RGB Composition of Dual Narrowband Images

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Using the L-eXtreme dual narrowband filter allows taking quite impressive pictures of various nebulae. However, it will create quite unnaturally looking star colors and will filter out features of the object that are not particularly emitting the spectral lines of Ha or OIII.  Some objects, due to their physical structure, look quite different in the narrowband than in images taken over the full RGB continuum. The crab nebula M1 is a good example for this.


M1 the crab nebula in HaOIII, 40x90sec
C9.25 on EQ6-R Pro, ASI294MC PRO, L-eXtreme


M1 the crab nebula unfiltered RGB image,
40x60sec, C9.25 on EQ6-R Pro, ASI 294MC Pro


It is interesting to look at and compare these pictures individually.  However, I wanted to try combining them in a meaningful way. Basically I had five different "channels" - red, green, blue, Ha and OIII and was looking for good ways to compose an image from that. There are several ways for doing that, some more flexible than others. I ended up using Astro Pixel Processor (APP) using the following steps:

  • register all raw images to align them
  • stack and color correct the RGB images as usual 
  • stack the filtered images and create an Ha and an OIII image using corresponding function un APP
  • use the APP function to remove stars on the Ha and OIII images
  • adjust the histogram on the Ha and OIII images to make the background black
  • use the RGB composition tool in APP to create various flavors of combined images


The RGB composition tool in APP
the individual channels can be flexibly mapped to 
RGB and Luminance


Composite RGBHOO image


The same exercise carried out with pictures of "Thor’s Helmet":


NGC2350 "Thor’s Helmet", 25x120sec
C9.25 on EQ6-R Pro, Reducer 1/6.3,ASI294MCPro, L-eXtreme 

NGC2350 "Thor’s Helmet", 25x90sec
C9.25 on EQ6-R Pro, Reducer 1/6.3,ASI294MCPro


Composite RGBHOO image of pictures above


















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