Vignetting and uneven field illumination should ideally be corrected in the telescope to remove their causes at the source. If this is not possible, they should be calibrated out with the proper flat-field frames. If flat-field frames are not used, vignetting or uneven field illumination may be present in the image.

Sky gradients may also be recorded along with the deep-sky object. These gradients may show up both as brightness and color variations across the frame. They can be caused by light pollution and other causes such as increased atmospheric absorption near the horizon. These types of gradients will not be corrected by flat-fielding because they are actual features of the sky.

Vignetting, uneven illumination, and sky gradients can be removed by the methods described previously for JPEG images, or with filtration in software as shown in this section.

Image with uneven background.

In this example image, I have added a strong red gradient that runs from the lower left to the upper right of the image.

We will use Russell Croman's Photoshop plug-in GradientXTerminator to reduce these defects.

We will apply GradientXTerminator in two steps. The first will remove any large scale coarse gradients, and the second will remove any fine-scale defects.

GradientXTerminator

Freehand Lasso Tool

The first thing to do is to make a selection of the galaxy to tell GradientXTerminator that these brightness differences are real and should not be corrected.

Simply draw a selection area around the galaxy with the free-hand lasso tool that can be found on the tools palette.

Selection visible in the image.

The selection is then inverted so the sky background outside of the galaxy is selected. Go to Select > Inverse. We want this area to be corrected by GradientXTerminator.

GradientXTerminator Dialog

Go to Filter > RC Astro > GradientXTerminator to launch the plug-in.

Set GradientXTerminator detail to medium and aggressiveness to medium.

"Balance background color" should be checked on if there are any color gradients present in the sky background.

Mouse Over

Hold you mouse cursor over the image to see a comparison of the image before and after the first pass of GradientXTerminator.

In the comparison above we can see that GradientXTerminator has done a pretty good job of correcting the defects in the background of the image.

If the sky background is not even after this first application of GradientXTerminator, a second application can be made. This time use the magic wand to select the sky background. Set the tolerance to about 25, and click on a blank area of sky background. This will select everything but the stars and the core of the galaxy. Now use the lasso and hold down the alt key to subtract, and de-select the galaxy again.

Set GradientXTerminator detail to fine and aggressiveness to high and run GradientXTerminator again.

Programs for Removing Vignetting and Gradients

• GradientXTerminator is a Photoshop plug-in written by Russell Croman that is excellent at removing vignetting and gradients and neutralizing the sky background.

• Background Subtraction Toolkit (Free) by Molecular Expressions