Use editing software to bring out hydrogen-alpha colours in Orion

Astronomy Photographer of the Year

Advice from a shortlisted entrant in the ‘Skyscapes’ category

My ‘Questions’ image was shortlisted in the ‘Skyscapes’ category at the Astronomy Photography of the Year competition in 2020. I created it after deciding to take an image linking our planet to the sky, but without featuring the usual Milky Way core. I started by shooting a mountain panorama during the Blue Hour – when the last remaining sunlight casts a blue glow into the dusk – to get maximum detail, and then set up my gear and waited for darkness and the Orion Molecular Cloud Complex.

I used a hydrogen-alpha modified Canon EOS 6D camera with a 40mm Sigma Art lens at f/2, mounted on a Star Adventurer mount and took 38 frames, using exposures of 90 seconds at ISO 800.

I processed the image using a Luminance, Red, Green and Blue (LRGB) editing technique in Lightroom and Photoshop to bring out the hydrogen-alpha colours from the emission nebula. My friend and fellow astrophotographer Ian Norman showed me the technique and I’m going to explain how it works here. It involves stacking multiple shots to reduce noise (unwanted artefacts), creating a high-contrast ‘Luminance’ layer and then overlaying it with a ‘Colour’ layer to produce an image that reveals the data hidden in the RAW image files.

To begin the process select the RAW images (like I did of Orion) and import them into Lightroom. Next, click the ‘Develop’ module in the top menu and scroll down from the displayed ‘Basic’ panel (on the righthand side) to find the ‘Noise Reduction’ panel and set all the fields to 0. Scroll back up to the ‘Basic’ panel and set a neutral white balance by increasing ‘Vibrance’ and ‘Saturation’ values in the ‘Presence’ section to +100. Then, in the ‘WB’ (white balance) section of the ‘Basic’ panel, find an even balance between the colour sliders (Screenshot 1), before resetting ‘Vibrance’ and ‘Saturation’ to 0.

Create a panorama

Next, press ‘Ctrl+A’ to select all the images and click ‘Sync’ at the bottom of the ‘Develop’ module, which brings up the ‘Synchronize Settings’ box. Here click ‘Check All’ on the bottom left and then click ‘Synchronize’ on the bottom right. Now use the mouse to right-click on the selected images and select ‘Edit in > Merge to panorama in Photoshop’. This launches Photoshop and loads the panorama window. In the ‘Photomerge’ option box untick ‘Blend Images Together’ and click ‘OK’. Next, select all the images that Photoshop has opened in ‘Layers’, by right-clicking the mouse and selecting ‘Convert to Smart Object’. In the top bar open the ‘Layer’ drop-down menu and click ‘Smart Objects > Stack Mode > Median’. Right-click the image and select ‘Flatten Image’ to reduce noise.

Now that we have a ‘Colour’ layer we can create a high-contrast ‘Luminance’ layer. Use the mouse to right-click the ‘Colour’ layer and select ‘Convert to Smart Object’. Next, press ‘CTRL+J’ a couple of times to replicate the layer, making three. Rename the two new layers as ‘Nebulosity’ and ‘Detail’ and then hide them by clicking the eye icon to the left of the ‘Layers’ tab.

The next step is to remove the stars in the ‘Colour’ layer. Select it and then go to the top bar and open the ‘Filter’ drop-down menu and click ‘Noise > Dust & Scratches’. In the ‘Dust & Scratches’ box set the ‘Radius’ to 8 (Pixels) and the ‘Threshold’ to 15 (Levels), as increasing the ‘Threshold’ will bring back detail. Doing this will eliminate all but the brightest stars. Smooth this layer by opening the ‘Filter’ drop-down menu again, and click ‘Blur’ > ‘Gaussian Blur’ and set the ‘Radius’ to 3 (Pixels) in the ‘Gaussian Blur’ box.

We now need to adjust the ‘Detail’ layer. Select and unhide this layer by clicking on the eye icon in the ‘Layers’ tab, then find the ‘Black & White’ icon in the ‘Adjustments’ panel above the ‘Layers’ tab and click it. Adjust each of the colour sliders in the ‘Black & White Properties’ box (see Screenshot 2) to bring out contrast – for example, increasing ‘Magenta‘ will reveal the hydrogen-alpha in your image. Next, click the ‘Curves’ icon in the ‘Adjustments’ panel and add contrast to your image with this tool. Under the ‘Layers’ tab, select ‘Curves’, ‘Black & White’ and ‘Detail’ and right-click the mouse on ‘Smart Object’. Rename the adjusted layer as ‘Detail’.

For the ‘Nebulosity’ layer, repeat the steps from the ‘Colour’ and ‘Detail’ layer (see Screenshot 3). Under ‘Layers’, select ‘Curves’, ‘Black & White’ and ‘Nebulosity’ and right-click on ‘Smart Object’. Rename the layer as ‘Nebulosity’.

Next, we combine the ‘Detail’ and ‘Nebulosity’ layers with an inverted layer mask. To do this, hide the ‘Nebulosity’ layer and select the ‘Detail’ layer. Press ‘CTRL+A’ to select the main image preview shown on the screen. Use ‘CTRL+C’ to copy it, and then re-enable the ‘Nebulosity’ layer and add a layer mask by clicking ‘Add vector mask’ at the bottom of the ‘Layers’ tab. Paste the copied contents into the new mask on our ‘Nebulosity’ layer. Hold ‘ALT’ and click on the mask, before pressing ‘CTRL+V’ to paste and then ‘CTRL+I’ to invert the contents. Group the ‘Nebulosity’ and ‘Detail’ layers by selecting both, then press ‘CTRL+G’ and set the group blend mode to ‘Luminosity’ – this creates our ‘Luminance’ layer. Now select the ‘Colour’ layer and click the ‘Vibrance’ and ‘Hue/Saturation’ icons in the ‘Adjustments’ panel to make increases. We can now combine our layers by right-clicking and selecting ‘Flatten Image’.

Finally, the mountain can be added by opening the image of it and using Photoshop’s ‘Magic Wand Tool’ to select the foreground. We apply a layer mask where the sky is black and the ground is white to create our final image (see the main ‘Questions’ image, opposite).

3 QUICK TIPS

1. The number values for this image may not fit your data, so alter them as necessary.

2. The key aim of LRGB processing is to enhance the hydrogen-alpha nebulosity in the image.

3. Be careful, as increasing the ‘Black & White’ luminosity too much may lead to data being lost.