Colour Calibration of Mono CCD's

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Colour Calibration of Mono CCD's

Postby martin_h » Wed May 18, 2011 1:02 pm

Someone asked me the other night how I calibrate colour when using coloured filters.
The colours we see are only there because our sun emits light in specific wavelengths that we convert to colour through our eyes and brain...Imagine if our sun was a red giant ! :-o
The sun that we know and love is classed as a G2V star, ie it emits what we know of as white light, so at night we have to find another star in the same class and it needs to be high in the sky to avoid atmospheric extinction (the scattering of blue light at low altitudes)
I use Mizar in U Major. What you need to do is take a single exposure of this slightly out of focus star through each filter (R,G,B) and measure its brightness, the exposuer needs to be high enough for a good signal but not saturated.
Using Photoshop or whatever software measure the brightness of the star and take a note of the readings, they should be slightly different for each filter. ie.

Red filter 19,000
Green filter 25,000
Blue filter 14,000
To determine the colour ratio we use the formula -

Red correction factor=1/(red value/max value)
Green correction factor=1/(green value/max value)
Blue correction factor=1/(blue value/max value)

In the above example Green is the max value at 25,000 so using the formula we get
Red = 1/(19,000/25,000) = 1/0.76 = 1.32
Green = 1/(25,000/25,000) = 1/1 = 1
Blue = 1/(14,000/25,000) = 1/0.56 = 1.79

So the ratios are 1.3 : 1.0 : 1.8 RGB

So for a 5 min exposure we would use Green = 5 mins. Red (5x1.3)=6.5 mins. Blue (5x1.8)=9 mins

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