After several iterations of Curves and Levels, we have been able to bring out that faint stuff that you couldn't even see in the original image. It was there, just hidden way down low in the histogram.
But we're not done. Notice how the center of the image is brighter? We have a gradient that shouldn't be there. This is vignetting -- an optical effect of the light rays being restricted through the telescope tube, then having to spread out to illuminate the camera's imaging chip. The further off-center the light rays are, the more they fall off. Instead of purchasing a $10K telescope that has less vignetting, I use a cheat...GradientXTerminator. This is a plug-in for Photoshop that analyzes the gradient and smooths it out.