How To Align and Stack Images in PhotoshopAlign and Stack images in Photoshop or similar graphics package, allows multiple photos to be combined to improve signal and reduce noise.
Aligning and stacking images in Photoshop or a similar graphics package allows multiple photos to be combined and increases signal while also reducing noise.
In astrophotography, we need to capture as much light as possible. With film, this is achieved by long exposure settings, but using long exposures on a digital SLR will create large amounts of noise as the chip gets hot quickly. Because of this, most digital cameras are very limiting with their exposure times. The longest is around 30 sec. But the average is more like 15 sec.
The preferred method of capturing more light in digital astrophotography is multiple exposures. This involves taking a series of 10-15-second exposures and 'stacking' them on top of each other. This is a beginner's guide that only uses the most basic features of Photoshop or Paint Shop Pro.
If you photograph unguided (without the camera on a mount or piggyback on a telescope), you can only realistically take ten or so 15-second exposures; otherwise, star trails form around Polaris. You can see this in the top area of my final image. If you are on an equatorial mount, this is not a problem, and depending on how good the tracking is, you may not need to align your images.
First, convert the image files to BMP or uncompressed format. This will prevent a loss of quality when we save the file. You can convert it back to jpg when you have finished post-processing. If you are shooting in RAW or TIFF, you can work from these or a backup.
You may want to remove any light pollution in your images before you start this process.
Loading the first image to Align and Stack
Load up the first image to process; in this example, I am continuing to use my one of Orion. I have nine other frames that have been processed for light pollution.
Load all the frames to be stacked
Open each frame in the sequence and copy it to the clipboard, then paste it as a new layer on the first image. When you are done, you should have the first image with nine layers + background. I have adjusted the opacity to show the star trail movement.
Aligning the photos
You should work on one layer at a time, hiding all layers not being worked on and keeping one base layer. Set the opacity of the working layer to around 50% so you can see stars on the layer and base, and then align one star over the other. Set the opacity back to 100% tog, glue the layer visibility on/off, and see if the star jumps. If it does, adjust the layer position as required until it no longer jumps around. It will probably wobble due to atmospheric disturbance.
Stacking the photos
When you have aligned all the layers, you have a choice of how to blend the images. You can use screen, which will 'add' the light areas together, making the image brighter, or you can leave it on normal to reduce noise. You will need to adjust each layer's opacity. I usually go (from base to topmost layer) 100%, 50%, 33%, 25%, 20%, 17%, and 14%. This does depend on the number of layers you have, and depending on the images, I will set 1 or 2 of these layers to screen, the rest being normal.
Final Align and Stack Processing
If the resulting image is too light, you can use the same technique for light pollution removal to alter the image. The final result will be far more impressive than a single exposure.
