Before attacking gradients, perform any final histogram and color adjustments until you are satisfied with the image contrast and brightness in your image. Many options are available to dispense with gradients. You may want to first try gradient reduction tools present in many astronomical imaging programs.
Within Photoshop, you can use the image itself to create a model of the sky gradient if the celestial target is either an open cluster or a well-defined nebula or galaxy that occupies less then a quarter of the image. Flatten the image (if not already flattened) and then duplicate the background layer. Choose the copy, and select color range - shadows. Select inverse, and then cut the selection. Apply a large Gaussian blur of about 50 or more pixels. Change the blend mode to difference, and reduce the opacity to about 80%. Your sky gradient should be almost invisible. This technique fails with larger faint nebula or galaxies because the faint detail can be erased and gradients superimposed directly on the target are not eliminated.
Other methods in Photoshop work for larger objects, using the gradient tool. This technique begins with a new blank layer that is "filled" with a gradient from sampled "foreground" in the brightest corner of your image to sampled "background" in the opposite corner. This gradient is then subtracted by changing the blending mode to difference at less than 100% opacity.
Another way to reduce gradients is with GradientXTerminator, which is a plug-in for Photoshop. This program creates a higher order correction to the background than can be achieved by simpler Photoshop routines and only takes a few minutes to perform. A 30-day trial of the software is available prior to purchase. The program includes a straightforward online tutorial.
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