Color intensity projections: A simple way to display changes in astronomical images

To detect changes in repeated astronomical images of the same field of view, a common practice is to stroboscopically switch between the images. Using this method, objects that are changing in location or intensity between images are easier to see because they are constantly changing. A novel display method, called arrival time color intensity projections, is presented that combines any number of gray-scale images into a single-color image on a pixel-by-pixel basis. Any values that are unchanged over the gray-scale images look the same in the color image. However, pixels that change over the gray-scale image have a color saturation that increases with the amount of change, and a hue that corresponds to the timing of the changes. Thus, objects moving in the gray-scale images change from red to green to blue as they move across the color image. Consequently, moving objects are easier to detect and assess on the color image than on the gray-scale images. A sequence of images of a comet plunging into the Sun taken by the Solar and Heliospheric Observatory (SOHO; NASA/ESA), and Hubble Space Telescope images of a trans-Neptunian object, are used to demonstrate the method.