In this astrophotography editing tutorial you’re going to edit my photo of NGC 7000 – The North America Nebula! I chose this target as a tutorial piece as it has a lot of nebulosity to work with, it’s a busy image to learn where to point your colour samplers, there’s a lot of stars to reduce but there aren’t any bright stars like Navi or Alnitak to make life even harder.
When I first began my astrophotography journey image editing was at the back of my mind. I was focused squarely on the actual photography side of it. It was only when I sat down to begin processing did I realize that I should’ve been honing my processing skills also. Editing deep sky astrophotography images is like nothing I had done before – requiring a completely different workflow, skill set and knowledge of the editing software.
In this Deep Sky Astrophotography Editing Tutorial you’re going to learn:
- Using Layers
- Curve Adjustment Layers
- Levels Adjustment Layers
- Star Reduction Techniques
- Selective Colouring
- Selective Contrast Enhancement
- Layer Masks
- Localized Contrast Enhancement
- Noise Reduction Procedures
Sounds like a lot? You won’t even realize how much we cover. The procedure flows quite naturally, but I know that in the beginning it may feel clunky, but rest assured you’re learning the core astrophotography editing skills you’ll need on your journey!
So get the image of The North America Nebula by clicking the link, open it in Photoshop and let’s begin!
If you fancy watching this in video format, then I have a tutorial on my YouTube channel. If not, then keep reading.
When you first open the image in Photoshop, you’re going to see something like this. This is because Deep Sky Stacker doesn’t apply a stretch to the image, and images taken with dedicated cameras such as the Altair Astro 183c Pro Tec camera I used for this image don’t do any in-camera processing that DSLRs may do. They are truly raw.
The First Levels Adjustment
To even begin editing this photo. We need to do stretch the data. Right at the beginning we’re going to do a Levels adjustment by going to: Image – Adjustment – Levels
Levels allows us to directly control the white gray and black points as well as their output intensities. This is handy for making an image brighter or darker, or otherwise controlling haze and gamma issues that may occur. Levels are one of the most important adjustment tools you learn to use.
So within the Levels adjustment screen you’ll see the sliders I mentioned. We’re going to pull the white point all the way in until we can see stars. Where you can see that there are no white lines on the level screen it means there’s no data there. This can be safely clipped out with the slider.
Establishing The Bright And Dark Points Of The Image
So now we can see stars, we want to find the brightest and darkest points of the picture. This is useful in order to maintain a colour balance to the photo, as well as making sure we’re not clipping data in the white and black points of the image. To do this we use a Threshold layer:
Layer – Adjustment Layer – Threshold
Using a threshold layer you’ll see a slider similar to that of the levels adjustment. You’re going to want to use the white point slider and move it to the left. This adjusts the brightness on the image until you can see where the darkest areas are and just before the image begins to blow out. Using this we see the brightest and darkest areas. The brightest areas are, naturally, going to be stars – bright ones. The darkest areas you want to sample are areas of empty sky with no dust or nebulosity.
The threshold adjustment layer allows us to see the bright and dark parts of the picture. Above: Threshold layer and colour sampler tool.
Now go to the sidebar of tools and select the eyedropper. Right click it and select the colour sampler tool. From here, select the mode to 5×5 averaging at the top. This will sample a square of 5×5 area and will take the average value of the pixels sampled. From here, sample a white point – a star, and a black point – the dark sky. Make sure your information panel is showing: Window – Info.
Once this is all done, delete the threshold adjustment layer.
The Final Initial Steps
Before we begin the editing proper, we have only two more steps. Another Levels adjustment, to make sure we have some nice data to work with and we can see what we’re editing. If you pay attention to your informational panel you’ll see the white points go up to and exceed 255. This means they’re blowing out and losing information. Using the Output Levels at the bottom, pull the white point down (I use about 190) as so to protect the highlights and white points.
Balancing The Background
This is going to be another recurring step, and is another one used within the Levels adjustments. So far, we’ve been using the RGB mode. Now we’re going to go channel by channel, red, green, blue. Within the Levels adjustment window, click the drop down box and select the channel. Then, using the white and black point sliders, manipulate them until you can balance your values. We want to make sure all the R,G,B white points remain at 190, and we want to pull the R,G,B black points to a common value as well. On this image, I matched it to the lowest black value – red 29. In the end I had R, G, B white values of 190, and R, G, B black values of 29.
We’re now ready for the actual processing to begin. We’ve already covered Levels, Threshold, Channel Manipulation and Balancing The Background!
Go ahead and do a crop now also. When the images are stacked there is usually a black border around the edges of freshly stacked images. This will skew your histogram. It isn’t too bad in this picture we’re practicing on, but it is of good practice to get these cropped out as soon as possible.
It’s also a good idea to do a crop now to remove the stacking artefacts around the edge.In my tutorial I did this step a bit later on – which is why the history looks skewed. In my video, I do it right at the end (since it didn’t make too much a difference to the histogram).
Stamp – This is a great shortcut. Control + Alt + Shift + N + E. What this does is stamp visible. Basically, everything you can see on the screen gets stamped and added to a new layer. We use this a lot. So remember this shortcut!
Curves are like Levels – extremely important to your skillset. Now that we’ve got our background sorted and a nice base to begin working with, its time to begin doing Curves. These allow you to manipulate and change the values of pixels within the blacks, shadows, midtones, highlights and whites of the image. Curves are amazing.
We’re going to use an Adjustment Layer though. When we go Image – Adjustment – Curves, the values we apply are burnt on and permanent. By using an adjustment layer, we can always go back to that adjustment and make further edits.
Now we see the histogram, with a diagonal adjustment line going through it. This is the line we’re going to manipulate. The histogram, if you didn’t know, shows the blacks to whites from left to right. Bottom is darker, higher is brighter. That’s why it’s a diagonal – you can’t get darker than pure black – and you can’t get brighter than pure white.
We’re going to do an aggressive Astro Curve. This is where we’re going to pull the shadows and black areas of the image brighter. You’ll now notice that the whites are blowing out again to 255+. So we’re going to pull the curve down to a flat to preserve the white values.
We’ve now increased our brightness and can see much more detail now on this image. However, you may notice this strange colour haze hanging over the image. This can be rectified with a Levels adjustment and pulling in the black point of an image. So go ahead and go to image – adjustments – levels. Whilst you’re there, balance the background again.
The Second Curve Stretch
So now we’ve got another base, one with a slightly better brightness/contrast and it’s already looking better, right? Let’s continue on that line and do another curve stretch. This is going to very much like the last stretch, however not as harsh and more towards the mid-tones of an image.
Once again now, balance the background and reduce the white haze by pulling in the black point within the Levels tab. With that done, Stamp a new layer.
Selective Curves and Levels Adjustments
I call this “Selective”, as we’re going to use tools that pick out the nebulosity from the background, and the background from the nebulosity. Instead of doing a blanket edit like we have been doing with the above Levels and Curves. We’re going to use selection tools and layers to achieve this. Let’s start.
Selecting Colour Range
The trick to picking out what we want from what we don’t want, in this situation, is using the tool Selective Colour. Go to Select – Colour Range. From the drop down box select Sampled Colours and use the eyedropper within the pink nebulosity of the North America Nebula. Change the fuzziness to make the selection larger and softer. For this image, I used about 100 fuzziness. Press okay. You’ll now see the selection appear.
Now we’re going to Feather this selection to make the edges softer, the transitions less harsh and refine the selection. Go to Select – Modify – Feather. I used a feather range of 10 for this. Now it’s been feathered you’ll see the marquee to be much softer and rounder.
Copy and paste this to its own layer by using Control + C and Control + V.
Now we’re only going to manipulate the nebulosity, since its on its own layer isolated from the background. You’ll quickly realize how powerful this process is for large targets such as this. It’s an easy and effective way of making a specific part of an image POP.
With the isolated nebulosity layer selected (layer 3 in the image above), make a Curves Adjustment Layer (Layer – Adjustment Layer – Curves). Make sure you hit the little box to make this a clipping mask – this way your modifications will only affect this layer and not the whole image (See below for picture).
The Contrast Curve
A big hitter with your curves, what is known as the ‘S’ curve. This curve excels at raising image contrast. To do the ‘S’ curve, towards the left-hand of the curves (in the shadows/blacks of the histogram), pull the line down slightly. A little does a lot.
Now, on the right hand side of the histogram (highlights/whites), bring that up slightly. Again, a little does a lot. What you’ll see happen is the red nebulosity will begin to pop out from the background. This helps separate the target from the sky, but overdoing this can lead to fringing and it looking unnatural. Be sure to do it a little at a time and see the results.
The final step we’re going to do at the moment on the isolated nebulosity layer is to brighten it up a bit in general. To do this, make a Levels Adjustment Layer and again make that a Clipping Mask. Move the white point slider in from the right to increase the nebula’s brightness.
Adding Contrast To The Background
Now we’ve made the nebula pop, we can darken the background a bit as well to further the contrast within the image. This step is very much to taste – if you like a very contrast heavy image, then make a deeper ‘S’ curve by pulling down more on the black part of the curve adjustment layer we’re going to make in a moment.
Make a stamp layer of your current image before proceeding (Control + Alt + Shift + N+ E)
I want you to take a moment right now to think if you can work out how to darken the background only. You’ve learnt all the skills you need to know right now to do this. Go ahead; see if you can figure it out. I’ll wait here.
You’re back? Excellent. If you managed to figure it out then congratulations! If you weren’t able to then no problem at all, it’s why we follow guides and tutorials. I’m going to show you how I would do it. We’re going to use the Select Colour Range tool again, and instead of selecting the nebula, we’ll select the background sky, adjust our range and then feather accordingly! Again, as before, once you’ve made your selection and feathered it, use Control + C and Control + V to copy and paste the selection into its own layer to be edited.
From there, much in the same way we made the nebulosity pop, we’re going to use a Curves Adjustment Layer as a Clipping Mask to edit the background only. Do you know what curve we’re going to use? That’s right – the ‘S’ curve again. We’re trying to raise contrast and – as mentioned – it excels at doing just that.
Just like before now, make a new layer and Stamp Visible, and then Duplicate that layer (control + J). We need two identical images here as we’re going to brighten the top layer, but to maintain control of just how much is applied, we’ll use the opacity slider (located on the layers tab, next to the drop down box that says normal).
So with your two identical layers, select the top most and apply a Curves Adjustment Layer. In this adjustment we want to raise the overall image brightness. So click and drag the curve up a bit in the dark and bright sections of the histogram. This raises the overall brightness, but it now is too much. Select your opacity and reduce it to a point that suits your taste. For me, that was about 50%.
Stamp another layer, and name it ‘Mid Edit’. You can also save your work here. In fact, you can save anywhere and as often as you want during the process if you so wish.
Duplicate the Mid Edit layer, we’re going to use that next.
Okay, so you may wish to skip this step entirely. It depends on your taste. Star reduction isn’t for everybody, but if you want to achieve a bit more separation between DSO and background, declutter the image a bit and just reduce the size of your stars then follow this step.
Okay, so to do this we’re going to use the Colour Range selection tool again found under Select – Colour Range.
Again, knowing what you’re beginning to learn about image processing and the histogram, where do you think we’re going to select stars? They’re the brightest parts of the image, right?
They’re located within the highlights of the histogram. But instead of having to use the Sampled Colours tool, we can use the drop down box to select Highlights. From here, we adjust the fuzziness (which affects the selection size and range as well as softening the edges), as well as the range. Range is sort of like the threshold adjustment from earlier. Range will select items of that pixel value or higher, the pixel value being its brightness. In this example a fuzziness of 50% is used and a range of 250.
Now we’re going to modify our selection a bit. Go to Select – Modify – Expand. Expand the selection by 1 or 2 pixels. This makes the selection marquee bigger, allowing us to encompass the halos around the stars also. What I like to do now is to feather the selection also. Select – Modify – Feather. I would use 2 or 3 as the range. I just want to make the edges a bit softer to reduce the harshness of our next action.
Okay, we’re selected, we’re expanded and we’re feathered. Time for the magic. Go to Filter – Other – Minimum. Make sure ‘Preserve Roundness’ is selected and adjust the radius to suit your needs. Don’t go too hard on this, a couple pixels radii is all you’ll need for most if not all but the largest of stars. If you go too hard, you’ll see dark donuts appearing around your minimized stars – and that just is a dead giveaway. Use your preview. A radius of 2 worked nicely for this image.
Try turning your top layer on and off now. See what a difference it made? it’s quite apparent. However it’s probably gone too hard now. So select your minimized layer and set it to an opacity that suits you. For this, 66% worked well for me – it had remained them reduced, but left a more softer result in general.
Go ahead and Stamp.
Next, we’re going to learn about the Camera Raw feature which is amazing for astrophotos.
Noise Reduction And Camera Raw
The Camera Raw feature in Photoshop offers some brilliant tools for very focused yet easy editing. It basically brings up the toolset available to you in Lightroom. Within Camera Raw you can adjust exposure, contrast, whites, blacks, highlights and more. We’re here for the sliders, but mostly for the noise reduction tools. We’ll begin with the sliders though.
These sliders help speed things up. We can do the same adjustments using Colour Range, Curves, Levels and other tools. But with Camera Raw we simply say we want to boost the contrast, drop the whites, tweak the blacks. It’s all right there available. Another note is the vibrance slider as well as clarity. Clarity is a great tool for adding crazy sharpness and contrast, but it’s very easy to over do it. Do very small tweaks here, it’s easy to get carried away and bang the sliders right up. But it takes some experience to learn when to reign it in. If you feel like you want to add “just a bit more”, chances are that you don’t actually need to add that bit more.
Noise Reduction however is so good here. Similarly, it’s done with sliders. So it’s easy. Again, it’s tempting to smash that slider to the max. But what Noise Reduction actually does, to an extent, is smoothen out the image. So whilst you add noise reduction, you lose fine details. It’s a trade-off. I do a very firm noise reduction as this isn’t a lot of data to work with. On the third tab, Details, you’ll find the tools. Under Noise Reduction, move the Luminance slider up and down until you’re happy. You can zoom the display preview in to get a closer look at what your adjustments are doing. Whilst you’re here, I recommend adding a spot of colour correction. This helps remove a bit of the colour noise that may be present in the image.
If you’re feeling it, you can now do one last Curves Stretch before we move onto the final step – Selective Contrast
I call this step Selective Contrast as we’re going to go in manually now and pick out the detail we want to highlight. In this image, for example, the focus point is the Cygnus Wall. But it’s beginning to get lost within our edits and doesn’t pop nearly as much as I want to.
Before we move on here. Stamp your layer, and make a Duplicate. This is very important as we’re about to go hard on contrast. Name the stamped layer Background or something that you can pick out, and the duplicated layer Contrast.
Select the contrast layer. What I do is add a strong ‘S’ Curves Adjustment Layer. Once I’ve done that, I select the Contrast layer again, and then go into Camera Raw again. From there I bump up the Contrast slider, and add a healthy amount of Clarity. The image is going to look quite strong now. Lots of sharp contrast, fine detail being lost. It looks like we’ve undone all our hard work. But this is by design. The Cygnus Wall will begin to stand out a bit more now. If not right now, it will soon. Very soon.
We’re now about to rescue this and begin to bend it to our desire. Select the Contrast layer, and then go to Layer – Layer Mask – Hide All.
What this does is add a mask of black over the Contrast layer. Anywhere which is black is hidden on the activated layer (Contrast) and allows the layer underneath to show though (Background). This is how we’re going to selectively add contrast.
A layer mask works on blacks and whites as well as grays. Blacks hide, whites show and grays kind of do the bit in-between.
Select the Brush Tool, and use a nice sized brush to allow you to pick out the details you want. Make sure its Hardness is 0% – we want a nice soft edged brush. At the top of the screen make sure Flow and Opacity are 100%. Now, with the Layer Mask selected (this is very important!) go in and paint White everywhere you want to show the high contrast layer through. This will, most likely, be over the Cygnus Wall itself.
Once you’ve finished the main parts, you can change the brush’s Opacity and Flow if you want to begin softening the transition even further. The softer you make it, the better generally it will look. But don’t worry too much about this. You’ll see where your Layer Mask is working by A) being able to see the high contrast layer, and B) the black Layer Mask window on the Layers will show the white.
We’re really close now. It still is a bit harsh to my eye. So we’re now going to change the Opacity of the layer. NOT the layer mask. But the actual layer with the contrast and nebula on. So click on that within the Layers navigator window and then drop the Opacity down to help blend it. I dropped mine down to about 33%, but you can change it to your own taste – if you decide to change it at all.
I love it. This was the final touch. We’re now basically done. So be sure to Stamp the image. We’ve got some nice contrast and detail on the Cygnus Wall, and we’ve blended it in nicely. This image, if you wish, is now done. I, however, do a couple more steps.
I go in finally to do one final Curves Adjustment Layer with a general image brightness curve. I just want it a touch brighter.
And then, at last, is to rotate the image. When I shot this picture at least, this isn’t the orientation I had in mind. It’s 180′ out from what I wanted. So go to Image – Image Rotation – Rotate 180′
And that is it.
No more steps.
We’ve covered a lot of fundamental image processing tools in this tutorial. Curves, Levels, masks, layers and so on. You can, of course, play with these values as you see fit though! All that remains is resizing the image for any media you want to share it on, saving it, and uploading it.
I hope you’ve found use in this tutorial. If you did, let me know what you think or if you have comments or questions, go ahead and ask. Share this with other astrophotographers you know starting their journey if they need some image editing help!
Clear Skies everyone!