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Old 13-02-2014, 08:03 PM
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some colour decisions in astro imaging

Hi
I am trying to work out a consistent approach to colour rendition, but have come across a few questions that do not have obvious (to me) answers. I would appreciate any comments on the following:

“green is bad” seem to be a widely used assumption. Clearly individual stars cannot be green, but surely some nebulae can have a greenish tinge. And what about a galaxy where unresolved old yellow stars mingle with new blue ones – wouldn’t green be the overall combined colour? is green really a no-no?

Galactic extinction can give far distant objects a red cast – the red is a real part of the object appearance, so would you compensate and let the foreground stars go blue, or leave it as is?

Unresolved combinations of emission and reflection nebulae in distant galaxies (other M20s) will have a magenta colour – the trend seems to be to add Ha and push them back to red. What is your view?

Stars can have only a very limited range of colours because they have essentially black body spectra, but it is common to see star images that seem to be of much deeper saturation than is possible. Is there any good reason why star colours should be realistic?

really appreciate any opinions. regards ray
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Old 13-02-2014, 08:43 PM
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I dunno, I'm just in it for the pretty pictures. Seems to be the way for deep sky anyway. For many of us it's more of a form of art than science. The colours rendered to the final image are not necessarily the "real" colours. But then that goes for both amateur and pro astronomers.

Do we want to please ourselves or our piers? In photography there's "rules". The rule of thirds, don't let a portrait subject's nose cross the cheek line, etc etc. Some colours go well together, some don't. Sorry Australia but green and gold together are puke! But that's my opinion.

Are there any "rules" for astrophotography? I don't think there is a correct answer for what colour something should be. That died long ago with the emergence of the Hubble palette lol.

P.S. I'm also mucking about with daytime IR photography. With channel swapping the colours can be really bizarre, but fun!
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Old 13-02-2014, 09:00 PM
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Interesting questions, Ray.

I often start with an eXcalibrator colour calibration which is based on objective quantitative measures. Then I tweak the heck out of it

We're not generally producing science images so I think some artistic licence is only fair.

Cheers,
Rick.
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Old 13-02-2014, 09:15 PM
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Ray,
Quote:
Stars can have only a very limited range of colours because they have essentially black body spectra, but it is common to see star images that seem to be of much deeper saturation than is possible. Is there any good reason why star colours should be realistic?
Hi Ray,
I now think that star colours should be correct & artistic license
allows an image processor to boost their colour.
I haven't always followed this but now try to.
It can be difficult due to halos when using Ha as luminance.

If you equalise the humps in RGB and after later doing a minor LAB colour boost
certain stars are either Red Yellow or Blue etc & then
when checking many other pics you find the same colours -
then boost them to at least show their real colour.

I think that adds authenticity to the image and is valuable data
that should be highlighted by a slight boost in colour.

I'll think about your other questions.

cheers
Allan
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Old 13-02-2014, 10:38 PM
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Redshift could pull a very distant blue star into the green zone.
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Old 14-02-2014, 07:35 PM
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Quote:
Originally Posted by cometcatcher View Post
I dunno, I'm just in it for the pretty pictures. Seems to be the way for deep sky anyway. For many of us it's more of a form of art than science. The colours rendered to the final image are not necessarily the "real" colours. But then that goes for both amateur and pro astronomers.

Do we want to please ourselves or our piers? In photography there's "rules". The rule of thirds, don't let a portrait subject's nose cross the cheek line, etc etc. Some colours go well together, some don't. Sorry Australia but green and gold together are puke! But that's my opinion.

Are there any "rules" for astrophotography? I don't think there is a correct answer for what colour something should be. That died long ago with the emergence of the Hubble palette lol.

P.S. I'm also mucking about with daytime IR photography. With channel swapping the colours can be really bizarre, but fun!
You make a great point with the Hubble palette - no realistic colours there at all

Quote:
Originally Posted by RickS View Post
Interesting questions, Ray.

I often start with an eXcalibrator colour calibration which is based on objective quantitative measures. Then I tweak the heck out of it

We're not generally producing science images so I think some artistic licence is only fair.

Cheers,
Rick.
Thanks for the ideas Rick - will have to try excalibrator when I have some free time
Quote:
Originally Posted by alpal View Post
Ray,
Hi Ray,
I now think that star colours should be correct & artistic license
allows an image processor to boost their colour.
I haven't always followed this but now try to.
It can be difficult due to halos when using Ha as luminance.

If you equalise the humps in RGB and after later doing a minor LAB colour boost
certain stars are either Red Yellow or Blue etc & then
when checking many other pics you find the same colours -
then boost them to at least show their real colour.

I think that adds authenticity to the image and is valuable data
that should be highlighted by a slight boost in colour.

I'll think about your other questions.

cheers
Allan
Thanks Allan. This philosophy seems to give nice results.

Quote:
Originally Posted by billdan View Post
Redshift could pull a very distant blue star into the green zone.
Might do at that ---will have to think about that

thanks for the ideas - there is clearly a diversity of opinion on how/if colour can be related to reality. I guess my hope was to use my scope as a "Starship Enterprise" so that I could get an idea of what things might really look like if I could get out there. Others see the production of a striking artwork as the goal and fidelity is not necessary for that. I guess my real question was about the various philosophies people have, but I didn't realise it at the time.

thanks Ray
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Old 14-02-2014, 07:48 PM
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Quote:
Originally Posted by Shiraz View Post
I guess my hope was to use my scope as a "Starship Enterprise" so that I could get an idea of what things might really look like if I could get out there.
Jay GaBany's view (or perhaps my misrepresentation of it) is that you can't ever see these things in colour with the naked eye. If you build a huge scope then the magnification increases along with the aperture. If you travel and get up close then these huge objects become larger but more tenuous. Therefore it's all false colour and you're free to tweak
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Old 14-02-2014, 08:54 PM
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tilbrook@rbe.ne (Justin Tilbrook)
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Hi Ray,

I love colour even strong vibrant colour, which many seem to think is a big No No!
I love the art side of imaging and the freedom of expression it brings. I've done the methodical thing with variable star observation - comet astrometry and comet hunting.
For me it's the different twist's that colour variations in each image posted that's the best thing.

Besides, life can be a little dull and colourless and when did people become so afraid of colour!!

Viva the colour!!

Just my point of view.

Cheers,

Justin.
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Old 14-02-2014, 09:26 PM
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Quote:
Originally Posted by RickS View Post
Jay GaBany's view (or perhaps my misrepresentation of it) is that you can't ever see these things in colour with the naked eye. If you build a huge scope then the magnification increases along with the aperture. If you travel and get up close then these huge objects become larger but more tenuous. Therefore it's all false colour and you're free to tweak
well, I clearly never gave that enough thought then - of course a galaxy will remain at mag 22/arcsec2 even if I can get closer to it. Even the galaxy we are in is too dim to see in colour, unless the stars are individually resolvable. Thanks for putting me straight on that one Rick. I am still not certain that I agree with the idea that this allows total freedom in tweaking (will have to think about it more), but at the very least least, the idea of an underlying visible reality goes out the window... food for thought. Maybe the most realistic approach is to only do monochrome - colour is an artefact..

Quote:
Originally Posted by tilbrook@rbe.ne View Post
Hi Ray,

I love colour even strong vibrant colour, which many seem to think is a big No No!
I love the art side of imaging and the freedom of expression it brings. I've done the methodical thing with variable star observation - comet astrometry and comet hunting.
For me it's the different twist's that colour variations in each image posted that's the best thing.

Besides, life can be a little dull and colourless and when did people become so afraid of colour!!

Viva the colour!!

Just my point of view.

Cheers,

Justin.
Thanks Justin. I had been in the category of people who favour less intense colour, but maybe I should think again.

regards Ray

Last edited by Shiraz; 14-02-2014 at 09:56 PM.
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  #10  
Old 15-02-2014, 07:44 AM
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David Malin's argument is that per his research into star colours he found that star colours are pastel shades not deep saturated colours.

Using that then as a reference you can then use it as a guide in determing overall colour of the other objects in the image.

Heavy colour saturation is usually something we all go through when doing processing. All that digital power to manipulate the colour is irrestible!

It has its place to emphasis features otherwise not seen like in narrowband.

But generally speaking, I have found that over the years I tend to pull back the colour compared to how I used to process and its generally better received.

So next time you reach for that saturation slider - try restraining yourself and instead of slamming it to 20 - relax, pullback a tad and slide it maybe to 5 or 8 instead!!

Its always a good policy to leave an image until the next day before you post it and see if you still like it. You can be too "in" an image to notice what garish wreckage you have created! If it passes the next day test then you are probably close to the money.

All good fun.

Greg.
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Old 15-02-2014, 10:35 AM
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Ray,
Quote:
Thanks Allan. This philosophy seems to give nice results.
Thanks Ray - I try.
As for colours - it is possible to turn up the colour too much.
I have 2 versions of Orion on my flickr photos now -
& I still can't decide which one to delete or whether to start again.

The one with more colour looks fake like chocolate box colours
but when reduced in colour it looks too pastel & lifeless.
I am still struggling with how much colour to apply.

cheers
Allan
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  #12  
Old 15-02-2014, 03:27 PM
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Quote:
Originally Posted by gregbradley View Post
David Malin's argument is that per his research into star colours he found that star colours are pastel shades not deep saturated colours.

Using that then as a reference you can then use it as a guide in determing overall colour of the other objects in the image.

Heavy colour saturation is usually something we all go through when doing processing. All that digital power to manipulate the colour is irrestible!

It has its place to emphasis features otherwise not seen like in narrowband.

But generally speaking, I have found that over the years I tend to pull back the colour compared to how I used to process and its generally better received.

So next time you reach for that saturation slider - try restraining yourself and instead of slamming it to 20 - relax, pullback a tad and slide it maybe to 5 or 8 instead!!

Its always a good policy to leave an image until the next day before you post it and see if you still like it. You can be too "in" an image to notice what garish wreckage you have created! If it passes the next day test then you are probably close to the money.

All good fun.

Greg.
that's a great idea Greg - using the stars as a reference. The allowable colours for black bodies are well documented, (eg http://www.vendian.org/mncharity/dir...blackbody.html ) so it is possible to get that side of things pretty right - and then tweak the object of interest from that starting point .

you sure are right about the morning after - "what was I thinking" is generally my feeling when I review processing from 11.30 the night before.

Quote:
Originally Posted by alpal View Post
Ray,


Thanks Ray - I try.
As for colours - it is possible to turn up the colour too much.
I have 2 versions of Orion on my flickr photos now -
& I still can't decide which one to delete or whether to start again.

The one with more colour looks fake like chocolate box colours
but when reduced in colour it looks too pastel & lifeless.
I am still struggling with how much colour to apply.

cheers
Allan
thanks Allan - the space between chocolate box/neon and wishy-washy is sometimes very fine - glad to hear it is not just me

Of course the "neon" look is perfectly suitable for some nebulae - they are based on low pressure excited gas, just like neon lighting, so should have saturated colours. Galaxies on the other hand radiate starlight, so one would expect much more muted colours (I guess).

regards Ray

Last edited by Shiraz; 15-02-2014 at 03:39 PM.
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  #13  
Old 15-02-2014, 09:15 PM
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For me colour is beauty, I don't care what Maestro Malin says about colour nuances, when I see a colourful image it makes me smile, when I see a "more natural" (??) pastel like image I often think oh yeah not bad I guess but perhaps a bit boring..?

I think there are a broad set of reasonably accurate colours for many objects like magenta (aka Pink) for most emission nebulae, blue for most reflection nebulae, yellower cores and bluer arms with pink HII regions for galaxies etc

RJ Gabany's bold colours look ok in my view as do those of Tony Hallas, what I do have an issue with though is using processing techniques that rely on painting in features that are often simply not there or exaggerate a faint feature in an arbitrary way and it is this quality about RJ's (and others) images I think is stretching (pardon the pun ) it too far.

Thing is, it is the spectacular colour saturated HST images that have brought the world of astroimaging aliiiive so go with the bold colours in my view ...but that's just my view, I would paint a car or decorate my home differently to others so there is no right or wrong but in the end make the choice and just go with the artistic flow

Mike
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Old 16-02-2014, 09:04 AM
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Perhaps a bit OT, but, the moment one uses the hipass filter calling something realistic is pretty far-fetched. Adding a lot of colour only compounds the issue in my view.

I've got a friend that imaged with a one-shot colour camera and very long exposures. His attitude was to do "nothing" to the image, arguing that the camera sensitivity to colour balance is set to give a faithful rendition to the human brain. Certainly there are things that will make the colours shift so adjusting the hue to match star colours ought to set it right. I'm sure there are arguments that can be made against this position; I'm just reporting what was said to me...

My own feeling is that looking at the night sky unaided one can see muted star colours. Yes, seeing an intensly coloured photo is initially attention grabbing but to me it is too far from realistic. I always prefer to see colour but not pushed too far. The argument reminds me of the dichotomy that often divides camps of classical musicians. There are those that "respect" the composer's score and the other camp that performs to the audience with a decided lack of respect for what the composer wrote. Sadly, in my view, it was more often than not the latter camp that took more money to the bank. Doesn't make it right though!

Peter
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Old 16-02-2014, 09:26 AM
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The only issue with that argument about one shot colour is that different camera and chip makers can offer different auto white balances and colour jpeg engines.

There is a decided difference between jpeg output from different camera brands. Subtle perhaps compared to what we are talking about but there.

You get those who swear by Canon colours, then Nikon colours and usually the popular winner is Fuji colours. But you certainly get preferences.

So even in that arena there are differences. Also the demosaicing process that turns the image into a colour is set by the manufacturer or software writer.

But yes in practical terms its far less of a variable and is a bit of a guide.

Subtle again but there. I see output from my CDK17 and it usually picks up the Ha areas in galaxies without even doing a Ha set. It also will show the core as yellow without any trouble. Some other scopes I have found on some galaxies the core does not look yellow. So some manipulation is required in these cases. How well is the scope supplying the colour information? Coatings, mirrors, lenses all have their limitations.

Also if you used a one shot colour with a poor refractor you would lots of blue ringed stars which are obviously not correct. Lens coatings on APOs can definitely skew colour one way or the other. The FSQ106ED I would argue skews colours to the mustard. It may be the lens coatings, it may be the internal black paint but there is a colour bias there.

Greg
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Old 16-02-2014, 03:50 PM
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Personally, I like to see a range of colours--from orange stars to blue stars and different colours across a galaxy. PixInsight's colour calibration tool is pretty good at getting this sort of wide colour spectrum.
Quote:
Originally Posted by strongmanmike View Post

RJ Gabany's bold colours look ok in my view as do those of Tony Hallas, what I do have an issue with though is using processing techniques that rely on painting in features that are often simply not there or exaggerate a faint feature in an arbitrary way and it is this quality about RJ's (and others) images I think is stretching (pardon the pun ) it too far.

Mike
Fully agree here Mike. Painting and arbitrary selection lassoes shouldn't be part of astro processing.
Geoff
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Old 17-02-2014, 09:41 AM
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Originally Posted by strongmanmike View Post

I do have an issue with though is using processing techniques that rely on painting in features that are often simply not there or exaggerate a faint feature in an arbitrary way and it is this quality about RJ's (and others) images I think is stretching (pardon the pun ) it too far.

Mike
Yes Mike but Ken Crawford wouldn't agree with you:

http://www.imagingdeepsky.com/Presentations.html

He "digs out the details" using masks.

I often use masks to get the desired result but in a different way e.g.
An inverted layer mask is great to get rid of noise in low signal areas
but what about when there is noise in a high signal area?
The only way is to:
reduce noise for the entire picture until the noise in the area you want is reduced
then make a hide all layer mask,
then paint on the mask for the area you want to be reduced.
You can see the results as you paint.
Then you can blur the mask & reduce the opacity to make a perfect seamless blend.

I wouldn't have known that unless I had watched all his videos.

You can also do the same thing to sharpen a certain area while
leaving other areas alone - also -
e.g. at the edge of Centaurus A it is very bright & the tiny details
get lost in this brightness.
A selective adjustment of curves along the edges will show those otherwise hidden details.
You can "dig out the details" in such ways without
having a false image - it's just using the data you have.

Also - if you want a 3D effect then it's great to make the front area of
a target ever so slightly brighter.

Ken's methods allow you to give more impact to your images.

I will continue to use such methods but only in special cases -
it's too much work to do an entire image that way.

cheers
Allan
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Old 17-02-2014, 10:07 AM
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Yes Mike but Ken Crawford wouldn't agree with you:

http://www.imagingdeepsky.com/Presentations.html

He "digs out the details" using masks.
Using multi strength sharpening layers causes little in the way of make believe, an over zealous user may reveal an over sharpened area sure but it doesn't reveal something that isn't there it may just reveal a noisy bit (although, sometimes this noise is mistaken for real detail)...however when multi layering techniques are applied to faint features (as opposed to fine details) here is where the make believe can creep in. Producing several variations of an image with different degrees of stretching and then layering them on top of each other and then painting (revealing) in the faint structures is where it can easily become make believe, is it a real feature or just blending in of a bit of the lighter background from one of the more stretched background layers...?

Do you get what I am saying?

Mike
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Old 17-02-2014, 10:23 AM
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Originally Posted by strongmanmike View Post
Using multi strength sharpening layers causes little in the way of make believe, an over zealous user may reveal an over sharpened area sure but it doesn't reveal something that isn't there it may just reveal a noisy bit (although, sometimes this noise is mistaken for real detail)...however when multi layering techniques are applied to faint features (as opposed to fine details) here is where the make believe can creep in. Producing several variations of an image with different degrees of stretching and then layering them on top of each other and then painting (revealing) in the faint structures is where it can easily become make believe, is it a real feature or just blending in of a bit of the lighter background from one of the more stretched background layers...?

Do you get what I am saying?

Mike

Not really Mike - just by stretching an image you're making it fake.
The naked eye would not see it like that.
The stretching does allow you to see the faint details.
Most images can do with a bit of sharpening.
BTW - have you watched all of Ken's videos?
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Old 17-02-2014, 10:54 AM
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Not really Mike - just by stretching an image you're making it fake.
The naked eye would not see it like that.
The stretching does allow you to see the faint details.
Most images can do with a bit of sharpening.
BTW - have you watched all of Ken's videos?
Ok, well imagine you stretch one version of a galaxy image and not the other, now one will have a lighter background than the other, now layer them with the lighter one below the the darker one, now pick an arbitrary place on the edge of the galaxy on top layer and very carefully start painting (revealing ) the layer underneath..you can very carefully reveal a galaxy arm in a place that is simply a lighter background and voila! a new faint galaxy arm YAY!...except there is no real galaxy arm there at all...of course while few would think of being so blatant the technique can cause some good make believe shapes and features

That's all I am saying
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