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.
Insert star information back into a tonemapped image.
Option 2: (click here)Add Ha star information to the SII and OIII channels.
