Determining a Centroid

The centroid of a physical object is its barycenter, or center of gravity. It is the point at which the object suspended from a thread or teetering on a fulcrum-balances. In an image, the centroid is the point at which the values of all surrounding pixels are balanced. What makes the centroid interesting is that even though light from a star is spread over many pixels, the distribution of light among them enables us to recover the exact center of the star image within a small fraction of a...

C72 Histogram Shaping

The previous tool, Brightness Scaling, operates on an image by examining the range of pixels in it and assigning new pixel values based on where each lies in the range between the values selected for black and white. Histogram Shaping, while looking superficially similar, works in an entirely different way. While both tools reassign each pixel value in the original image to a new value, Histogram Shaping does it by trying to force the histogram of the image, as a whole, to a new shape. While we...

Morphological Operators

Isophotes Photoshop

Morphology deals with form and shape in image processing, morphological operators are tools that aid the observer in defining, extracting, and manipulating the Figure 15.9 Isophote contour lines encircle a region having pixel values greater than the isophote pixel value. This highly enlarged section of the M101 image shows a knot in the spiral arm, an isophote contour line drawn around that knot, and the same isophote line superimposed on the image. Figure 15.9 Isophote contour lines encircle a...

B31 Astrometry Books and Articles

Guide to Minor Body Astrometry, Internet article at Description of how to carry out measurements and report observations of asteroids This guide is intended for those observers interested in undertaking an astrometric CCD-observing program of minor planets and or comets. Everhart, Edgar. Constructing a Measuring Engine. Sky and Telescope September 1982 279. Describes building a two-axis precision engine for measuring positions from photographs great insight into state of the art of amateur astrometry before CCDs. Konig, A. Astrometry with Astrographs. In Astronomical Techniques. Ed. W.A. Hiltner. Chicago University of Chicago Press, 1962. Quite possibly the clearest exposition of astrometry available appears in a short chapter of an impeccable source book on astronomy. March , Jordan D., II. Measuring Positions on a Photograph. Sky and Telescope July 1990 71. Instructions for doing astrometry from photographic prints includes program in line-numbered Basic for reducing x, y data using...