Original Bicolor Technique for combining Ha and OIII images

All Images and Content  Copyright Steve Cannistra unless otherwise noted.

Please note:  I keep this page for historical reference only, since I have modified this technique as of 12/31/05.  The recommended bicolor technique can be found here.

   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).

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.

Modification 12/31/05:  Please click here for another version of this technique which uses the Hue/Saturation adjustment layer instead of the Color Balance layer.

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 hereLabel 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 "lighten" blend mode to the OIII (blue channel) layer; assign "multiply" blend mode to the OIII (synthetic green) layer; assign "screen" blend mode to Ha (synthetic green) layer.

6.  Create a clipping mask using the OIII (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
.  First, click on the Ha (red channel) layer, go to "Layers, New Adjustment Layer, Color Balance".  Be sure to assign this as a clipping mask.  You now have a Color Balance Adjustment Layer for the Ha (red channel)Label as indicatedDon't adjust it now- we will do this shortly.

8. Do the same for the other two channels (note the location of the Color Balance layer for the synthetic green group!)Label as indicated.

9. Now we are ready to colorize each channel
.  This step involves adjusting the sliders of each Color Balance layer.  Here is a suggested slider profile that will serve as a good starting point:

Color Balance (red) layer:
Shadows: +90, 0, 0
Midtones: +90, 0, 0
Highlights: 0, 0, -20

Color Balance (green) layer:
Shadows: 0, +90, +20
Midtones: 0, +90, -70
Highlights:  0, +20, 0

Color Balance (blue) layer:
Shadows: 0, 0, +30
Midtones: 0, +20, +20
Highlights: 0, +50, +50

10. Addendum (10/29/05)- I have used this technique quite a bit now and find that these starting values do not need further adjustment.  Rather, additional adjustments at this point should be made in 3 ways: 

    A) If there is clipping in the R, G, or B channels, simply decrease the opacity setting of the respective color adjustment layer for that channel, until it is corrected.

    B) If the histogram shows that the color channels do not start at the same blackpoint, simply adjust them individually using a "Levels" adjustment layer so as to achieve a neutral background (i.e., bring the black point up to the histogram rise for each channel, so that all colors start at roughly the same point on the histogram curve).

    C) The most important adjustments are now done using the
"Selective Color" layer as a top layer (no clipping mask).  You can also right click next to each layer in order to assign it an identifying color, which makes it easier to work with.  Here's an example of a final layers window.

11.  You may also choose to flatten this image and use it as color for the Ha luminance channel, for a final composite of Ha(Ha:sG:OIII).

12.  Here is an example of the technique in action, using only the Ha and OIII channels from a Hubble Palette image of M8 from Richard Crisp:

Bicolor Processed Image

Richard Crisp's original

These two images should allow you to directly compare the Bicolor Technique with
Hubble Palette processing.  Although there is loss of information when using data from only two channels as opposed to three, the Ha:sG:OIII image still captures a lot of detail and has a pleasing quality to it (although this is subjective).  I have also used this technique to process red and blue broadband filtered images obtained from the Digitized Sky Survey, as shown here.

The Ha:sG:OIII technique takes some patience to master, but I believe that it represents a nice way of achieving a pleasing narrowband image using data from only two filters (Ha and OIII).

Happy moonlight imaging!


All Images and Content  Copyright Steve Cannistra unless otherwise noted.