Although a lot of the work done here could possibly be done in a single application, I find the most comfortable workflow involves the use of a number of programs. Most of the work will be done here in GIMP 3 but pixelmath will be done in Siril. Stacking, de-noising star removal and de-linearising were done in PixInsight.
Creation of a palette from grayscale images captured through Ha, O3 and S2 narrowband filters.
The data used here are of the Tadpoles nebula IC410, LBN807. Captured by AstroDMx Capture with a QHY Minicam8 through a William Optics 81mm APO refractor with a 0.8 reducer/flattener. The Ha, O3 and S2 data were stacked and part processed in PixInsight.
The monochrome stacked images have their stars removed and are de-noised. Then the starless images are stretched with Curves and then Levels in GIMP 3 so that their histograms are similar and have the same means as closely as possible. This gives the images similar ranges of brightness from darkest to lightest,
H-alpha image
O3 image
S2 image
Producing an SHO image by mapping the S2, Ha and O3 images to R, G and B
Assigning monochrome images to RGB channels
Produces the resulting SHO image of the Tadpoles nebula
The stars can be returned by pasting the stars image onto the starless images and select Screen mode. With the pasted layer in focus, Curves can be used to adjust their prominence to the desired degree before flattening the image.
The other seven narrowband palettes: OHS, OSH, HOS, HSO, SOH, HOO and SOO can be made by the same method in GIMP 3.
There is another group of narrowband palettes that are produced by pixelmath and that was done here in Siril. These palettes are the Gendler palette, the ForaaX palette, the Natural Palette and pixelmath variations on these.
The Gendler palette constructed by pixelmath in Siril
The Gendler palette of the Tadpoles nebula RGB rendering
The stars can be added back as before
Because each channel is a complex pixelmath construct, another process called channel shifting is used to contruct a further five palette renderings from a palette such as the Gendler palette.
The Gendler palette image is decomposed into its RGB channels as layers
Decomposing to layers
Decomposed image with the three monochrome layers shown at the right hand side of the screen
The RGB channels are re-mapped in a different order to BRG (channel-shifting)
The RGB image has been channel-shifted to BRG a totally different rendering
By this procedure, it is possible to produce all five of the channel-shifted renderings of the Gendler palette in addition to the original RGB rendering. The stars are Screened into the image as before, and their prominence adjusted with Curves before flattening the image.
Gendler palette BRG rendering
Gendler palette GBR rendering
Gendler palette BGR rendering
Gendler palette RBG rendering
Gendler palette GRB rendering
It is, of course, possible to similarly construct channel-shifted variations of any pixelmath generated palette.
It is no surprise that the six basic narrowband palettes: HOS, OHS, HSO, SHO, SOH and OSH are simply channel-shifted versions of each other, although they are more conveniently constructed by channel combination from the original monochrome Ha, S2 and O3 images.
It is worth exploring the channel-shifted variations of any pixelmath generated palette as they each reveal the structure of the nebulosity in different ways and some may be more aesthetically pleasing than others.
