After taking some photos of the Perseids meteor shower, I wanted to blend all the images so all shooting stars would be shown together in just one image, so it seems they crossed the sky at the same moment in time. I’ve got four separate images, three with a shooting star on each and a fourth one with what I think is an Iridium flare, which is shiny object in the sky as a result of the reflection of the Sun on the big antennas of an Iridium satellite orbiting the Earth.
The process in Photoshop involves opening the four images as layers, using one as the base image (the one which shows the ‘still’ stars) and masking the other three images except for the shooting stars (and Iridium flare). To make the photo blending more accurate in relation to the position of the sky where the shooting stars came from, I align the three layers so the firmament of all photos match.
Often blending in Photoshop is not difficult if you have used a tripod, because it will involve only moving the layers around. However, shooting the sky at night makes it a bit trickier because of the relative turning movement of the stars. You will need to rotate the layers around Polaris, and move them too in case you had repositioned your camera between shots, like I did.
Loading the images as layers
First I need to open the four images as layers in Photoshop. In my case I was editing the photos in Lightroom, so:
Select the three pictures and right click> Edit In> Open as Layers in Photoshop
If you prefer to use Photoshop directly you should go:
File>Scripts>Load Files into Stack.
I usually rename the layers, it takes a moment and it’s a good practice so you don’t get lost when dealing with lots of them. I turn off visibility on the two layers above and focus on the first two.
Aligning first two images
The first step is aligning these two images, click on “Perseids 2” and change its opacity to 50%, a different value may work better for you, just play with it keeping in mind that you want to see both images properly (Perseids 2 and Perseids 1 in this case).
Then I select the Move Tool [V] and start moving and rotating this second layer until the stars on both images match. Once this is done you can change the opacity back to 100%.
“Perseids 1” is my base image, so to show it fully I create a full transparent mask on it. This step is not completely necessary but good practice too.
Click on “Perseids 1” > Create Vector Mask
Now I need to hide everything from “Perseids 2” except the shooting star so first I create a full opaque mask on the second layer holding Alt/option key:
Click on “Perseids 2” > Alt + Create Vector Mask.
And now use the brush [B] using a white colour to paint over the shooting star which will make it viewable. Just be careful to do this on the mask and not on the image. If you paint too much around the shooting star press [X] to change colour to black and mask that area again.
That’s it, it seems more complicated to explain than to actually do. After this you will need to repeat the steps with the other images. Every time you do this process your base layer is the mix of the layers below.
At the end, to merge all the layers as one select them all and click [Cmd+Alt+Shif+E].
This is the final image showing the three meteors and the Iridium flare:
The Iridium flare is on the far left of the picture and looks like a short meteor.