Ingo,
If your neb turned out yellow, adding more blue is only going to turn it green. Sounds like your colour balance ratios are out for the other channels too. Based on your exposure lengths, you are using a basic 1:1:2 ratio. As Steven points out, the ratio is likely to be higher in the blue, so you’ll need to increase the length of your blue exposures. Keep an eye on the R and G channels as well. I doubt they are 1:1 respectively.
If you are planning on using exposure lengths for the method of balancing images, keep in mind that this method is dependant on S/N ratio. i.e the channel (say blue) will have a high signal that the R and G channels for the image to colour balance correctly. While this sounds relatively simple in theory, I’ve struggled to balance images well using this technique. The problem is…when you combine your individual R, G, B exposures, the combine method you use will naturally alter the S/N. The only time I could get this method to work was using the a Sum combine – which in all honesty…is crap, because it does little in the way of noise reduction. Median or Sigma Reject/clip is a far better options, but these alter the S/N.
By all means try the exposure length method, but you may wish to start with taking equal exposures through the individual filters, then colour combine them using different ratios.
To find your correct colour balance based on a G2V star, follow these steps. 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.skymap.com/g2v.htm
- 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 (don't forget to dark,bias,flat process them!).
- Register and average or median (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 encompass 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. My method above method can be also used for colour imaging with UVBRI filters – in particular BVR (not so much U and I as they are out of the visual spectrum).
Finally, make sure you "normalise" you individual combined R,G,B files. If the average background ADU is brighter in one of the channels, the image will not balance well. MaximDL has a automatic normalisation feature, though you usually get better results normalising manually. Analyse each of the R,G,B combined files - look at the darkest area (same location) in each image, then use pixel math to subtract a determined value from each of the images so that the average background ADU is equal (or close enough) to 50 (others may use a different value, but 50 works ok for me). The component images should then balance correctly. Make sure you don't measure the average background ADU where there is nebulosity or a gradient is present as this will through out your figures. Sample a few corners of the image usually works.
A little pain for long term gain... Welcome to the world of monochrome imaging with R,G,B filters! Those OSC guys have it too easy!
UPDATE: I've attached a DB search dump from TheSky listing the G2V stars - enjoy
