The tutorials use stacked images of a moving object with the Screen Ruler to identify which of the underlying unstacked images can be rejected due to the moving object being too close to field stars and galaxies.

Tutorial I stretches the Ruler along the length of a star trail that is interfering with the image of a moving object, allowing the images around where the trail crosses the moving object to be identified. The method is quick and simple but may not identify more than the most obvious interfering background trails.

Tutorial II uses three different stacks from the same underlying set of images to more rigorously identify all field stars and galaxies that get in the way of the moving object. The stacks are:

       - a stack of the star background (stacked with zero motion) to reveal fainter sources more clearly

       - a stack of some or all available images stacked using the object's motion that allow the object to be seen (even though it may be partially obscured by star trails)

       - a stack of just the first and the last available image, again stacked using the object's motion, to accurately define the length of star trails

Tutorial III uses the same method as Tutorial II but uses information automatically extracted from the underlying FITS files to mark on the ruler where the moving object is located on each image. Images can be rejected by dragging the mouse along the ruler where background stars are seen, the remaining set of selected FITS files can then be copied to a separate folder to make re-stacking easy.

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