I would like to present some results for the Ha/OIII combination with the Heaviside function bases on the Wizard nebula (a gas nebula with the open cluster NGC 7380), located in the northern hemisphere. The wizard nebula has a large OIII area, overlapping with larger Ha regions, which makes it a wonderful object for testing the formula.

The images were taken with my standard equipment for nebula at full moon (SQM 19.5). Integration time about 9 hours.

 

Results for stacking with Siril, using a script for Ha/OIII extraction with background reduction for each individual image:

Wizard nebula: Ha

Wizard nebula: OIII

Comparing the two images, it is obvious that the fainter OIII areas will be 'swallowed' by the Ha signal - after a LinearFit the OIII becomes even fainter. Now let's prepare the two images for our PixelMath blending. My typical workflow consists of...

  1. ScreenTransferFunction
  2. Crop stacking artifacts (do this on Ha, then apply the same cropping to OIII)
  3. DynamicBarkgroundExtraction
  4. Apply the individual STF to a HistogramTransformation, making images non-linear.
  5. Linear fit for OIII with Ha as reference.
  6. Starnet2 on Ha and on OIII with creating a star mask from the OIII image.
  7. Noisereduction

Step 6 is the most important step. Blending becomes much easier as we do not have to take care of the star color later.

Remark: Other PixInsight users might prefer a more sophisticated workflow - but this one works for me quite well. Suggestions are welcome any time.

 

Let's have a look at the results;

Now we apply the PixelMath formula. We start with boost = 0, which is identical to the expressions from 'The Coldest Nights'.

We see that the background is too bright, but this can be improved in later workflow steps, e.g. with a proper blackpoint.

But where is the blue? As the OII signal is much lower than the Ha (at least in my setup with dualband and OSC camera), it does not show in the image at all.

 

So let's try some boost.

include = 0.2, reject = 0.99, slope = 20, difftoha = 0.4
mix = 10, boost = 1

We have now successfully intensified the OIII signal without shrinking the Ha regions too much. However, the OIII image has high grey values in the background, which is not caused by the nebula.

With the magnifier tool (left mouse button), an average value of 0.23 to 0.26 can be estimated. We can exclude these pixels from the boost by setting include = 0.33.

 

include = 0.33, reject = 0.99, slope = 20, difftoha = 0.4
mix = 10, boost = 1

This setting removes the blue in the background by still retaining the OIII signal in the nebula, but there is still too much blue in the lower part of the nebula.

Lets soften the slope and increase the include parameter a bit,

 

include = 0.4, reject = 0.99, slope = 5, difftoha = 0.4
mix = 10, boost = 1

We now have a good separation between OIII, Ha and the background, but the OIII could be more 'blue' and Ha more 'red'. We need a better separation between the two.

For this, we create a steeper threshold between OIII and Ha by increasing the mix and reduce difftoha .

 

include = 0.4, reject = 0.99, slope = 5, difftoha = 0.2
mix = 30, boost = 1

Let's take this image as a result, combine it with the stars, set the blackpoint and do some final color adjustment.

Each nebula, each filter, each color camera will require its own set of parameters for a 'good' image. My thoughts on colors in astrophotography can be found here.

Please reference thus page if you are using this approach for your Ha/OIII combination in published images.