Adventures with the Orion Nebula Part 4

So far in this series we have used different cameras and filters to produce images of the Orion Nebula (M42) that maintain details in the core of the nebula, including the Trapezium stars, and show the amazing outlying structure. In this concluding tutorial Nik Szymanek shows how to use RGB filters to create a stunning final image.

Nik Szymanek

When capturing images of deep sky objects using broadband filters, we would normally call on a clear luminance filter to capture as much detail as possible and add the colour using RGB filters in the time-honoured way. A few years back I acquired the narrowband data of M42 that we worked with in the last issue. Just after that I switched to RGB filters and shot a dataset.

Not expecting the results to be as good as the narrowband view, I did not shoot any luminance data. I ascertained that I could take 120-second exposures without burning out the core, although the brighter stars would be saturated. This scenario matches last issue’s workflow as we will be scaling the data separately to get the best of both worlds one final time.

Step 1 shows the three RGB FITS files with the larger images scaled using PixInsight’s Screen Transfer Function tool in auto-scale mode. Unsurprisingly, they all look quite similar as the nebula is emitting strongly at all broadband wavelengths.

When scaled in this way we can see all the faint outer nebulosity, but the core appears overexposed. Using the same tool in manual mode I rescaled the image to retain core details and these are shown in the insets.

Next, I combined the RGB files into a colour image and created three versions of it utilising different scaling and arranged them in three different layers in Affinity Photo . In this way I was able to show the Trapezium stars and inner nebulosity on the base layer, a mid-brightness version for the second layer and the full brightness image as the top layer. As always, the Erase Brush Tool was used to blend the layers together. Step 2 shows the bright version of the image on the left and the blended version to the right, with no further processing. Even at this early stage the image looks good but there is so much more that we can do.

Taking it to the next level

As this is the final article in the series, I would like to spend a bit more time on the processing and we will need a bit of time travel to get the best result. I went back to the original RGB FITS file and called upon my favourite processing tool, RC Astro’s Blur Xterminator . It is a supremely effective and simple-to-use deconvolution process. At the present time it is only available for use within PixInsight but there are other programs beginning to offer their own variants, sometimes free of charge ( GraXpert , for example). Once applied, it was easy to see how the stars had been tightened up and nice detail appeared in the nebula. Of course, we must repeat the earlier procedure of combining multi-scaled versions, but we can jump in our trusty time machine and bypass that step and show the future deconvolved and blended version ( Step 3 ).

I switched back to PixInsight and applied Star Xterminator to the RGB TIFF file to remove the stars from the image. It is a very simple procedure to apply and shortly I was able to view the starless version of the nebula and a separate image made of just the stars. With this procedure we can process the nebula separately from the stars, which allows a lot of flexibility and can be done using Affinity Photo or Photoshop . We will save both images as 16-bit TIFFs and jump back to Affinity Photo again.

I opened the starless image and applied the Tone Mapping Persona (another simple procedure). As always, the result was astonishing, magically conjuring low contrast details from the background and enhancing the structural detail of the nebula ( Step 4 ). At this stage it is always worth applying a small amount of noise reduction as the Tone Mapping procedure also enhances low-scale pixel noise. The Luminance slider found in the Denoise tool can be increased to smooth away the noise. I found that a value of 37% worked well. I really rate this tool; while it is not AI-based, it always does a remarkable job and is comprehensive yet easy to use.

Now we can start to apply the cosmetic enhancements on the starless version. Tone Mapping slightly desaturates the image so we can start with a global colour boost and shift using the trusty Colour Balance menu. With the Midtone slider selected I boosted both the blue and red colours and then switched to the Selective Colour menu.

Here I adjusted the strength of the red tones (using the red and magenta sliders) and then the blue tones (using the cyan and blue sliders).

Another technique I like to use is to create a duplicate of the starless layer for targeted processing. To do this, right-click on the nebula layer and from the drop list select Duplicate. This will place an identical copy in the layer menu. Hide the middle layer and click on the base layer so it is highlighted in blue. I called upon the Brightness / Contrast menu and boosted both the brightness and contrast to enhance the fainter nebulosity. This will make the brightest core area of the image show too much contrast, but we can now revert to the upper layer and use a soft-edged brush with a low opacity setting to erase parts of the top layer to see the higher contrast below. To blend the two layers back together, right-click on the upper layer and choose Merge Down. Next, I used the Dodge Brush, set on Midtones, to gently brighten the faint outer wispy nebulosity. Use a low Opacity setting for this (around 10%) and be careful not to introduce anything too unnatural. When this processing is finished, open the star layer and paste it on top of the nebula. To blend both layers together choose Add or Screen from the blending list at top right of the Layers menu. Step 5 shows my results, this time with north up.

I hope you have enjoyed this series on processing techniques for the Orion Nebula. We have covered a lot of ground and worked with many different cameras and filters. With care you will be able to produce a lovely image with just about any equipment.

Nik Szymanek is the author of Shooting Stars and Shooting Stars II , which are Astronomy Now books about modern astrophotography techniques.