Thread: star colours
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Old 17-11-2014, 04:04 PM
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Shiraz (Ray)
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Originally Posted by irwjager View Post
Hi Alistair,

The conclusion above would be true for all software (that I know of) *except* StarTools.

StarTools (if you let it) recovers true RGB ratios by using the Color module which, thanks to the whole Tracking thing, knows how to 'unstretch' all your data, grab the right colors and apply them after stretching everything back. This is the reason why it is highly recommended to do your final colour calibration *after* you've brought out all your detail (something that would be pretty much nonsensical in other software as colour calibration normally requires linear data).

The benefits of this are pretty massive. As it actually allows for colours that are wholly independent of who shot it, independent of exposure times, independent of your luminance processing and independent of the characteristics of your gear. This is why it's called 'Scientific' mode; it is based on the assumption that colour *ratios* out there are constant, don't change and therefore should not change in your image no matter its luminance characteristics.

Then there is the procedure of colour calibration. In StarTools there a no less than 6 ways to calibrate your image.
  1. using a white reference and simply clicking on it
  2. using a suitable galaxy as a white reference in a mask
  3. using a wide enough star field as a white reference (in a mask)
  4. using a G2V star as a white reference
  5. using the aggregate of any diffraction spikes in a mask or by eyeballing
  6. using reasoned (by knowing about the composition of an object) calibration combined with the MaxRGB mode (which even lets you calibrate using non-calibrated screens)
The latter is (afaik) a new technique where (reasoned) channel dominance in known areas are used to calibrate the image. In the case of your NGC6744 image for example, we know that the core should be dominated by yellow (older stars) whereas the outer rim should be blueish (dominated by younger more luminous stars). The disk will be peppered by purplish HII areas (predominantly caused by a mixture of red Ha and cyan-ish Hb emissions). Lastly we know that green-dominant features are very rare (with some notable OIII-rich exceptions, such as M42's core or the Tarantula nebula). If green is dominant, 9 times out of 10 you should be backing off on the green ratio. A little green sprinkled around in your image is fine as long as it fits the colour noise signature of your image.

Finally, we also know that in a wide enough field all star temperatures should be represented equally, e.g. there should equally as many red, orange, yellow, white and blue stars scattered around the image which can serve as a final sanity check for your colour balance.

Hope this helps somewhat!
I am a bit confused here and would be grateful for some feedback from Ivo - hope you will allow this question Alistair, but I think that it fits with your original post.

Thanks for the very clear explanation Ivo, but my basic problem in understanding it is this: if colour is processed separately from brightness, then surely dynamic range must be affected - I can't see how you can simultaneously have full dynamic range and deep saturation. For example, if a part of an image is saturated red, that means that the blue and green emitters in the screen are turned off. Ignoring what the eye does, this means that fully saturated colours can never produce more than 1/3 the brightness of white regions (where all 3 emitters are on). Once you get above 1/3 max scene brightness, you cannot display deeply saturated colours and the brightest parts of an image can really only be white if you want to use the full dynamic range of a display. Have I got this right and if so, what does it mean for colour in astro images? I think that it means that the brightest parts of an image (eg star cores) should not be coloured, but am I missing something here?

Last edited by Shiraz; 17-11-2014 at 05:12 PM.
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