Adding Ha to RGB/LRGB images using Pixel Math ( PixInsight Tutorial)

 Pixel Math is one of the most powerful tools available in PixInsight. Then it shouldn't come as a surprise that even the simplest of codes in Pixel Math can do so many things ranging from creating NB combinations to performing magenta star removal. Here we will discuss about using Pixel Math to blend Ha data to RGB. This method works best with Linear ( Non-Stretched ) data so make sure the data is not stretched. There are two different ways to perform this although the results are nearly identical .Both method involves manipulating the Red Channel with the Ha data. If you want to use the NBRGB script in PixInsight, the tutorial is here.

I) Light Vortex Astronomy Method : 

                           I came across this method in the Light Vortex Astronomy site. The steps are as follows : 

1) Using the channel extractor tool, extract the Red Channel from the RGB color image as shown below 

Rename the extracted Red Channel as RGB_R and the Ha data as HA. Its better to rename it to avoid PixInsight file name related errors. Name is case sensitive.

Now use the following exrpression in Pix Insight : 

RGB/K : ((HA * R_bandwidth) - (RGB_R * HA_bandwidth)) / (R_bandwidth - HA_bandwidth)

Symbols: R_bandwidth=40, HA_bandwidth=3

Make sure the "Use a single RGB/K expression checkbox is checked and Create new Image radio button is selected as shown below : 

Now that you have created the RGB_R_HA composite use the following expression in Pixel Math and drag and drop it onto your main RGB image(From which Red channel was extracted).

R/K : $T + ((RGB_R_HA- Med(RGB_R_HA)) * BoostFactor)

G: $T

B: $T

Symbols: BoostFactor=1.0

Radio Button " Replace target image" has to be selected this time and drag the triangle onto the image(RGB). This is how the expression looks in Pixel Math : 

You have successfully blended the Ha with the RGB. Now the one parameter that you may try to fiddle with here is the BoostFactor setting. You can try increasing the values if you want to add more Ha to the Red channel regions as the Green and Blue channels are left untouched in the RGB image.The left image shows a BoostFactor of 1 where as the right shows 5.

II) A modified Pixel Math method : 

 This is a bit modified method whose first steps are almost identical with the previous method. Here use the following expression in Pixel Math (Assuming the Red channel is named as RGB_R and Ha is HA)

RGB/K : HA - Q *(R - med(RGB_R))

Symbol : Q = .143

Rename the file as Ha_Clean

Make sure the "Use a single RGB/K expression checkbox is checked and Create new Image radio button is selected as shown below : 

In the next part use the following expression and drop the Pixel Math expression onto the RGB image : 

R/K : $T + B * ( Ha_Clean - med ( Ha_Clean))

G : $T

B : $T + B * .2 *  ( Ha_Clean - med ( Ha_Clean))

Symbol : B = 3 

Radio Button " Replace target image" has to be selected this time and drag the triangle onto the image(RGB). This is how the expression looks in Pixel Math : 

The results are almsost identical and as you imagined, you can play with the B value to boost the saturation levels if intended. The image on left has B on 3 and the right one has B value of 5

Hope you guys enjoyed this tutorial. Let me know how it worked for you and also holla at me if you need help.