Hi Raymo, I think I know why you have blue in the centre of your Lagoon. A lot of this is probably (as a saying from where I come from) "teaching a grannie to suck eggs", due apologies if so ... but it helps for both me and hopfully others to put out what I think I understand, especially if I get it wrong . I think there are two reasons:
1: In an image such as the one you presented, you should not centre all the histograms over each other. Over 50% of your image is dominated by the red Lagoon Nebula, and a histogram of this image should have a significantly larger than average red/pink component. Images of smaller DSOs that have a lot of neutral dark grey/black sky background are much better approximated as being a neutral colour balance - red, green and blue curves should roughly overlay each other. You should only overlay the histogram 'cones' in cases where most of your image ought to be neutral. A simple example would be a picture of a large red circle surrounded by a dark grey rectangle - the red histogram is skewed to the right of the others by the presence of so many red pixels.
In your Lagoon picture, you are suppressing the red in your image by centring the histograms into a single cone, and so the Lagoon loses some of its red colour ... but it keeps the blue:
2: Hidden behind the H-alpha in the Lagoon is a solid component of blue-green H-beta, especially where there is strong emission and less interstellar dust in the way. Your image demonstrates this. The ratio of H-a to H-b is used in astrophysics to estimate interstellar reddening by intervening dust is Hα/Hβ/Hγ= 2.86:1:0.47, an example is NGC3576 and NGC3603 in Carina (Mike Sidonio's great image of it). NGC3603 is much further away, and much more reddened by dust as the H-beta and gamma are scattered along the way. A pure hydrogen emission colour is pinkish, even dangerously closer to the dreaded 'magenta' that imagers try and avoid [but it's not exactly magenta either, it's more delicate than that]. A hydrogen emission nebula is only pure red if there's a lot of intervening dust, otherwise it has the blue-green and blue components that make it more pinkish than pure red.
Put the two together - for a big cloud of dominantly reddish hydrogen emission, if you force the red, blue and green components to have the same weighting, a lot more of the blue will come out so long as there's not too much intervening dust. It's not the same as the varying camera/DSLR detector sensitivity to H-a and H-b, but a similar resulting effect). Yet another problem is a reddish sky background caused by light pollution, which, when roughly neutralised to grey, can take out some of the red from a nebula. Photographing red nebulae is not so easy!
It doesn't mean it's "wrong" (colour interpretation, what people like, what different equipment can do, can all be pretty subjective, so go with what you like ), but I think this is why many Lagoon images are reddish and this one of yours has more blue.
And before anyone talks of pots, kettles and black, I definitely get the colour balance thoroughly skewed in most of my imaging attempts!!
Thanks for the comprehensive reply Andy. You were partly right about
sucking eggs; I understand the physics of it all, having been doing
astro for over 60 years, BUT, and it's a big but, I have almost no digital image processing skills at all, having always struggled with computers. No amount of video tutorials, or hours spent reading
PS books, can get it to stay in my head. The end result is that I work
with JPEGS, no separate darks, flats etc: and almost entirely in DSS.
I wasted my money buying PS, can't get my head around it. Ditto
to a degree with Backyard EOS. I had an attack of a thing called transient global amnesia several yrs ago, and ever since, my short term memory has been c--p.
Incidentally, with the DSS histogram, if I put one colour slightly to the right of the rest of the cone, the whole image takes on a hint of that colour, not just the part of the image that is naturally that colour.
cheers raymo
Andyc: I think you mean to talk about oxygen III emission and blue dust reflection. Hydrogen beta almost always appears as a fraction of hydrogen alpha emission (http://en.wikipedia.org/wiki/Balmer_series) and results in the pink colour (red + blue)
Raymo: Nice pic. I think you've pushed it a bit hard though. Ease off on the colour saturation and contrast and it'll be a cracking pic.
Cheers,
Cam
Quote:
Originally Posted by andyc
Hi Raymo, I think I know why you have blue in the centre of your Lagoon. A lot of this is probably (as a saying from where I come from) "teaching a grannie to suck eggs", due apologies if so ... but it helps for both me and hopfully others to put out what I think I understand, especially if I get it wrong . I think there are two reasons:
1: In an image such as the one you presented, you should not centre all the histograms over each other. Over 50% of your image is dominated by the red Lagoon Nebula, and a histogram of this image should have a significantly larger than average red/pink component. Images of smaller DSOs that have a lot of neutral dark grey/black sky background are much better approximated as being a neutral colour balance - red, green and blue curves should roughly overlay each other. You should only overlay the histogram 'cones' in cases where most of your image ought to be neutral. A simple example would be a picture of a large red circle surrounded by a dark grey rectangle - the red histogram is skewed to the right of the others by the presence of so many red pixels.
In your Lagoon picture, you are suppressing the red in your image by centring the histograms into a single cone, and so the Lagoon loses some of its red colour ... but it keeps the blue:
2: Hidden behind the H-alpha in the Lagoon is a solid component of blue-green H-beta, especially where there is strong emission and less interstellar dust in the way. Your image demonstrates this. The ratio of H-a to H-b is used in astrophysics to estimate interstellar reddening by intervening dust is Hα/Hβ/Hγ= 2.86:1:0.47, an example is NGC3576 and NGC3603 in Carina (Mike Sidonio's great image of it). NGC3603 is much further away, and much more reddened by dust as the H-beta and gamma are scattered along the way. A pure hydrogen emission colour is pinkish, even dangerously closer to the dreaded 'magenta' that imagers try and avoid [but it's not exactly magenta either, it's more delicate than that]. A hydrogen emission nebula is only pure red if there's a lot of intervening dust, otherwise it has the blue-green and blue components that make it more pinkish than pure red.
Put the two together - for a big cloud of dominantly reddish hydrogen emission, if you force the red, blue and green components to have the same weighting, a lot more of the blue will come out so long as there's not too much intervening dust. It's not the same as the varying camera/DSLR detector sensitivity to H-a and H-b, but a similar resulting effect). Yet another problem is a reddish sky background caused by light pollution, which, when roughly neutralised to grey, can take out some of the red from a nebula. Photographing red nebulae is not so easy!
It doesn't mean it's "wrong" (colour interpretation, what people like, what different equipment can do, can all be pretty subjective, so go with what you like ), but I think this is why many Lagoon images are reddish and this one of yours has more blue.
And before anyone talks of pots, kettles and black, I definitely get the colour balance thoroughly skewed in most of my imaging attempts!!
Thank you Cam; it is one of the first images I took when trying out
digital imaging for the first time. I just haven't got round to imaging it again with my now slightly improved processing. I'll find the subs from
that image and have another go at them.
raymo
Hi Cam, I meant not to talk about OIII or reflection, just the colours in hydrogen emission but please don't take this comment as argumentative! I've expanded about it because Raymo put up the picture directly in response to a discussion on Lagoon colour on another thread. And of course I should've said- Raymo, very nice pic . I'm not at all criticising Raymo's picture, as I thought he was putting up the picture to try and work towards understanding the colours we get when we point a DSLR or CCD at nebulae with particular colour properties. And I may well be wrong in some respects!! [though hopefully not entirely so]
H-beta is of course always in hydrogen emission as a physical property of the emission process at the ratios I mentioned, but whether it is in the observed nebula on Earth depends on how much intervening dust there is between Earth and the nebula. So we observe hydrogen emission nebulae ranging from pinkish to fairly deep red, but I don't think blue ever dominates if it's pure hydrogen.
Cyan OIII (reflection nebulosity is very different, I'll leave that out) is a complicating factor. For example NGC346 in the SMC has a high amount of OIII emission and so often appears very blue/teal, unusual for an emission nebula - the same process affects the Tarantula as well and highlighted by DSLR images of it. Most nebulae presumably have some OIII, but it's not generally that prominent in galactic emission nebulae. If the Lagoon has a significant component of OIII, then it should be more blue ... but only if there's not too much scattering dust in the way!
Yet another complication around the Lagoon is that background stars near the galactic centre are somewhat yellowish/orange. There's a lot of them in this area (!) which means that a fair bit of the background of any Lagoon image is from the masses of older stars. Finding a real neutral colour calibration point for any images in this area is really hard! There's no reason to expect the histogram should have red, green and blue in the same place if the whole image, both nebula and background, has a little extra red. But identifying where the balance is - that's a dark art!
So that leaves us with:
1: Areas of pink/red nebula colour comprise a large part of the frame of a telescopic Lagoon nebula image as opposed to black background, but with an unknown (usually smaller) blue component in the nebula. So the histogram should be red-heavy if the nebula really is mostly reddish.
2: Light pollution potentially makes the background reddish, also affecting the red channel of the histogram, but in a way we do not want!
3: Old galactic centre stars make a non-trivial number of background pixels yellow/orange around the Lagoon, altering the histogram again, but in a way we do want. There's very little true 'black' sky here. A correction to neutral overall colour would remove red from places we know ought to have a lot of red in their spectrum, not just in the nebula.
4: Different cameras record colours in different ways. A DSLR records relatively less H-alpha than a CCD, but is closer to what the eye would see (our colour sensitivity at differing wavelengths are not so far from the DSLR sees, but the CCD is better in deep red. So a CCD and DSLR don't 'see' hydrogen emission as the same combined pinkish/reddish colour. Our DSLR image of hydrogen emission should be a bit more blue ... if there's little intervening dust!
Because of all this, I'm absolutely not going to criticise anyone harshly for their colour interpretation or colour balance. I'm always looking to learn more as I'm often not quite satisfied with the colour outputs in my images, which comes down to how you work the processing. I'm trying to grasp what we expect of an object's spectral colour on the detector, before we try and interpret it and make a pretty picture.
I'm fiddling around with a Lagoon pic I took this week to see if I can reproduce the various colour effects of different Lagoon images that arise just from our histogram adjustments, and boy it's not easy to calibrate the background
Thanks for the comprehensive reply Andy. You were partly right about
sucking eggs; I understand the physics of it all, having been doing
astro for over 60 years, BUT, and it's a big but, I have almost no digital image processing skills at all, having always struggled with computers. No amount of video tutorials, or hours spent reading
PS books, can get it to stay in my head. The end result is that I work
with JPEGS, no separate darks, flats etc: and almost entirely in DSS.
I wasted my money buying PS, can't get my head around it. Ditto
to a degree with Backyard EOS. I had an attack of a thing called transient global amnesia several yrs ago, and ever since, my short term memory has been c--p.
Incidentally, with the DSS histogram, if I put one colour slightly to the right of the rest of the cone, the whole image takes on a hint of that colour, not just the part of the image that is naturally that colour.
cheers raymo
Hi Raymo, just saw your reply - sorry to hear about the short term memory, you really have my sympathy for that. And you do very well with your images! I wish I had good advice about the histogram sliders in DSS for you - areas like this are so tricky.
I hear you about putting an overall cast on the image, I'm guessing a bit, but I think that's in part because the wanted and unwanted colour distribution is not uniform across a single channel of the histogram, so uniform changes don't always correct for things like light pollution in a way we'd like. I'm not quite sure what to suggest - I've had a lot of joy with the various colour balancing utilities in PixInsight (which doesn't help you) - but I suspect their colour balancing is not as simple as clipping and gamma (middle slider) each colour channel. And that's the reason I'm so interested now - I have a Lagoon pic with nice eggy stars and a difficult colour balance that PI's colour tools don't work so well on as there are no good black and white reference points.
The end result is that I work
with JPEGS, no separate darks, flats etc: and almost entirely in DSS.
I wasted my money buying PS, can't get my head around it.
Hi Raymo,
just thought it might be worth posting this for you. It seems like a much more simplified approach to image processing (does away completely with darks & flats etc) and if you can manage to start using raws then Camera Raw is a lot simpler than PS itself from what I've seen.
I want to have another go at the Lagoon if we ever see another clear night again in the Sydney area.
The last shot I took of it was a couple of months ago with my little Fuji camera which, while not being modded, certainly has a different colour response from my Canon...which IS now modded Anyhow, it turned out much more purple than before (see attached), and purple comes from red and blue.
Not to put too fine a point on it, but raymo's exploits from his DSLR were a strong influencing factor in me buying a DSLR in the first place
just thought it might be worth posting this for you. It seems like a much more simplified approach to image processing (does away completely with darks & flats etc) and if you can manage to start using raws then Camera Raw is a lot simpler than PS itself from what I've seen.
One point he lists in his processing workflow...Carboni's actions...they're great for folk who have Photoshop and want some common astro features but don't want to have to get their hands too dirty with the nitty gritty.
One point he lists in his processing workflow...Carboni's actions...they're great for folk who have Photoshop and want some common astro features but don't want to have to get their hands too dirty with the nitty gritty.
Thanks for highlighting that Dunk. I'll have to investigate those.
One thing about PS is that there are often many ways to achieve the same result.
I had a go with my data - ah the joy of posting small images on IIS is that the really egg-shaped stars are almost invisible! This is my un-modded EOS 60D, and 8 x 5 minutes at ISO800, with moderate light pollution and a setting 65% Moon. I thought I'd share roughly my processing to show how I didn't end up with a very blue Lagoon.
I initially automatically adjusted the histogram in PixInsight, which, rather interestingly did overlap the red, green and blue curves (more fool me ), and it modified their intensity so that all the curves were as near as possible in both height and shape. But that gave an image with too much blue all over, so I adjusted the combined curves, reduced the blue and green curves separately just a bit, then as a last point I did a nonlinear stretch of curves on the red, concentrating on the highlights. [I blended in an HDR transform version to pick out some of the nebula details, but that's largely irrelevant here.]
It taught me a lot about the colours of this region, and my dumb eyes have probably stuffed it up and it'll appear green on everyone else's monitor . My aim was to have something of a golden colour on the area around the globular to the lower left, to have the little banner of reflection nebulosity at the top left quite blue, and have the fringes of the main nebula as quite red (I think these come out as deeper red in most images, either due to more dust or less OIII). But boy is it subjective when you can't colour calibrate straightforwardly. There's certainly some blue in the middle, and I couldn't honestly say how much I feel should be there (more than I first thought?). And weird colours appear so easily when you fiddle with it.
I've attached 3 images - the first is the 'final' image, though the star shapes probably stop me from publishing it much bigger. The second is the basic stacked image with all channels stretched equally, showing a lovely orange light-polluted cast. The last image is the basic stack with channels stretched and matched by PixInsight so that red, green and blue overlap each other at the same intensity (it's a histogram gamma stretch and clips only).
Sometimes I wonder if black-and-white is the way to go...
. I think there are two reasons:
[but it's not exactly magenta either, it's more delicate than that]. A hydrogen emission nebula is only pure red if there's a lot of intervening dust, otherwise it has the blue-green and blue components that make it more pinkish than pure red. 
), but I think this is why many Lagoon images are reddish and this one of yours has more blue. 
and a difficult colour balance that PI's colour tools don't work so well on as there are no good black and white reference points.
), and it modified their intensity so that all the curves were as near as possible in both height and shape. But that gave an image with too much blue all over, so I adjusted the combined curves, reduced the blue and green curves separately just a bit, then as a last point I did a nonlinear stretch of curves on the red, concentrating on the highlights. [I blended in an HDR transform version to pick out some of the nebula details, but that's largely irrelevant here.] 
