This is a tutorial for using Photoshop to remove Light Pollution gradients from astronomical images
	This tutorial should serve as a basic guideline from which to experiment.

Is it really possible to get this image...

...from this one?

Start by opening the image in Photoshop. In this tutorial, I'm using Photoshop CS2 but the tools needed to accomplish the tasks can be found in many other versions and similar programs. The screen captures in this page are reduced 50%. To see the full image, right-click on it and select "View Image" from the pop-up menu. The first step is to apply a large Gaussian Blur to the image. This removes all small-scale details.

In this example, I blurred the image 80 pixels. You will need to find the optimal amount for each image individually as there's so much variation in scale depending on your imaging medium, the main subject, and degree of light pollution. Experiment! Once you've blurred the image, save it with a new name.

Re-open the original image so you can work with both the normal and blurred images.

Next, use the "Apply Image" command to "Add" the inverse of the blurred image to the original image. The result is removal of all the large-scale color and luminance features seen in the blurred image.

Convert the blurred image to "LAB" mode and copy the luminance data to the clipboard.

Return to the original image, create a new adjustment layer for levels and set your new high and low points.

Click on the "Channels" tab and paste the luminance data from the blurred image as a layer mask for the new Levels adjustment. The red indicates the areas of less effect of the adjustment.

You can now discard the blurred image. Create a new Levels adjustment layer and make additional adjustments as needed. Remember, this is just a starting point of the general technique. Experiment with different values for blurring and try using curves adjustment if you think it gives you better results.   Good luck and happy imaging!