Use Selection Checkbox

Click the checkbox or press U to toggle.

The appearance of this checkbox notifies you that a selection was made in your image editor at the time the plug-in was started, and allows you to include or exclude the pixels that were not a part of that selection.

Note that no matter whether a selection is made, the entire image will be flattened; the selection affects only the pixels analyzed as input.

• If the [  ] Use Selection checkbox is checked, only the pixels that were selected will be included in the analysis of the image. Those that were not selected will be ignored. This is useful if you want to have the plug-in avoid considering pixels such as borders or stacking remnants in the flattening analysis.
• If the [  ] Use Selection checkbox is NOT checked, all of the pixels in the entire image will be used for the flatness analysis. In other words, the selection that was made before the plug-in was started is ignored. This could be useful if you inadvertently left a selection made when starting the plug-in.

AstroFlat Pro is pretty good at separating image details from the background without any selections. However, an example of where you might want to select pixels before starting the plug-in would be to make a rectangular selection excluding the outer edges of the image to cause the plug-in to avoid using stacking remnants or edge noise in its analysis.

If this checkbox shows on the main dialog, you may have made a selection you didn't intend to be used for the flattening process. In that case, just uncheck the box.