Author Topic: About our color calibration methodology  (Read 48581 times)

Offline Juan Conejero

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Re: About our color calibration methodology
« Reply #30 on: 2010 November 18 01:28:06 »
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Credibility can be diminished not when someone cheats, but when they don't provide enough proof (burden of proof).
And that is what Vicent did this time with his first post, just three jpegs and "have a go at it"... which is the post that caused me to warn him to be careful.

I disagree. I think Vicent provided a good amount of proof:

- He published the raw image, the image calibrated with eXcalibrator, and the image calibrated with PixInsight. These are not "just three jpegs"; they are stretched JPEG versions of the working images. Obviously, since the working images are linear —because a color calibration procedure cannot be carried out with nonlinear images—, the JPEG versions have been stretched and slightly processed in order to evaluate the results; otherwise they would be almost black! We figured that both facts —linearity of the data and the need to show stretched versions— are so obvious that it was unnecessary to comment on them. Of course, both resulting images received strictly the same treatment, except the color calibration step which is the target of the test, because otherwise the test would be invalid. That's completely out of question and again is so obvious that no explanations seemed necessary.

- Detailed numeric data about the eXcalibrator calibration: the number of stars used (12), the survey (Sloan), the dispersion in the results (0.02 in G and 0.03 in B), and the resulting weights (R=1, G=0.746 and B=0.682).

- Detailed data about the color calibration performed with ColorCalibration in PixInsight: the galaxy used as white reference (M66) and the resulting weights (R=1.055, G=1.1 and B=1).

We are used to work —especially Vicent— in academic environments, so we really know something on how to make objective tests and comparisons. Many examples and tests published in professional academic scientific papers provide less data than what I've described above.

Of course one can ask for more explanations, detailed descriptions and more quantitative data (although in this case there are really no more quantitative data beyond the numbers that have already been published). Those requests are logical; I know Vicent is working on an elaborated and explanatory step-by-step example right now, including screenshots.

The quality and value of an objective test can be criticized or questioned based on objective technical criteria. It can also be discussed based on conceptual and philosophical considerations. However, what cannot happen —should never happen— is that the credibility of the author gets diminished because the JPEG versions of the resulting images are not as somebody would expect, or because they just "look weird". That is an unscientific attitude.

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The talk-behind-the-back in this discipline (and so many others) sometimes is quite bad, unfortunately.

That's true but I am not interested in that. So case closed for me too.
Juan Conejero
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Offline georg.viehoever

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Re: About our color calibration methodology
« Reply #31 on: 2010 November 18 08:04:25 »
Regarding the question why we usually dont see green casts in normal images: This might come from the fact that using HistogramTransforms reduces the strength of colors for bright objects. I did experiments on this in http://pixinsight.com/forum/index.php?topic=1689.msg10371#msg10371, see top row of screenshot. You can recover some of the color by using a strong saturation boost, and color then is most prominent in not-so-bright portions of the image. This might be the reasons why in Vicent's image, the green cast in mostly visible in halo of stars or with rather dim stars.

Just a theory.

Georg
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Offline edif300

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Re: About our color calibration methodology
« Reply #32 on: 2010 November 19 14:37:11 »
Even if we aren't able to see them in green colour, are there any green stars in the universe ?

With a perfect filters... on which colour would be registered in a ccd?
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Offline georg.viehoever

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Re: About our color calibration methodology
« Reply #33 on: 2010 November 22 01:33:00 »
Hi Edif,

actually, the sun is a star that emits most of its energy in green light. Only our visual system is calibrated to see it as white .

All stars (as opposed to some nebula, such as H-alpha) emit a continuous spectrum of light. The balance of light depends on their temperature - this is why we perceive hot stars as blue and cool stars as red. Depending on how we weight the different contributions of light, we will see different colors - it is easy to choose a white balance that will turn Arkturus into a white star...

See http://pixinsight.com/forum/index.php?topic=2562.msg17275#msg17275 for additional information.

Georg
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Offline edif300

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Re: About our color calibration methodology
« Reply #34 on: 2010 November 22 13:17:25 »
Thanks Georg for reply.

Best regards,
Iñaki
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Offline neuling

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Re: About our color calibration methodology
« Reply #35 on: 2012 June 10 09:30:30 »
What do you think about the methode used in Theli, the images are registered on a star catalog and a colorcalibration is also used on it.

Offline Geoff

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Re: About our color calibration methodology
« Reply #36 on: 2012 June 10 15:21:12 »
Hi Edif,

actually, the sun is a star that emits most of its energy in green light. Only our visual system is calibrated to see it as white .
And our visual system can change its calibration very rapidly. Move to a planet orbiting a red dwarf and it will look white within a day.  Another example I like is the experience of looking at a computer shielded by a red screen while photographing. The screen looks quite normal once you have been looking at it for a while. Glance quickly at Jupiter after looking at the screen and Jupiter is a nice green colour.
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Offline Juan Conejero

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Re: About our color calibration methodology
« Reply #37 on: 2012 June 13 00:27:27 »
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What do you think about the methode used in Theli, the images are registered on a star catalog and a colorcalibration is also used on it.

Another implementation of the G2V method. The problem is not with the accuracy of star measurements (although there are significant errors in the photometry data of large catalogs). Color in deep-sky astrophotography is a purely conceptual matter. The accuracy of an implementation is much less important than the reasons and the understanding behind the decision of choosing a particular type of objects as a white reference.

I have already sufficiently explained my opinions about the ideas of "real color" and "natural color" in DS AP. I have also said what I think about the G2V method many times. Color in DS AP isn't a matter of "how", but a matter of "why".
Juan Conejero
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