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During an HST exposure of a
celestial object, the camera occasionally
picks up light from objects other than the
intended source. During one of the several
HST exposures that were combined to make the
color image of NGC 4013, a bright artificial
satellite (possibly one used for weather,
communications, or military purposes; we have
not determined which satellite it was) crossed
the field of view. This resulted in a bright
streak across the image, visible in this reproduction
of a single exposure, just above the nucleus
of the edge-on galaxy and near the bright
foreground star.
During the processing that created
the final color image, artifacts that appear
on only one of the exposures are removed;
thus the streak does not show up in the final
picture. As you can see by clicking
on this single-exposure image for a closer
look, the picture is also covered
by a multitude of small specks and short trails,
almost as if dust had been sprinkled on the
picture. These are caused by charged particles
above the Earth's atmosphere, called cosmic
rays, which strike the detector during the
exposure. Telescope astronomers always try
to obtain at least two exposures of the same
object in the same filter. With the aid of
computer software, they can remove the cosmic
rays as well as other artifacts such as satellite
trails from the
final presentation of the image.
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