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Old 17-06-2015, 08:53 AM
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Star colours what is correct?

I know there are programs that measure and using G2V stars for references etc etc but what about the saturation levels of colour for stars?

In the digital age a few sliders can shift things dramatically, in the film era colour was more or less set by the object and influenced less by the film.

So what is a correct level of colour for stars? Are they hardly coloured at all or are they fairly coloured? Our sun can look very yellow at times for example.

Greg.
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Old 17-06-2015, 09:18 AM
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I think this is a highly debatable subject. I'm pretty sure you were at the AAIC meeting where there was a lively discussion on the subject of colour. I know you are speaking about stars, and I assume stars close by in our galaxy. Anyway, wasn't it said at that meeting that no matter how close we could physically get to a nebula or other galaxy that we wouldn't be able to see colour? And, isn't that a reality just because our eyes are insensitive to colour as the brightness dims? I guess this is where the controversy begins; CCDs being so much more sensitive than our eyes reveal colour that our eyes naturally cannot see. To me this crosses over into the realm of art and there just isn't a right answer but is more a matter of taste. Personally I like colour more muted and less gaudy and that goes for stars too. I'm sure everybody will have a different opinion!

Peter
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Old 17-06-2015, 09:25 AM
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You can represent visible star colours pretty accurately, but most choose to accentuate the saturation a bit in imaging - they are actually only moderately colourful.

This is the best site I have come across on actual star colours. http://www.vendian.org/mncharity/dir3/starcolor/

as you say Greg, local atmospheric and galactic dust effects can alter these - for example, it is not unusual for stars to appear more red looking than they should be and to have bluish scatter halos.

Last edited by Shiraz; 17-06-2015 at 11:16 AM.
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Old 17-06-2015, 12:31 PM
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I've seen that link before Ray. Its a good reference for colour in which is the topic of this thread. The notes on the page rightly point out that intensity or brightness is not dealth with which is therefore the third dimension (hue, saturation and intensity).

"This page is concerned only with chromaticity (hue and saturation), but not brightness. The luminous power per unit area of star classes varies greatly. If we were doing brightness, and the G class star color were as above, then the M class star color would simply be black." http://www.vendian.org/mncharity/dir...intensity.html

As has been said before, I think this will remain a subjective topic and ultimately up to personal choice as to what the imager wishes to convey. I think we can all agree green stars don't go, and for those narrowband aficionados who enjoy their magenta stars we'll just have to make an exception for.
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Old 17-06-2015, 01:50 PM
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Greg,

My thoughts on a subject that can have no single right answer.

If you have performed a G2V calibration and removed any colour gradients, what you have left ought to be a fair representation of the "real colour" . . . assuming you havent done much in the way of non linear transformations with your colours or blown out any channels and attempted to capture equal amounts of each colour channel in the first place - considering that different channels may have different signal to noise ratios given the weak amount of actual data (photons collected) we rely on.

What you do from there on is your business !

But even with G2V calibration, the spectral sensitivity of your CCD, the filters and your optics as well as the local atmospheric colour extinction etc will all have an influence on the relative colours and brightnesses.
The fact that we frequently enhance Red with Ha, as well as dramatically exaggerate our luminance channel with Ha to draw out interesting contrast detail (and therefore even more of the Red channel) and that our CCDs convert some IR into Red and some UV into Blue means that colour integrity is ultimately destroyed.

But in saying this - what are you trying to do - Was colour the purpose ? or rather the use of colour and contrast to portray a particular target in the best possible way to illustrate whatever features of interest your image is intending to show and highight ?

As already stated, our eyes cannot see most of it in the first place - its just too dark, but our eyes can only detect a relatively narrow part of the whole spectrum in any case - so what is real ?, what is right ? and more importantly what is wrong ?

For Astro Imaging its more a question of the philosphical approach of the imager for that particular target.
Should we eliminate spectrally aesthetic features because we cant see it with our own eyes ?
Should we ignore visually interesting detail just because our camera happens to detect more IR and UV than someone elses ?

Unless you are attempting to repesent your images as having scientifically accurate colour, your image is what you make it.
And generally it wil be judged by you and maybe others in that way.

AP is not like terrestrial photography to the extent that the image processor (you) is required to process and manipulate vast amounts of colour channel information and the ultimate aim is usually to show interesting detail rather than represent any subject or colour faithfully and use the interplay of light and composure in an interesting way.
But just an a terrestrial photography uses shadows and highlights in his image along with much greater opportunities for composure, perspective and depth of field etc and to create an image that provides us with a pleasing view - they too are manipulating the colour balance and brightness levels
We just dont have the opportunity of the latter to play with (ie depth of field, composure, perspective etc)

I prefer colour that looks realistic, not exaggerated or flouro, not oversaturated, I prefer images with stars that look like they are generally correct, but thats just me and that still leaves plenty of room for variation.
I thik the black art of Ap is to produce a pleasing visual image that retains elements of natural colour and beauty, but maybe also teases out the really interesting details and visual effects that make that image personalised and unique.

Rally
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Old 17-06-2015, 04:20 PM
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Really interesting thread and great responses.

As it has been explained before, when looking through the atmosphere, from the ground, as we humans normally do, the same stars will have different colours depending on their declination, so as the Earth rotates, the colours of the stars will change due to atmospheric scattering (as it happens with our Sun; it is yellow/red at sunrise/sunset and whitish/yellowish in the middle of the day). For this very reason, at least how I understand it, it is impossible to represent the colours of the stars that we would perceive with our eyes in one definite way.
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Old 17-06-2015, 05:30 PM
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The real question here is not so much what colour they should be as when you have your histogram balanced and normally imaged objects look like most images then you are pretty set.

Its more how saturated should star colours be. David Malin has said he did a study on star colour and wrote a book about it. That stars are pastel colours.

Greg.
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Old 17-06-2015, 06:43 PM
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This discussion is why I want to get a set of Johnson-Cousins UBVRI photo metric filters. Using one of many photometric catalogs (some by Hubble's WFC) allow for accurate magnitude figures which can be transformed into colour. It is something that I am yet to actually do myself as I have only very recently bought my first CCD imager, got LRGB but no photometry filters yet :-)
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Old 17-06-2015, 09:04 PM
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Quote:
Originally Posted by gregbradley View Post
The real question here is not so much what colour they should be as when you have your histogram balanced and normally imaged objects look like most images then you are pretty set.

Its more how saturated should star colours be. David Malin has said he did a study on star colour and wrote a book about it. That stars are pastel colours.

Greg.
I saw a talk that David Malin gave to the Astronomical Society of South Australia earlier this year. At the end of the talk he was specific about star colours. He was quite adamant that star colours appear as pastel, showing vague colour. Not any where near the saturation levels most of use display. Generally I would say no one produces star colours like this on this forum. Everyone tends to like a bit more saturation.

For G2V I used CCDautopilot which has a feature which can test how your frames ought to be imaged. Use that if you are really keen on getting it exactly like G2V.

In the end its all a matter of personal taste. What one person might like in star colour might be disliked by others and I think that nearly everyone is producing Art and not science related images here. That is perfectly ok from my perspective and I am sure it would be from most other people as an observation.

A thought provoking discussion.
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Old 17-06-2015, 09:43 PM
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For my 2c worth it's simply a matter of what looks best aesthetically- unless you're trying to match an image from the Hubble, in which case take onboard all of the scientific knowledge available and lay your colour cards on the table!

I'm an unashamed "pretty picture" maker, if an image looks better to my taste with saturated or Rgb stars then so be it! Personally I have no love of magenta stars, they just don't work in any way to my perception- but as to whether or not a star is the 'right' colour, well I'll leave that to the experts to decide, meanwhile I'll just keep on attempting to make them look good, not distract from the image and celebrate their richness of colour!

Many of us are making 'technical art', but what comes first is the real debate, Technical or Art?

Good discussion, look forward to continuing it with you over a decent glass of red one day

Last edited by Andy01; 17-06-2015 at 09:47 PM. Reason: Typos
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Old 17-06-2015, 09:56 PM
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For my 2c worth it's simply a matter of what looks best aesthetically- unless you're trying to match an image from the Hubble, in which case take onboard all of the scientific knowledge available and lay your colour cards on the table!
When it comes to actual science, the images aren't pretty. RAW files are calibrated (Bias, Flat and Dark) and that's it. No other modifications can be done because anything else manipulates the pixel intensities. Flats also become pretty much the most important calibration frame because pixel to pixel sensitivity variations can cause real havoc :-)
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Old 18-06-2015, 07:21 AM
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I think the technical art argument works the best. So what I get from this discussion I have permission from everyone to produce some green star images and if anyone complains it'll be met with - but its technical art

So really its technical art but don't get too arty eh?

Greg.
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Old 18-06-2015, 05:31 PM
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Go for your life! Your creating art
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