Right Declination Astrophotography

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

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)

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

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