When we left off in part one, we had stretched our M16 using a masked stretch to not blow out the core, and then done some colour correction with a DeMagenta PixelMath script, followed by some SCNR denoising. Now we will continue with the rest of my PixInsight workflow, up to where we export the image for final tweaks. I use Affinity for my final touches, but most photo editing packages have equivalents.
9. HDR Multiscale Transform
This next step is about teasing out as much of the detail in our highlights, so we’re going to reapply the mask we generated in the last tutorial (or regenerate one now), and ensure that the core is exposed to changes while the outer nebulosity is masked.
Once again we want a preview window, so that we can test out our High Dynamic Range (HDR) transform. With our preview defined and then selected from the sidebar tab, open up the HDRMultiscaleTransform tool.
The default setting on HDR Multiscale Transform are a good place to start, so I usually run it once, and then start tweaking. This is essentially an aesthetic choice, so you’re looking for something that recovers data from the highlights and adds structure, and that’s a subjective decision to make. Best advice is not to overdo it.
I will usually bump up the iterations to 2, run it again, then increase the number of layers by 1 and run it again, and keep adding layers until I feel like it’s overcooked, then back off. Next I will try adding a little overdrive and see if I like that or not, then lastly have a play with Preserve hue and To lightness checkboxes.
On this occasion, I’ve found some results that I like the look of with only a couple of tweaks from the defaults. So it’s time to go back to the main image tab, and run the transform for real. This has really brought down the brightness within the core of the nebula, and has really darkened up the oxygen blue colour as well. So I’m going to go back to my histogram tool, and tweak the stretch to bring a bit more brightness into the image.
10. ColorSaturation
With my mask still applied to select only the core section, Use the IntensityTransform -> ColorSaturation tool to raise the saturation of the blues in the core.
You may need to apply the saturation a couple of times to get the tealy blue colour I’m aiming for; that said though, SHO processing is false colour processing, so don’t get hung up on “right” or “wrong” colouring.
Note how there are dots along the line to keep the other parts of the spectrum in the middle. I’m trying to isolate just the blues at this point. The colour saturation curve takes a little getting used to, especially when you realise that the red part of the spectrum is at either end of the line, and so lifting the line on the right will affect the curve on the left! We will see that shortly.
Removing the mask from the image, it’s time to lift the orange hue in the main nebulosity. As you see, lifting the line in the reds on the right has lifted it also on the left. I’ve brought the rest of the spectrum back down to the middle line (neutral impact) and even down a bit lower in the greens. Again, this is mostly a question of personal preference and taste. When the preview shows you colours that you like, hit the apply.
11. Working with nebulosity
It is the beauty, structure and detail of Hydrogen Alpha emissions that I love the most about narrowband imaging, and so teasing out the detail in this channel is where a lot of my time goes. In order to achieve this, first we need a mask that is biased towards the Ha content in our image… and luckily, we have a whole channel that is 100% hydrogen alpha.
It is time to return to our H channel image, that we last used in the previous tutorial in order to put together our SHO image in the first place. The 3 narrowband images are all linear still, we have not stretched them. But now we want to start making use of this Ha content in our stretched image. In order to do this, we are going to borrow some steps from the previous tutorial, applying them instead to our hydrogen image: deconvolution, noise reduction, and stretching.
Once we have a nicely stretched Ha image, we need to clone it. This is as simple as clicking on the name tab of the image, dragging, and then letting go.
We will come back to the H image shortly, but for now we want to work on the H_clone, as we are going to use it to make a mask to work on the Hydrogen in the SHO image. And for this mask to be as effective as possible, we need to introduce a lot more contrast to turn the areas that we don’t want to work on black.
For this step I return to the Histogram Transformation tool that we used to stretch the H layer. Now working on the clone instead, move the far left (black level) triangle towards the right until the peak of the top histogram curve is at the far left of the chart.
As you can see in the above image, by shifting the black point so far, a lot of the fainter detail has been blacked out. This is perfect for creating our mask, and we will keep the H layer as it is for later use.
Drag the high contrast H_clone onto the SHO image as a mask, then bring up Process -> IntensityTransformations -> CurvesTransformation.
This curve is what is known as a contrast curve or S curve – you pull the line down a bit on the left end of the histogram (making darks darker) and up a bit on the right side of the histogram, making lights lighter and thereby increasing the overall range of the image.
With curves it is best to make a small change, apply, and then repeat.
12. More Hydrogen
Remember our stretched H alpha image? Now is its time to shine. For this we need the LRGBCombination tool. Drag the label tab across into the L textfield (or just type H in there, assuming that’s the label on the image), and untick R, G, and B.
Next open the Channel Weights section, set the L slider down to 50%, and then drag the triangle onto our SHO image. Adding the H Alpha layer in as luminosity can really give the nebulosity some punch, but once again this is an artistic decision in many respects, so season to taste, as it were. This is one of those times I might clone my SHO image 3 times and apply the H Alpha to them at differing strengths, say 33%, 50%, 66% and then keep the one I like the best.
13. Star minimisation
The first step for this approach to star minimisation is to clone our SHO image and rename the clone to Starless. Then, as the name suggests, we need to remove the stars from this cloned image. StarXTerminator is my preferred tool for star removal, but StarNet is a favourite of many, and doesn’t cost extra.
One thing about star removal is that it takes a whack of time. So while it is running, here are some resources that you can check out for what we’re about to cover.
This is Shawn’s video where I first learned about these star reduction scripts. But these scripts are not Shawn’s work, but Bill Blanshan’s hard work. You can download Bill’s scripts from this share, and I highly recommend subscribing to his YouTube channel while you’re about it! In fact it looks like Bill has some newer scripts so there may be an even better technique available to us, and I’ll be sure to put out another guide once I’ve had a chance to try it out. But for now, let’s keep going with this. Download the scripts and from the Process menu, open up the process icon file you have downloaded.
There are three different star reduction algorithms in this pack, so I make three clones of my SHO image and label them V1, V2, and V3. Double check that your starless image is labeled Starless (capital S) and then let’s click on the first script.
As usual, once you’ve opened the V1 star reduction script, drag the triangle across to your V1 clone, and let it run. You can adjust the S variable in the script, but at this stage I prefer to run all 3 on their defaults, applying each version to the clone of the same number.
At this point, I line up the 3 different versions, and see if any stand out to me as being clearly better, or clearly worse. Again – this is aesthetic, there’s no right answer.
I will zoom in on an area that has both large and small stars in it, and just check the detail, the halos etc, and see if one is a stand out candidate. In this instance, though versions 1 and 2 reduced the stars slightly more, there were halos left that I did not like compared to the version 3 result.
14. A touch of sharpening
This is the last step in my PixInsight workflow, and it is to add a little sharpening just to crisp up some of the nebulosity. Make a clone of your image (in my case, it is the V3 image with star reduction applied), and open the MultiscaleLinearTransform tool. This tool is an absolute beast and I can’t begin to explain everything that it can do, but we are going to use it twice to get ourselves a little sharpening.
The first step is to set it up as per the screen on the left – double click layer 1 (which will disable it, note the red x), click on layer 2 and use the Detail Layer section to set it to -1.0, and use the same technique to set layer 3 to -0.5. All the other sections can be left unchecked. Drag the triangle onto your new clone and apply the transform. This applies a very light blurring to the image, which will make it a perfect mask for the second step. So drag the label of our clone onto the main image sidebar to apply it as a mask.
Now to reset the MultiscaleLinearTransform tool for sharpening duties, set it up as follows. Again layer 1 can stay disabled, and set layers 2, 3, and 4 to small positive values as shown. (If when you apply the sharpening you feel it is too strong, these are the values that you will lower to reduce the sharpening effect).
Note also that Deringing is checked, with the Dark value set at .1. If you notice nasty artefacts around your stars after applying the sharpening, you may need to tweak these values. Again, set up a preview window so that you can rapidly test different values and see their impact.
Once you have set up your values for the sharpening, double check that the mask is applied to your SHO image, and drag the triangle across to apply.
What I’m looking for is just a slightly crisper definition to the tendrils of nebulosity. The impact is subtle, but it’s there, and that’s the balance that I’m looking for.
15. Save for final touches.
Select your finished image, in this case min is the V3 image. CTRL + SHIFT + S will open up the save dialog, and I will save this out as a TIFF file, so that it can be opened for final touches in Affinity, PhotoShop, or your photo editor of choice. I will also save the starless image at this point because they’re just so damn beautiful!
Part 3 will be coming up soon, where I’ll take you through the last final touches that I apply before considering the image to be complete. Thanks for following along, and clear skies!
