ASTROPHOTOGRAPHY by Joseph Cali

TECHNIQUES -  Manual stack and average images in Photoshop
Joe Cali

Sometimes, auto stacking programs just don't work.  Computers can do what they're told but they're not particularly smart! Take the example of a moving comet against a fixed background of stars.  If you track on the comet and the star images are not circles or points, most programs just won't register. As long as the number of images to stack is modest, they can easily be stacked in Photoshop. In this tutorial, I'll explain how.

1. Load all the files into a layer stack.

2.  Click the browse button and navigate to your images.
3.  Find the images and highlight(select) the batch.

I prefer to lightly batch process my raws in Lightroom, then export them as 16 bit photoshop files before importing to Photoshop.  But you can stack raw files in exactly the same manner.

4.  The file names will appear in the file window.  If the batch of files are all identical, then you can check the "Attempt to automatically Align Source Images." In the moving comet example, we don't want to auto align. Leave it unchecked.

Click OK.  If it is a big batch, it will take some time for the computer to complete this step. The length of time will depend on your computers hardware. A computer with plenty of RAM, and an SSD will complete the task much quicker than a magnetic external drive and minimum RAM. 

5.  Now you should see all the layers stacked up. If not, open the layers window
by either pressing F7, selecting from the windows menu or clicking the layers
icon in the sidebar  -

6.  Press and turn off all the little 'layer visible (eye)' symbols next to all the upper layers, leaving just the bottom two layers visible.
7. Set the layer blending mode to 'DIFFERENCE' mode.

This will subtract the difference layer from what image detail lays underneath.
8. Initially you will see two images of each star and two ghostly comet images. Ignore the stars and using the move tool,   align the two images until the two comet images disappear.  Magnifying the image to 200-300% makes it much easier to see when the images disappear or at least come to a minimum.
9.  You can use the mouse for big drags or the arrow keys for single pixel shunts until the comet images disappear or at least come to a minimum.  The stars are not aligned and remain as double images.
10.  Change the layer blending to 'NORMAL.' and then click the eye on the next later up and repeat steps 7-10 for this layer and each layer above until all layers are aligned.

11. When complete, you should see a full layer stack
and all layers should be set to normal.

12. Select all layers except the bottom layer using
SHIFT+MOUSE CLICK. Slide the layer opacity slider
to a value where the image smooths out the noise.
In my example, the optimum value was 28% for each
layer. I look for an opacity that fades the stars and
optimises the smoothness of the comets luminosity.

13. Click on the add adjustment layer icon and select the LEVEL option

14  Adjust the LEVELS to your liking. The advantage of an adjustment
layer over just using levels is that the adjustment layer acts on the
integrated image formed by the visibility of all images below.

15  Zoom back to 100% or less and voila your image is finished.  The image size will be big.
For big stacks my image can be a couple of gigabytes.  Flatten image will flatten
all the layers including the adjustment layers.  If I am finished adjusting the layer
opacity, I will collapse the stack by selecting all the image layers, and merging them down.
I usually leave the image as 16 bit and leave the adjustment layers saved as a Photoshop file.


This example  of 7 images, took 14 mins to complete from start to finish
including the time to take the screen shots for this tutorial at each step.