User:CorniceBowlSkier/Aperture (image mask)

In astronomical image analysis, an aperture is a two-dimensional area used to mask off a portion of interest in a photograph or digital image of an observed region of the celestial sky, most commonly the night sky. It is an artificial device that limits investigation or scientific analysis to only the image-data pixels that detect electromagnetic radiation from a particular astronomical object seen in the image. Although the shape of an aperture is arbitrary, it is a circle, square, or rectangle in its simplest forms. Often, the shape of an astronomical object, such as a spiral galaxy viewed at an oblique angle, will determine the aperture shape (e.g., elliptical) that is optimal for its scientific study. Besides geometrical considerations, sometimes photometric criteria determine the aperture's shape (e.g., all pixels with image-data values greater than some   threshold).