This color separation is due to the varying angle of the light rays that hit the line scanner. In the center, the three color channels rgb coincide, whereas at the top and bottom the red and blue channels get shifted by a couple of pixels horizontally, see diagram below. An example image is shown in the next image, left part. This is a 220 pixel portion of a 5594 pixel high scan extracted from the lower edge of the image.
Correction is accomplished by shifting the color channels horizontally against each other.
This shift is accessible in the 'Correct' submenu, option 'Radial Shift'. The button 'options' opens the dialog window below, which is used to provide the necessary parameters. Note that the numeric values are partially hidden in the screen shot. Clicking 'ok' initiates the transformation which results in the right image above. To determine suitable values check the formula in the diagram above.