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The blob menu

  Nova image being measured The blob menu

This menu only appears on image windows which have been displayed by clicking on or near a detected object (ie, blob) in another frame. "Blob" is the term used in GRIP for a connected group of pixels that has been detected in some way. Some other systems use the term ROI (region of interest) for the same thing. One aim of the present menu is to identify blobs as stars and then to be able to measure the magnitudes of some of the stars by using others as known references. There may well be non-astronomical uses too, along similar lines. More details of the procedure for magnitude estimation are available here.

The (generally small) window that appears when a detected blob is clicked on (or near) in a parent frame appears as shown above. The blob in this case is a rather faint star (Nova Aql 2009). It is magnified so that its individual pixels can be seen, each as a 6 x 6 square of pixels. The two cross hairs run through the measured centre of the blob and the heavier green dots show the boundary of the blob as initially detected. Generally the boundary does not cover the whole blob because it had to be detected at a level higher than the background. Outside the blob are the outlines of two circles, shown by less heavy green dots. These measuring circles can be used for determining the brightness of the blob accurately, by means of options on this menu.

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