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Colour calibration, an idea..?

ollypenrice

By ollypenrice
in Imaging - Discussion

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ollypenrice

ollypenrice

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The best G2V star available is the sun. Surely we could find a way to make it possible to calibrate our RGB values using that?

Plan A for discussion;

-Make an extended tube that fits over the objective.

-The outer end of the tube is completely blocked by a disc.

-In the disc are pierced a small collection of tiny pinholes.

-Behind these are two or three diffusers as used in making light boxes. They must be chosen so as to have no effect on the spectrum, perhaps frosted glass, perhaps white T shirt, whatever. I have a hunch that perspex might distort the spectrum.

This would allow a well diffused and minimal amount of sunlight onto the chip.

From this point on you follow the G2V routine as usual, adjusting your colour balance until the flat field you have taken looks white.

What do you think?

Olly

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RobH

RobH

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I still reckon it might be too bright Olly, but maybe worth giving it a go....it'd have to be around noon to avoid atmospheric refraction.

TBH, I question the whole G2V logic though....the sun isn't white so if you calibrate on a sun type star, your colour balance will be off.

Maybe calibrating on a pure white lightsource would be better?

Rob

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ollypenrice

ollypenrice

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Hmmm... but what scientists call White Light is the full solar spectrum.

I don't think it would be too bright because not much gets through a pinhole and even that would be greatly diffused. Keeping light out of the camera in the daytime is often a real beggar for me but I think it could be done.

As for atmospheric refraction, I see your point but that is going to apply to the stars in your astro images as well, isn't it? And the sun really only reddens drastically when very low.

Olly

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brianb

brianb

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I don't like the idea of using a diffuser, there's bound to be some spectral shift.

Why not just use the reflection of the sun in a ball bearing?

It doesn't matter whether the sun is "white" or not, so long as the colour is the same. (Same balance of red / green / blue of the pixels within the disc representing the star's image.) Calibration should help you get this right.

And G2V is a fairly coarse specification - there is a small spread of photospheric temperatures in the continuum between what would be G1V and what would be G3V, and the metallicity of the photospheric composition can vary quite a lot depending on when & where the star formed. The effect on colour balance is not large, probably you wouldn't be able to detect it by eye even if you have exceptionally good colour vision.

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ollypenrice

ollypenrice

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Teadwarf, I was thnking that it would be easier to find, bright enough(!!!) and would not consume imaging time, something that I hate to waste even though I get a lot of it.

Brian, the BB idea is interesting. I have used this method for collimation. You would need a big black background behind the BB and I wonder whether stray light would, even then, wipe out the image. But yes, I will try it some time soon.

Olly

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brianb

brianb

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You would need a big black background behind the BB and I wonder whether stray light would, even then, wipe out the image.

I don't see why ... if you take a circle of pixels within the FHWM of the focused image of the reflection, there will be very little "unwanted" light in there ...

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mcrossley

mcrossley

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Hmm... what we subjectivity call 'white' - a piece of white paper for example - in sunlight is our human white reference is it not? This is what we would calibrate a 'normal' daytime photograph to, even if G2V stars are classified as slightly yellow. Now unlike cameras, human eye's are remarkably adaptive and a piece of 'white' paper remains white in late afternoon, or early morning when atmospheric extinction has shifted the colour significantly as far as a camera is concerned. So we should calibrate our white around midday to minimise extinction - and tough on those that live at high latitudes! :D

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ollypenrice

ollypenrice

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All good stuff here. Thanks for the feedback. Maybe not such a great idea after all!

I may drop Gerd Neuman a line to see if he could come up with a ready made RGB calibration device of some kind. It is right up his street I'd have thought.

Olly

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brianb

brianb

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6500 Kelvin seems to ring a bell.

Hmmm .... solar photosphere is approx. 5800K, the "sunlight" colour balance on DSLRs is around 5200K - the difference is that much blue light is scattered by the atmosphere. If you set your custom white balance to 6500K and take an image in normal sunlight, it will be noticeably orange hued.

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RobH

RobH

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Just arrived home from a long journey and picked up on the thread, and glad it's turned out interesting with lots of feedback :D

Brian, a great idea with the ball bearing.

Merlin66's white paper approach is my favourite due to the fact that there's no potential brightness issue really and the fact that as mcrossley pointed out, we view white as the colour of white paper.

I would say that this is close enough, certainly closer than our respective screens will be showing.

I also agree that my premise of G2V stars etc not being completely white is probably such a fine distinction as to make no real difference too.

It would be great if we could try different methods and compare notes. There's a lot of ability and experience in this forum, and I'm sure we could refine certain aspects of the art we practice with a bit of research from time to time :)

Cheers

Rob

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brianb

brianb

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You'll find 'daylight' lightbulbs/flourescent tubes are 6500k

Don't know which ones these are ... "white" fluorescent tubes are around 4000K, electronic flash is around 6000K and is noticeably bluish in daylight ... 6500K would be a mix of about 1/2 "white cloud" illuminated by sunlight and 1/2 light from a clear blue sky. You need several mireds colour shift to correct this with daylight balanced colour film, 81B or 81C filter will do nicely. Of course with digital cameras you don't need the filter, but you still do need to balance the colour at some stage.

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roundycat

roundycat

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I found the basic data for this a while back but have never put it to any kind of test. I seem to recall it was done by somebody that was cleverer than your average bear. Personally I would not recommend using values of 255 at any stage in the processing but it might be allowable right at the very end after sharpening. Dropping each value by 4-5 would leave basically the same colour, just a bit less liable to saturation. If you have such a star and know its 'proper' colour (Spectral Class) using the high value ink dropper in Curves will allow you to make global changes to the colour of the picture. (double click to get the colour picker). Depending on how uneven your initial stretching was your mileage may vary.

Dennis

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