Color Ratio Calculator
To test
the relative color balance of your imaging system, you will need to image a
solar analog star and measure the variation in brightness through each
filter. This is easily done. Imager Bart Declerq recommends imaging
an out-of-focus G-type star, preferably near the zenith. If no star is
available so high in the sky, an atmospheric extinction correction factor can
be applied using the chart shown in the next section.
Measurement
of the star brightness can be done using the Information tool in Maxim DL function in other image processing software.
20 second exposure
Take one exposure through each color filter, red, green, and
blue.
Choose
an exposure that yields a brightness between 10,000 to 50,000 ADU(bright enough for a good
signal but not saturated).
Use the identical exposure for each filter.
Measure
the average brightness of the out-of-focus star in each image. The value
should be slightly different based on the characteristics of the CCD chip and
filter set. For example, the measured value of the star might be as
follows:
Red
Value: 19,000
Green Value: 25,000
Blue Value: 14,000
Green Value: 25,000
Blue Value: 14,000
The
color ratios are determined as follows:
Red
Correction Factor = 1/(Red Value/Maximum Value)
Green Correction Factor = 1/(Green Value/Maximum Value)
Blue Correction Factor = 1/(Blue Value/Maximum Value)
Green Correction Factor = 1/(Green Value/Maximum Value)
Blue Correction Factor = 1/(Blue Value/Maximum Value)
In the
above example, the green value is the maximum value so the correction factors
would be:
Red
Factor = 1/(19,000/25,000) = 1/0.76 = 1.32
Green Factor = 1/(25,000/25,000) = 1/1 = 1.00
Blue Factor = 1/(14,000/25,000) = 1/0.56 = 1.79
Green Factor = 1/(25,000/25,000) = 1/1 = 1.00
Blue Factor = 1/(14,000/25,000) = 1/0.56 = 1.79
These
value yield the 1.3:1.0:1.8 RGB ratio used on the Whirlpool Galaxy example
image above. Most cameras have the greatest sensitivity in green or red
and therefore green or red is normally the basis for comparison, but some
cameras (notably the popular ST-2000) have higher blue sensitivities and might
yield a ratio more like 1.7:1.3:1.0 in RGB.
You might
use exposure times of 130 minutes, 100 minutes, and 180 minutes in red, green,
and blue, respectively, to obtain proper color balance.
using the
green (1) as the base
Green = 1
Blue = 1.8
Red = 1.3
If taking
10 frames x 600 second, I use the same ratio to apply the difference exposure
time.
For example
Green
= 1 = 10 x600 seconds = 6000 /60 = 100 Mins
Blue =
1.8 = 10 x 1080 seconds or 18 x 600 = 10800 =180 mins
Red =
1.3 = 13 x 600 = 7800 sec = 130 mins
Summery
Color Factor
Green Blue Red
1 1.8 1.3
Instead of
changing the color ratio you can change the exposure time which will do the
same effect as changing the ratio
Exposure
100 min 180min 130min
