Aligning RGB Channels

There is a check box in Deep Sky Stacker's options to do this automatically for you but, if you find you've already created your stacked images and the colours aren't aligned, read on.

 

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If you find that your colour channels weren't properly aligned in your stacking software, there's a very simple way to rectify this in Photoshop and this should be done as soon as you become aware of a misalignment, as it will skew your images if you do it later on in processing.

You'll most likely notice the RGB misalignment when you're using curves and are surprised to see that there are multiple grey peaks rather than the usual single peak in the histogram - see the diagram (left). Or you may just realise visually that the background sky or foreground object colours are a bit iffy. But, fear not, here's the remedy:

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1. If you click on the Histogram icon (indicated by the top/middle arrow in the diagram on the right) you'll see the mis-alignment in the top left window where the R,G&B peaks overlap rather than being combined into a single peak.

By seeing how far the the individual R,G&B peaks are from the left hand side, in the 3 other windows on the right, you can tell how misaligned they are.

What we want to do in this case is lower the red levels and raise the blue levels to match the green level.

Firstly click on the 'Channels' tab on the palette (indicated with the right hand side arrrow).

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2. The colours are combined as default so select the red channel in the palette and change the channel dropdown box option to 'Colors' (shown by the top left hand arrow). 

Then open Levels using ctrl+l and increase the Input Levels' black point (shown by the middle arrow) until the red peak sneaks in behind the green peak in the 3 peak window (top left). Try to get the red peak to fit evenly behind the green so that the middle point in the red peak aligns with the centre of the green peak.

Don't worry if one peak is wider than the other, just align them as precisely as you can. 

When happy, click OK.

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3. Next up is the blue channel.  Select the blue channel in the palette and select 'colors' in the channel dropdown box (as Photoshop resets this setting).

Then, again, open Levels using ctrl+l but this time increase the Output Levels' black point (shown by the middle arrow) until the blue peak sneaks in behind the green and red peak in the 3 peak window (top left). Try to get the blue peak to fit evenly behind the others.

You want to finish up here with one single peak with as little showing either side as possible. When happy click OK.

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4. When your histogram window looks like this, you've aligned your red, green and blue channels properly. 

Now click the Histogram icon (greyed out on the bottom left of this smaller image) to close this feature as you wont need it anymore. 

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5. Make sure you click on the RGB channel in the palette (right hand side of the screen marked by the arrow) otherwise when you make any more changes it will continue to only make changes to the selected blue channel!

Finally, click on the LAYERS tab in the palette to return to the screen where you can continue with curves - but now with RGB aligned images.