Smoooothing Your Interstellar Background

Ever process your astroimage data, only to find that when you finally get your DSO looking just right, with all the stretching you've done, your deep space background is a bit noisy/blotchy/nasty?

Good deep sky images usually present the interstellar background at a level somewhat above pure black. Mostly this facilitates viewing the dimmest details, as computer monitors and some printers are notorious for hiding things in the shadows, but it also helps the image look more natural. After all, we're shooting through the Earth's atmosphere, and like it or not, some sky fog just seems right, visually. Unfortunately, this may also make background noise visible.

Of course, if you're processing Hubble space telescope images, please disregard this tip.

Here's a way to get a really nice looking, smooth background without losing perceptible detail nor reducing the natural appearance of the image.

First, you have to make sure the background of your entire image is essentially flat (no gradients). For this you may use flat calibration, my actions (e.g., Light Pollution Removal), and/or your own methods. At some point in your processing you should ensure your image is flat.

The first thing we will do is add a "Curves" adjustment layer to "turn on the bright lights" and let you easily see the background... This is a good idea to help you with your editing in general.

1. Make sure your top image layer (if you use layers) is selected, then choose Layer - New Adjustment Layer - Curves, and pull up on the curves to brighten everything up and to allow you to easily see the background. It may not look as smooth as you'd like!
2. Select the image layer.
3. Choose Image - Adjust Curves from the menus.

4. Grab the black point - usually at the lower-left of the graph.
5. Pull the black point up toward the white point, keeping the Input and Output values the same.
6. Stop when you *just* see the background noise get blanked out. You may have to release the mouse and enter values from the keyboard for Input and Output manually to get this just right. The Input and Output values should be identical, and *just* large enough to blank out the noise (this may differ from image to image).
7. Press [OK] to commit the Curves operation.

8. Choose Edit - Fade Curves from the menu, then enter 75% or so. Avoiding this step will leave you with an almost perfectly smooth background, but this may look unnatural.
9. Delete (or disable the visibility of) the adjustment layer you created in step 1.

That's it! Please look closely at the images at the graphic illustration of the technique in action.