Use the image-processing function to stack and adjust an image of our home Galaxy

An essential part of an astrophotographer’s processing toolbox, the ‘Layers’ function exists in most processing software, including Adobe Photoshop and GIMP. ‘Layers’ can be used to stack multiple image files; a technique that reduces noise (unwanted artefacts) in the final image. It also allows us to make gradual enhancements to our image files along the way. As the ‘Layers’ function provides the ability to delete small processing errors at any stage of our edit, it means we don’t have to start again if we make a mistake. In this article we will show you how to use the ‘Layers’ function in Photoshop to stack and process an image of the Milky Way (see the ‘before’ image, above) to bring out its wonderful, starry detail. We start by opening the image files that we want to stack in Photoshop. Click ‘File > Open’ and navigate to the image file location. To open multiple image files in Photoshop, we hold the ‘Ctrl’ key and select al the required files from the file location, before clicking ‘Open’. Photoshop then uploads each of the selected files in separate windows (see Screenshot 1). Each of these files will be used to form layers within our stacked image. The file on the far left of Screenshot 1 – labelled ‘Start image’ in our example – will form our base image. We will copy and paste all the other image files onto this one.

Getting ready to stack

To combine these files into a single image ready for stacking, first start with the file window to the right of the base image – this will become ‘Layer 1’. Click on the tab for this image, as highlighted in Screenshot 1, and then press the ‘Ctrl and A’ keys at the same time. You will now see the image highlighted with dashed lines. Alternatively, you can click ‘Select > All’ in the image window. By pressing the ‘Ctrl and C’ keys, this image is copied (or click ‘Edit > Copy’). Next, click back on the base image window, and press ‘Ctrl and V’.

This will place your first layer on top of your base image as ‘Layer 1’. You can now delete the window that you copied this file from by clicking the ‘X’ symbol in the corner of the tab and proceed onto the next image file. Next, repeat the copy and paste steps for all the image files you wish to stack, so that these become layers on top of the base image.

Now we need to merge these layers into a single stacked file – reducing the number of layers to one. To do this, select every layer by holding down the ‘Ctrl’ key while clicking each layer, so that they are all highlighted (see Screenshot 2). Next, click ‘Layer > Smart Objects > Convert to Smart Object’. Once Photoshop has processed this command, click ‘Layer Smart Objects > Stack Mode > Median’. You’ll see the image change as the noise level is gradually reduced.

We can now use the ‘Layers’ function in a slightly different way, to edit this stacked image. The most effective way to use ‘Layers’ for editing is to create a duplicate of the preceding layer, so that we can use a new layer to make a separate adjustment each time. To do this, hold down the ‘Shift, Ctrl, Alt, N and E’ keys simultaneously. You will now see a new, identical layer form (see Screenshot 3). You can now select any item from the ‘Adjustments’ menu – for example, ‘Adjusting levels’, or ‘Brightness/Contrast’ – which we have highlighted) and make an alteration specifically to that layer. Once satisfied with the adjustment you’ve made, use the mouse to right click on the layer and select ‘Merge Down’. This will combine your adjustment with the layer.

Easily adjust your edits

By creating a new layer before making a new adjustment, you are separating your process into stages. The advantage of creating these layers for all the key steps, is that you have the flexibility to go back to any point and quickly review all of your steps to see where something might have gone wrong, or to mark what you did right. If you decide an adjustment has made a change that you don’t like, you can remove that layer by right-clicking on it with the mouse and selecting ‘Delete Layer’. If you feel the adjustment was slightly too aggressive, you can also solve this by adjusting the ‘Opacity’ for that layer. Just move the ‘Opacity’ slider (see Screenshot 3) to reduce the effect of the alteration.

To ensure full control of your edit, repeat the above steps on every adjustment you make. When you have finished, you can use the mouse to right-click on a layer and select ‘Flatten image’. This ensures the finished image is in a suitable format to export as a JPEG or TIFF file (click ‘File > Export as…’).

We now have the completed image, where the Milky Way is revealed in all its glory.

3 QUICK TIPS

1. You can change the effect of an adjustment layer by amending the ‘Layers’ mode; popular types are ‘Colour’ and ‘Luminosity’.

2. Try ‘Hide All’ and ‘Reveal All’ masks, where each adjustment layer is applied to the image.

3. Use the ‘eye’ icon next to each layer to establish whether your most recent layer has improved the astro image before keeping or deleting.

ALL PICTURES: CHARLOTTE DANIELS