Hey guys, I'm not getting as much time under the stars as I'd like, so it's taken quite a few weeks to complete this one....!
I found this one difficult to process in one way. Not entirely happy with the star colours, but having trouble correcting it! Although I think it turned out retry good, considering it was imaged pretty low in the sky, and over the towns skyglow.
11 hours of Luminance, and 5 hours each in RGB.
Feedback welcome and if anyone has any suggestions on my star colours, let me know
I like your image a lot. This galaxy has famously low surface brightness making it far harder than you'd expect, especially in light pollution, so top job.
You asked about the colour. I'm the world's worst to comment, because I'm colourblind, but that means I've spent more time "doing it by the book" and thinking about the astrophysics, rather than subjectively. Doesn't mean I get it right, but here's a possible recipe which is heading in the right direction:
- Measure the average colour of bits of starless background. It ought to be close to neutral and very dark but nowhere black. Set the zero point for each channel accordingly. Do this first. If you can't measure the actual RGB values of the averaged background, looking at the histogram can help a lot. Set the black point for each channel so that the bottom of the histogram is almost, but not quite, zero.
- Measure the average colour of the entire galaxy. Magellanic galaxies can be very blue, and distant, dust-obscured galaxies can be very red, but most face-on spirals should on-average be approximately neutral. The brightest, most conspicuous clusters of stars in the spiral arms will be OB stars and therefore blue, and the stars in the core will mostly be very old stars and therefore salmon pink. But don't try to make them that way, just adjust the white point on each colour channel until the average colour of the galaxy is neutral. Then, hey, presto, the blue spiral arms and the salmon pink core will spring out automatically. You can then (according to taste) carefully increase the saturation in order to bring out subtle differences which have been hidden by a strong nonlinear stretch.
I had a go at doing this with your shot, and it works. The core ends up more salmon pink, the spiral arms more blue, and the field stars come out a pleasing mixture of subtle colours.
One trap to watch using the method I've described is that some galaxies have a strong mixture of near-magenta in the spiral arms, and warmer orange in the core, and the above approach done blindly will put too much green in the green channel. So just make sure you don't have any turquoise stars.
Another trap with increasing the saturation to show subtle true colours in the galaxy is it can end up creating background colour noise. There are ways around that, using a blurry mask to select just the bright regions for restoring the saturation, leaving the dim regions unchanged, and pro rata in between. That's not necessary in the case of your image.
Others can advise you far better than I can, but that might be a starting point.
Nice image, Erik! I had a go at 6744 a couple of years ago and as Mike said, it is a tricky one.
eXcalibrator (http://bf-astro.com/excalibrator/excalibrator.htm) might be worth looking at if you want a semi scientific (well, it does involve fudge factors) way to figure out the colour balance.
Looks good Erik. I am trying to learn better star control re color and size so I am far from an expert but I have found a couple things that seem to work. I use MT with a mask in PixInsight to reduce star size a bit. I also mess with the STF function if an auto stretch blows out the color. I have had some luck with Masked Stretch but not consistently, operator error I'm sure! In PS, I've tried using the magic wand tool set to 11x11 to select bright stars of similar size and apply curves to reduce the brightness and then feather out to blend in. Also in PS I've tried the minimum filter with some success.
I like your image a lot. This galaxy has famously low surface brightness making it far harder than you'd expect, especially in light pollution, so top job.
You asked about the colour. I'm the world's worst to comment, because I'm colourblind, but that means I've spent more time "doing it by the book" and thinking about the astrophysics, rather than subjectively. Doesn't mean I get it right, but here's a possible recipe which is heading in the right direction:
- Measure the average colour of bits of starless background. It ought to be close to neutral and very dark but nowhere black. Set the zero point for each channel accordingly. Do this first. If you can't measure the actual RGB values of the averaged background, looking at the histogram can help a lot. Set the black point for each channel so that the bottom of the histogram is almost, but not quite, zero.
- Measure the average colour of the entire galaxy. Magellanic galaxies can be very blue, and distant, dust-obscured galaxies can be very red, but most face-on spirals should on-average be approximately neutral. The brightest, most conspicuous clusters of stars in the spiral arms will be OB stars and therefore blue, and the stars in the core will mostly be very old stars and therefore salmon pink. But don't try to make them that way, just adjust the white point on each colour channel until the average colour of the galaxy is neutral. Then, hey, presto, the blue spiral arms and the salmon pink core will spring out automatically. You can then (according to taste) carefully increase the saturation in order to bring out subtle differences which have been hidden by a strong nonlinear stretch.
I had a go at doing this with your shot, and it works. The core ends up more salmon pink, the spiral arms more blue, and the field stars come out a pleasing mixture of subtle colours.
One trap to watch using the method I've described is that some galaxies have a strong mixture of near-magenta in the spiral arms, and warmer orange in the core, and the above approach done blindly will put too much green in the green channel. So just make sure you don't have any turquoise stars.
Another trap with increasing the saturation to show subtle true colours in the galaxy is it can end up creating background colour noise. There are ways around that, using a blurry mask to select just the bright regions for restoring the saturation, leaving the dim regions unchanged, and pro rata in between. That's not necessary in the case of your image.
Others can advise you far better than I can, but that might be a starting point.
Cheers,
Mike
Thank you Haha, well that makes it difficult for you! I will take your advice and have a play with it I think the actual galaxy is ok, its just some of the brighter stars which looked a little odd to me?
Quote:
Originally Posted by alpal
Looking good -
I would be tempted to use LAB mode on it:
I did after you mentioned it! It made it quite vibrant! Will post a re-pro at some stage.
Quote:
Originally Posted by RickS
Nice image, Erik! I had a go at 6744 a couple of years ago and as Mike said, it is a tricky one.
eXcalibrator (http://bf-astro.com/excalibrator/excalibrator.htm) might be worth looking at if you want a semi scientific (well, it does involve fudge factors) way to figure out the colour balance.
Cheers,
Rick.
Thanks Rick Don't think excalibur will work for me on that mac....I'll have to load it onto the Windows partition and give it a go.
Quote:
Originally Posted by NOMH
Looks good Erik. I am trying to learn better star control re color and size so I am far from an expert but I have found a couple things that seem to work. I use MT with a mask in PixInsight to reduce star size a bit. I also mess with the STF function if an auto stretch blows out the color. I have had some luck with Masked Stretch but not consistently, operator error I'm sure! In PS, I've tried using the magic wand tool set to 11x11 to select bright stars of similar size and apply curves to reduce the brightness and then feather out to blend in. Also in PS I've tried the minimum filter with some success.
JB
Cheers' I'll give it a shot. Haven't really used the MT tool, but will check out some tutorials!
Quote:
Originally Posted by strongmanmike
I think that's a very fine image Erik, I like the brightness of it
Mike
Thanks Mike
Quote:
Originally Posted by marc4darkskies
I agree - a fine effort on a tough target (to capture and process)!
Thanks Marcus, yeah it was a bit tricky. I was only managing 4 hours a night before it was getting to low.
Be my guest Allan, would you like a copy of the linear FITS, or work off the JPEG?
Thanks Erik,
I just thought I'd have a quick go with the jpg you posted.
I tried to get a bit more color & increase the contrast in the brightest part of the galaxy.
I'll let you do a full re-process if you feel the need.
Thanks Erik,
I just thought I'd have a quick go with the jpg you posted.
I tried to get a bit more color & increase the contrast in the brightest part of the galaxy.
I'll let you do a full re-process if you feel the need.
cheers
Allan
Looks good Allan, certainly brought out the magentas
and if anyone has any suggestions on my star colours, let me know 
