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Narrowband Processing Workflow: Colored Stars from Tri-Color Narrowband Filters

Like many astrophotographers today, I battle a certain amount of light pollution in taking long-exposure astro-photos. Narrowband filters greatly reduce the issues associated with light pollution, as they only allow a narrow wavelength of light through the filter and block wavelengths of light associated with artificial light sources. 

A wonderful and well-known method of processing tri-color narrowband images is JP Metsavainio's "Tonemap" method. I've been using it for a couple of years now and believe that the Tonemap method has become a minor revolution in the area of processing narrowband astro-photos. ( Link to Tonemap Powerpoint Presentation).

Just as a matter of taste I prefer not to overlay stars with RGB color over a narrowband image, so I have developed two processing workflows that aim to produce aesthetically pleasing star color from the original narrowband data, without having to overlay stars on top of an image. Both of the techniques listed below are basically a cross between traditional narrowband processing in which the weaker SII and OIII channels are stretched to match the Ha channel (resulting in magenta stars) and the Tonemap method (which results in white stars with little color).

I make no claims about the scientific accuracy of the resulting images using these methods. I aim to produce "pretty pictures" and the color rendering of narrowband images, for both the object being imaged and the stars in the field, is a subjective endeavor.

Option 1: (click here)

Insert star information back into a tonemapped image.

Option 2: (click here)

Add Ha star information to the SII and OIII channels.