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All of your sky images are soft-focus, which makes the Dust & Scratch filter very appropriate and fairly easy. For other images where the surrounding areas are sharp and need to retain detail, it can be trickier. Here is a general guideline for how to use the D&S filter:

• Always use it on a new duplicate layer, in case you ever need to refer back to what you originally started with.
• When you first open the filter, set both the Radius and Threshold sliders all the way to the left.
• Now very slowly increase the Radius setting until your dust and scratches—or in your case the star trails—are blurred away and disappear. Keep the radius as low as possible while still getting the job done.
• Next, go to the Threshold slider. The more you slide it to the right, the more it "undoes" the blurring you just did with the radius slider. The trick here is that you want the Threshold setting to be as large as possible so that it restores the sharpness of all the other "good" parts of the image, while still keeping the blur on the actual dust and scratches so that they're not visible.

On some images, this balance is not easily achieved. In that case, use the filter to get rid of the dust and scratches as best you can, and then apply a layer mask to this layer. Fill the mask with black, and then use the brush tool with white to paint away individual dust & scratches. (At this point you probably don't yet know what a layer mask is, so just save this tip for later.)