Hi Guy,
That is all under user control on the Multi-Aperture setup panel. If you want to use WCS coordinates to place apertures, select the option "Use RA/Dec to locate aperture positions". If that option is disabled, pixel coordinates are used to place the apertures from one image to the next. If the option "Reposition aperture to object centroid" is enabled, then after the initial aperture placement by RA/Dec or pixels coordinates (depending on the Use RA/Dec setting), a centroid operation will be performed to tweak the placement of the aperture to the centroid of the flux within the aperture radius (if the flux is very low, centroid may fail). Centroid will track target and comparison stars (translation or rotation) as long as they are within one aperture radius of the initially placed position. In pixel placement mode, the pixel coordinates update from image to image, so as long as the image movement from one image to the next is less than the aperture radius, the stars will be tracked. If the images are plate solved and the "Use RA/Dec" option is enabled, the stars will be tracked regardless of the the amount of field shift or rotation from image to image, as long as the plate solution is correct within one aperture radius.
Karen
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