Tutorial RGB weight calculator

In order to have the right color balance in your astronomical images you need to know the amount of light passing through each color filter related to the other ones. The easiest way to do this is to image a purely white star (G2V star), the amount of light for each color filter should be equal. In practice there is always some difference between each color filter. Therefore we have to determine the weighing factors of each filter. I make use of IRIS for measuring the intensities through each color filter. Step one is to make images, through each color filter, of a G2V star. A list of available bright G2V stars is available in the excel file under tab [G2V Stars]. Important is that NO processing is allowed otherwise the balance would be disturbed.

Load the red channel image into IRIS (see figure below).

Then select "Analysis/Aperture Photometry" from the toolbar (see figure below).

Then select "3" under "Circle number" in the pop-up menu (see figure below).

Then point the selection tool on the G2V star and select it (see figure below).

Now you got detailed red light information for the star, the mentioned intensity information has to be put in the table, repeat this also for the other filters (see figures below).

Due to atmospheric extinction the intensity of each color will be influenced by to the altitude of the object.
For this example the altitude of the object was 70 degrees above the horizon.
The corrections factors for each color can be found in the table under tab [Extinction factors] (see figure below).

Put these correction factors in the table and the normalized (to the red filter) weighing factors will be calculated (see figure below).

Ones done the calculator is defined for your filterset. Now you can use the calculator during your imaging sessions.
Simply choose the altitude from the drop down list and fill in the desired exposure time for the red channel (see figure below).

Copyright Rob Kantelberg