the stars seem to be bigger in the rgb and the colour is slightly off, could this be a collimation problem ?

Red can be taken lower to the horizon but the blue should be taken closer to the zenith to help prevent bloating,
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  #3  
Old 15-09-2018, 06:26 PM
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Slawomir (Suavi)
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Thank you George. I shall experiment with the next target and hopefully will improve the quality of data.
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  #4  
Old 15-09-2018, 07:58 PM
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The finest dust lanes, especially on the near edge, remain wonderfully sharp and clear and contrasty, and this is perhaps the greatest strength of the image.

We wonder if something went very slightly wrong during the colour processing. We can see from the striking and topographically correct colour in the main galaxy - blue OB clusters in the spiral arms and beautiful salmon pink in the nucleus, that the underlying colour data is very good.

Conversely, the brighter stars are on average white but show much colour variation within a single star. I don't think it's registration or chromatic aberration. Perhaps the star colours got clipped to white, and then an increase in saturation produced colour artefacts and a somewhat all-or-none effect in the galaxy.

(I'm nervous about saying this, because I had a good look at our NGC 253, and the colour is very dubious indeed. You're doing better than we did!)

Despite that minor niggle, the colour has added greatly to an already lovely image.

Best,
Mike
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  #5  
Old 15-09-2018, 08:00 PM
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Atmos (Colin)
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Not quite sure what has happened with the star colours there, they've gone a bit rainbow. The galaxy itself looks pretty good though
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  #6  
Old 16-09-2018, 06:02 AM
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Quote:
Originally Posted by Placidus View Post
The finest dust lanes, especially on the near edge, remain wonderfully sharp and clear and contrasty, and this is perhaps the greatest strength of the image.

We wonder if something went very slightly wrong during the colour processing. We can see from the striking and topographically correct colour in the main galaxy - blue OB clusters in the spiral arms and beautiful salmon pink in the nucleus, that the underlying colour data is very good.

Conversely, the brighter stars are on average white but show much colour variation within a single star. I don't think it's registration or chromatic aberration. Perhaps the star colours got clipped to white, and then an increase in saturation produced colour artefacts and a somewhat all-or-none effect in the galaxy.

(I'm nervous about saying this, because I had a good look at our NGC 253, and the colour is very dubious indeed. You're doing better than we did!)

Despite that minor niggle, the colour has added greatly to an already lovely image.

Best,
Mike
Quote:
Originally Posted by Atmos View Post
Not quite sure what has happened with the star colours there, they've gone a bit rainbow. The galaxy itself looks pretty good though
Thank you Mike and Colin.

I certainly lack experience with LRGB imaging. I attempted photometric colour calibration on this image but it didn't help with star colours. I think Mike is spot on with diagnosis - I overexposed many stars and that clipped colour data, so I tried to recover that by boosting saturation quite a lot. Next LRGB image will have shorter exposures (not 15-minute ones).

Thank you again for taking time to look at my image.
Suavi
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  #7  
Old 16-09-2018, 06:16 AM
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codemonkey (Lee)
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Nice work Suavi.

I had similar issues with one or two of my recent images, though not as pronounced. I hid some of that by using SCNR to nuke some green fringing which helped somewhat.

I note that your stars are a bit diamond shaped as well... have you tried drizzling your data? I think you might be reasonably undersampled and you might see an improvement in the stars with drizzling.
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  #8  
Old 16-09-2018, 06:20 AM
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Hi Suavi,

That doesn't seem as sharp as the Lum you shared earlier? Nice colour, apart from the big stars.

Cheers,
Rick.
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  #9  
Old 16-09-2018, 08:38 AM
markas (Mark)
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Very nice galaxy, but the big stars don't show so well, as others have noted. A quick fix is just to desaturate them - easy, if a little tedious, in Photoshop.


Mark
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  #10  
Old 16-09-2018, 01:48 PM
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cometcatcher (Kevin)
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Looks great Suavi! We're getting a few nice clear nights lately. Soon it will all be over and we will have cloud for months on end.
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  #11  
Old 16-09-2018, 02:37 PM
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Looks like the RGB channels would benefit from some sub pixel re-registration...not sure what software you are using, but both Pixinsight and MaxIm will allow you to nail down the registration of all channels...and fix the rainbow fringes in the data. Just a few tweaks and you’ll have a much better result.
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  #12  
Old 18-09-2018, 02:49 PM
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Nice colour and detail Suavi. Some miss-registration on the stars but everything else works fine.
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  #13  
Old 20-09-2018, 06:09 PM
Ross G
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A great looking galaxy photo Suavi.


So much detail and very nice colours.


Ross.
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