Pendleton, OR
ASG Observatory
NGC2244
IC405, IC410, IC417, NGC1931
NGC 3372
NGC 1499
NGC7000
M45
Horsehead & Flame Nebula
Barnard 33
There are literally unlimited ways and methods to stack and process astrophotography images these days. Tutorials to stack, tutorials to process, and with all the various combinations of software available, it can be daunting. As a teacher for 10 years in high school classrooms, and a computer software engineer for 20+ years, I will try to keep it simple and just explain MY process from start to finish.
Let’s start by breaking down into sections, and you may find one useful, while another you skip and use an alternative solution. I personally have tried many other solutions and settled on this solution, but some may like Deep Sky Stacker, or PixInsight, or Gimp, or SharpCap… so use a part of this or all of it as you feel fit!
So you collected your narrowband images and now it’s time to stack and combined them into one image… I have compared both Deep Sky Stacker (free) and Astro Pixel Processor and both have advantages, but APP is by far better for complex stacking of your data. You took so long to capture it, why not get the best results you can when it’s stacked?
APP does a great job not only on creating your stack, but also combined them into an RGB image toward the end. While not a tutorial guide here on stacking, I do prefer to do a final RGB Combine in APP and to keep my stretch minimal at around 10% before saving. I prefer to use Photoshop to control the stretching to my liking.
Want to learn more on APP or compare DSS vs APP? Follow links below.
Final APP stacked and RGB combined image with 10% stretch.
So we will assume you have made it this far and have your images and stacked them into an RGB image. If not, there are lots of tools to get to this point, whether you stack and combine in Photoshop or in APP is very negligible at this point, both work fine to get you an stock image with raw data in RGB mode here.
The thing about narrowband imaging is that you are taking a sliver of light curves, and when we talk about combining into an RGB image we are talking about 3 channels required for photo editing software. Narrowband images can really be combined into any format you like, there really isn’t a hard or set rule. Hubble for example became famous for creating its own palette because it shows contrast between gases so well. This really is the key for narrowband.
Some users like SHO, while others prefer HSO, and yet others prefer HOO. I have used HSS in some cases with good success as well as mix them up. Sulfur is deep red, Hydrogen Alpha is bright red, and Oxygen is a very blue/green mix. If you were to do this as the eye sees it, you get essentially a lot of red/purple with Sulfur and Ha and very very little contrast. I say play with them, most if not almost all of our astrophotography images are for visual appeal and artistry and little is used for scientific data like photometry, so explore the palettes.
I typically will use the Hubble color palate in SHO (Sulfur = Red channel, Hydrogen = Green channel, Oxygen = Blue channel) but you can flip them to whatever you like at this stage.
For this step though, we want to get our stars separated from our nebula and background. Others try to stretch ever so gently and I don’t like that technique. Pull them, separate them, and then I can deal with my nebula and not worry about star bloating at all. We can even repair what’s behind or glowing from them in the nebula as artifacts of our process.
I started out using StarNet as it was free and works very well, but recently purchased and made the switch to StarXTerminator by Russell Croman. This little extension simply plugs into Photoshop or PixInsight and works much better than StarNet (in most situations). It’s so simple to use, appears in photoshop -> filters, and runs in very little time compared to StarNet.
You can look up this software here: www.rc-astro.com
In this guide, I’ll use StarNet as it’s free, popular, and it does take a little more instructions to learn to use, but a good tool, whereas StarXTerminator is simple, costs money, but doesn’t really require much tutorial.
As a software developer, I can appreciate a good command line piece of software 🙂 It’s not pretty, but it sure does a good job and is simple to use. Setup using this quickstep guide.
It’s that simple… and now we can do what we need to with both images.
Can you create just a star image? NO, not in starnet, only remove stars, but that doesn’t matter, if we have both images, we can do that in photoshop easily with old tricks!
This a Hubble based version, but I didn’t crop it in this example used to run starnet. Note the overpowering green in this SHO starting image because Ha is mapped to green and is always likely the most powerful signal.
This is final version from starnet that has no stars. I prefer to crop it before running starnet just to help with speed.
The old trick used here is to utilize photoshops subtract feature. This has been used for years to remove artifacts, people, buildings, you name it from an image, similar to how green screens can be replaced with other details. Photoshop also offers a ‘Difference’ feature but gives you little control.
I really prefer to use ‘Subtract’ for my image and this way I can also reduce star size at this stage with really good control. Just open both .tif images (stars and starless) in photoshop and go to the Image -> Apply Image feature. Choose your starless image and remove from your star image.
With “Subtract” selected, you can adjust offset slightly and think of it as taking off a little each time from your stars, it does a good job evenly and experiment here to your liking. Imagine your image is in 3D, and the terrain is coming upwards towards the peaks being the white stars… you want to just remove and subtract all the data below that, which is what we do here. Using the ‘Difference’ method is like taking the exact image and removing it, but with ‘Subtract’ and offsetting it, we can move that clipping point up and down slightly to get more into the stars, or even go more down into the nebula peaks.
I prefer an offset of -10 to sometimes as much as -30 or -40 depending on my image. It is the best and optimal place to minimize your stars at this stage rather than other tricks like trying to select color ranges and minimize on a processed image.
NOW you have separated stars from the nebula, just highlight this whole image of stars, including the black background and paste this layer onto your ‘Starless’ image. Blend using the ‘Linear Dodge (Add)’ and you have a nebula layer, and a star layer…
Best of all you can stretch levels and curves now from your nebula layers without any worry on bloating your stars. Too often I see people trying to do slight curve stretches or trickery to a curve stretch to minimize impact on stars, when it needs to be masked or better yet, layered to fully prevent such problems.
After completing Phase I of stacking and combining our SHO into an RGB image, we are ready to start working on our image. We can do Phase II which is separating stars with StarNet as in our example, or as our video here shows we do it with StarXTerminator as well. Both do a great job.
Phase III is where we begin working on our image and processing it, either in PixInsight or in Photoshop. In this example, we will walk through narrowband SHO hubble color palette editing in PhotoShop. Including a quick section on StarXTerminator, Camera Raw, editing individual channels, Removing optical purple stars, High Pass adjustments, and more.
Our first step is to get those stars separated from our nebula. As in Phase II, we remove them with StarNet or we use StarXterminator here in this Phase III video tutorial. With StarXTerminator, we duplicate layers, we run StarXTerminator to get the nebula, we subtract that layer, then stamp a new layer with stars only. The Star layer will be ‘Linear Dodge (Add)’ and the nebula will be a ‘Normal’ layer.
The first thing I like to do with the raw nebula is open each channel and copy to a new document. Here I can run the healing brush on bad star artifacts found from star removal, I can also Camera Raw this specific channel and stretch lights, darks, mid-tones, denoise slightly, or whatever I need to get this channel specifically looking better.
Notice in this above image we are working on a single black and white channel, but we have also opened the 16-bit and RGB properties on the image. This gives us full access to all the PhotoShop tools we need to touch the image.
Heal your Nebula
I love photoshop for this next step. Some hard core enthusiasts will cringe, but the Spot Healing Brush on the left tool menu lets you run around your nebula and heal poorly addressed areas from star removal. Often a blown out star leaves halos, but this is perfect time to touch it up and remove. Some good tool shortcuts here.
I like to make my healing spot brush with spacing around 25% and my hardness around 50%. I create a new merged upper layer to work on here so I don’t destroy or can revert to my lower layer if I don’t like it. I just run around with my [] bracket keys, adjusting the size of my brush, spot dabbing on poor starnet stars and halos. A good tip here is you can also learn to drag your healing brush and it blends really really well some poor areas.
Notice these poor star artifacts left behind from Starnet, you can clean these up and blend well using the healing tool. You may spend a lot of time here, I try to not change too much data and keep it realistic as possible… but in reality, we started changing natural data of an image when we started… so… I’ll just leave it there on that topic.
You should have a good looking nebula and a good star layer at this point. I still leave my stars turned off at this point and keep creating new merged visible layers. Now we are ready for another fun part, which is Camera Raw in photoshop.
Camera Raw is fun to play with, I start in the basics and I tend to do the following settings above on all images. Blue temps, redder tins, more highlights and whites, blacker shadows and blacks. A little texture and clarity increase go a long way, so do vibrance and saturation.
DON’T OVER DO IT!!! It’s fun, but start small and save it, keep creating new visible upper layers when you make changes and go back as needed.
I stay away from noise and optic features here. I will touch those when I have stars added back in. I do like to go to the calibration section and loosely play with the ton of the coloring here.
The last steps we take are bringing our stars back… we didn’t forget about them… they have just been neutralized… as they should be. Turn your star layer back on and hit Camera Raw again one more time.
I like to run the defringe tool which is designed to remove purple halo effects in imagery, and is perfect because that is what we have here. If you run a different color palette than SHO you may have purple elsewhere, so be careful there, but if you do run SHO palettes, you likely will only have purples in your star halos, so turn that down toward the gray side. I like to leave a little, it’s sort of natural looking.
This is also our last final steps, so I don’t mind running noise reduction under the details section as well. My personal preference is that if I do want to make other color changes… do so before running this last Camera Raw process. There is nothing wrong with running Camera raw, target one specific change or area, save it, then run it again. I have seen some odd effects when you try to do too many features in Camera raw all at once.
I always try to save my denoise till the very end to just add a little smoothness to my final output!
At this point we have separated stars and nebula, touched up the nebula channels individually, enhanced our nebula with Camera Raw, High Pass, and Noise reduction. We have optimized our stars with a little less purple using purple optics adjustments, and now recombined our image into the final SHO version.
We might run through and combine the entire image back into one and hit Camera Raw one more time to touch up or enhance overall tones and colors to our liking. In all the image can be processed this way in as little as 30 minutes or redone multiple times to your liking.
Relax and have fun with it… it’s a hobby, not astrophysics.
Some final thoughts… This is artwork, and you can freely do what you want. Don’t let others tell you that you can’t change something, if you notice and like an adjustment that goes against everything everybody says on the interwebs, we get nowhere. It’s all adjusted data to some degree in my book. I try to do a satisfactory job keeping data as it is, I don’t add stars, or put three moons in there, but if you like it, go for it… don’t take any of it too seriously!
Also, this is the realm of narrowband, and when you have slivers of light spectrum to work with, you are looking for contrast, you want to see that difference between the two. Hubble was famous for devising a color palette with such great contrast amongst gases that otherwise would look similar. We will cover RGB as well as RGBHSO or RGBSHO imagery processing in our other tutorials.
