Shooting flats to get a colour balance ratio... I'd question the accuracy of such a process!
There are a few ways of calculating your colour balance for a specific imaging train. Ideally, you should perform the calibration through each optical system (telescopes, lenses etc) that you intended to use. Don’t expect the weighting for one optical system to be the same for others – even if you’re using the same CCD camera and filters!
There are many resources on the web that tell you how to do perform the calibration and there is more than one-way of doing it. Here’s how I do it. It may seem a little daunting at first, but once you’ve done it a couple of times it’s straight forward.
- Firstly, look up a table of G2V stars and select one close to zenith. You want to minimise as much extinction factor as possible. Also choose a star that isn’t going to saturate easily. There is a great reference table here - http://www.gemini.edu/sciops/instrum.../g2vstars.html
- Once you’ve selected your target star, begin to focus the star as best you can through your optical system. Proceed to take 10 or more equal exposures through each individual filter (R, G, B). I generally go for 30 second exposures, but its up to you.
- Register and average (combine) the frames.
- In MaximDL or other astro imaging software, I then measure the intensity of the G2V star in each frame (R, G, B). Make sure you’ve selected the right star!!! This sounds stupid, but when you’ve got a huge FOV you may need to reference your astro planning software or star charts. You need to ensure the measurement encompasses the entire star, not just the core.
- Take note of what the intensity values are;
As an example:
R = 600,000
G = 700,000
B = 500,000
- So based on these (example – don’t use these!) figures, you can see the example optical system is most sensitive in the green wavelengths. What ever is the most sensitive channel, make that ratio value of 1. So now that we have established G = 1, we can compute the R and B weights. If you divide the green number (700,000) by the red number (600,000) you get 1.16. Divide the green number by the blue number you get 1.4.
- So your calibrated RGB weightings are R:1.6, G:1,B:1.4 for that specific optical system/filter/camera combination.
From this point on, you can take equal exposure lengths as you image certain objects and use the correct weightings for each channel to obtain the right balance.
You should perform this even if you’re using true-balance filters such as those made by Astrodon. You’ll find the spectral response of your entire optical train will not deliver a RGB of 1:1:1. There are likely to be subtle variations.