So, you either didn't follow the 600 rule, you're bad at math, or you made a mistake!
Now you've got a shot that you absolutely love but the stars look like eggs, or worse yet, they are mini trails!
Don't scrap that photo without at least trying this little know trick of the trade.
In this tutorial I will teach you how to remove small trails to make your stars look crisper.
**** Does it always work?? Nope ****
But heck, why not at least give it a go before deleting that photo.
The Original Image
Here is my original image opened in Photoshop. You will notice that the stars look like mini trails.
This particular image was exposed for 43 seconds (23 seconds longer than I usually expose an image).
The Original Image Magnified
Here you will notice how the stars are trails and not as crisp as they should be.
Normally most people would throw this image out.
Stars Layer Selection
Start off by selecting the sky. I used the marquee tool but you can use any tool that will make that selection for you.
Copy the selection.
Pasting New Layer
Next I paste the copied section into a new layer. You will note that I've used the eraser to feather the edges of the copied layer. If you don't feather the layer it will be noticeable in the final image.
Selecting the proper blending option.
It is imperative that you use the correct blending option for the star trail removal layer.
The correct blending option is "Darken". This blending option is what allows us to remove the trails.
Once you select Darken as your blend, start moving the layer around slightly (use the arrow keys), you will start to see the stars get smaller. You want to be careful, because you can actually remove all of the stars from the sky using this method. Move the layer around until you get the desired (or best) result.
If you are good at blending and more advanced with photoshop, you could also split the sky into a few sections to help remove the trails that are moving is slightly different directions (this is due to field rotation).
Here you can see that it has done a pretty good job at removing the unwanted trails. The image isn't an award winner, but it is one that looks pretty good compared to how it looked prior to the adjustment.
Again, this is the image prior to the adjustment. You can see the adjusted one above.
The Final Image