Sunday, September 13, 2015

Bringing out OIII and SII details in narrowband images

Most of the time, the Ha signal in emission nebulae is MUCH stronger then the OIII or SII signal. Take my recent image of SH2-112 as an example:

Ha:

OIII:

SII:

All these are stretched and look similar in strength. But combining them results in this:

Increasing the red component brings out more SII details but also leads to a very red background:

To avoid that, I extract the luminance of this image:

Using HistrogramTransformation, I Auto zero shadows and highlights and use this as a mask for this image. The same red stretch now results in:

The sky stays dark and more SII details in the nebula. I don't worry about the magenta stars as I will fix them at the end anyway.