This photoshop tutorial shows how to correct out of focus stars.
Night photography often involves a compromise, often the compromise is depth of field because large apertures (such as f/2.8) are commonly used to capture stars before they begin to trail. This means that the focus can be on the foreground, or the stars and the DOF is unlikely to include both – even with a large focal length.
If you have a shot where the stars are just out of focus blobs of light, don’t despair! This easy fix will suffice for a web image and with some extra work, might work for a large print.
The video shows how I duplicate the layer and set the blend mode to darken. I then use the move tool to nudge the image. The smallest increments can be applied with the cursor keys and the amount of movement is determined by how much you are zoomed into the image. The smallest increment is 1 pixel. It might be necessary to repeat the process with different nudge directions for different parts of the sky.