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   Finds the centroids of bright objects in the pixelMap
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   Form:
   starMeasurement = PixelMapCentroid( pixelMap, threshold, show )
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   Inputs
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   pixelMap        (1,:)    PixelMap file name or 'demo'
                            If empty will open a dialog box
   threshold       (1,1)    Fraction of maximum value Default is 0.99  
   show                     If entered, will show pictures

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   Outputs
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   starMeasurement (1,1)    Data structure
                             .pixelLocation  (2,m) Coordinates of centroid [x;y]
                             .intensity      (1,m) Intensity of each object

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