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Modified Bicolor Technique for combining Ha and OIII images
All Images and Content  Copyright Steve Cannistra unless otherwise noted.
Please note:  As described below, I have made two modifications to the original bicolor technique.  As before, the bicolor method creates a synthetic green channel, but the following modifications affect the way that each channel is colorized:
1) Instead of using "lighten" mode for the OIII (blue channel) layer, use "screen" mode.
2) Instead of using the "Color Balance" adjustment layer, use the "Hue/Saturation" layer as a clipping mask in colorized mode.  This strategy was developed by Travis Rector and is more fully described by Ken Crawford (download Ken's step-by-step procedure here).

Introduction:   Narrowband filters such as those that pass SII, Ha, and OIII light permit moonlight imaging of supernova remnants, planetary nebulae, and several diffuse nebulae (such as IC1805, M16, IC1396, etc.).  Combining images from narrowband filters is often done using the Hubble tricolor palette, in which SII, Ha, and OIII are assigned to R, G, and B, respectively.  Another technique adopted by the Canadian-France-Hawaii Telescope team (CFHT) is the sequence Ha, OIII, and SII (for R, G, B, respectively).  With either of these techniques, the resulting image is portrayed in "false color" and takes some getting used to.   In addition, star colors are not well-preserved, and many images using the Hubble palette are characterized by red halos that are the result of greater bloating of stars with the SII filter (partly due to the need for increased stretching of this channel).

I have developed a processing method that only requires data from Ha and OIII filters, in an attempt to 1) decrease the amount of imaging time necessary for each target, 2) produce a more "realistic" looking color image, and 3) still preserve some of the unique appearance of narrowband imaging.  The technique described below accomplishes these goals by creating a novel synthetic green channel ("sG"), by using data from Ha and OIII.  I refer to the final color composite as Ha:sG:OIII.  The colors are pleasing and are representative of Ha (red) and OIII (blue), ionization fronts are yellow, and the stars do not have halos (although they are relatively colorless, which is a characteristic of this technique).  The technique may also be applied to conventional Red and Blue broadband images, such as those derived from the Digitized Sky Survey (DSS).  Please click here to view DSS images that I've processed with the bicolor technique.

Brief overview:  Ha is used for the R channel, and OIII is used for the B channel.  The synthetic green channel is created by multiplying the OIII layer with the Ha layer.  Construction of the color composite is done using the layer method in Photoshop CS and should be followed exactly as described for best results.

Method (please click on the indicated links for an illustration of the steps):
1.  Obtain your best Ha and OIII images in the usual manner.  Process as usual.

2.  Layer the images as shown here.  Label as indicated.

3.  Change the Mode to RGB (i.e., "Image, Mode, RGB Color").  When prompted, choose "Don't Merge"!

4.  Duplicate the Ha and OIII layers in anticipation of creating the synthetic green channel.  Label as indicated.

5.  IMPORTANT- Assign "SCREEN" blend mode to the top layer (OIII Image blue channel); assign "MULTIPLY" blend mode to the second layer (OIII Image synthetic green); assign "SCREEN" blend mode to the third layer (Ha Image synthetic green).   Bottom layer (Ha Image red channel) remains in "NORMAL" blend mode.

6.  Create a clipping mask using the OIII image (synthetic green) layer (i.e., click on this layer, then go to "Layer, Create Clipping Mask"). 

7. We will now assign an adjustment layer to each color group, in order to colorize them. 

8.  First, click on the "Ha Image (red channel)" layer, then go to "Layers, New Adjustment Layer, Hue/Saturation".  Be sure to assign this as a clipping mask.  Label this layer "Hue/Saturation (red)."  Don't adjust it now- we will do this shortly.

9. Next, click on the "OIII Image (synthetic green)" layer, then go to "Layers, New Adjustment Layer, Hue/Saturation".  Be sure to assign this as a clipping mask.  Label this layer "Hue/Saturation (green)."  Don't adjust it now- we will do this shortly.

10.  Finally, click on the "OIII Image (blue channel)" layer, then go to "Layers, New Adjustment Layer, Hue/Saturation".  Be sure to assign this as a clipping mask.  Label this layer "Hue/Saturation (blue)."  Don't adjust it now- we will do this shortly.

11. At this point, here's how the layers should look.  Now we are ready to colorize each channel as follows:

A) Open the "Hue/Saturation (red)" layer:   Click "colorize" box, set hue to 360 (or zero), saturation to 100%, lighten to -50 (just a starting point, see #12).  Here's how it might look as you adjust the sliders for the Ha layer.
B) Open the "Hue/Saturation (green)" layer:  Click "colorize" box, set hue to 120, saturation to 100%, lighten to -20 (just a starting point, see #12).
C) Open the "Hue/Saturation (blue)" layer:  Click "colorize" box, set hue to 240, saturation to 100%, lighten to -40 (just a starting point, see #12).  Here's how it might look when you have adjusted each of the three Hue/Saturation layers.

12.  At this point, it's important to look at the histogram for each channel to determine whether the highlights are clipped.  If you have clipping in a given channel, simply open the "Hue/Saturation" layer for that channel and reduce the intensity of the lighten slider until the clipping disappears (do this as you watch the real-time histogram for that channel).  Repeat this for the other two channels as needed.  This is a very important step.
 
13.  You may also notice that the black points of the histograms for each channel might not line up.  This needs to be corrected, since you want each channel to start at the same place.  Simply apply a "Levels" adjustment layer to the image (i.e., place it on top of the other layers), and adjust the black point of each channel so that it is moved to the beginning of the histogram rise.

14.  Finally, I find it useful to apply a "Selective Color" adjustment layer to the image so that you can adjust the red, magental, blue, and cyan to taste.

Have fun!


Steve

All Images and Content  Copyright Steve Cannistra unless otherwise noted.
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