These images can consist of mosaics of anywhere from two to hundreds of 2MASS Atlas Images. The Atlas Images are the final 2MASS image product and are produced by combining the raw survey data frames (which are taken six-deep in any direction in the sky) into a final uniform image. The raw survey data is taken with pixels which are 2´´ in size. The combination of the six raw frames, which are slightly offset relative to one another, into a final image permits the final image to be presented with pixels that are only 1´´ arcsecond in size. The "coadded" Atlas Images from which these "gallery" images are produced are the same ones that are delivered by the 2MASS image server at the Survey Visualization and Image Server tool, with the exception that the image server currently delivers slightly degraded 20:1 lossy-compressed images because of disk space limitations.
The colors in these images arise from assigning red, green, and blue to the three infrared "colors" in which the Survey images the sky. In analogy to color vision, blue (the shortest wavelength color) is assigned to the shortest wavelength channel for 2MASS, which is the 1.2 µm J-band; green is assigned to the 1.6 µm H-band; and, red represents the light detected in the 2.2 µm Ks-band. Cool stars will appear redder than hot stars. Interstellar dust artificially makes distant stars appear redder (in analogy to the setting Sun). The effects of this "interstellar reddening" are particularly apparent near the edges of dark obscuring clouds of dust.
The images contain a number of artifacts of the camera and telescope system. Bright stars tend to be embedded in diffraction spikes (faint crosses). These spikes are produced by light passing the cross-shaped secondary mirror support structure at the top of the telescope. Bright stars also produce trails of "persistence ghosts". This effect occurs because bright stars leave a latent image on the 2MASS detector arrays. Since these pictures combine many 2MASS raw data frames, and since these raw data frame are taken as the telescope scans smoothly in the declination (vertical on the images) direction, the "ghosts" appear as a trail of increasingly fainter images either above or below bright stars.
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