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Artifacts in Images

If you see something unusual in an image, be assured that it is one (or more) of the following well-known and understood artifacts and not a UFO. These artifacts are caused solely by the equipment used to make the image.

Blooming

In the image below, the "tails" below the two brightest stars are an effect known as blooming. This is caused when an excessive amount of light (as from a bright star) causes an "overflow" into adjacent elements of the CCD array.

Diffraction Spikes

In the image below, the brightest stars exhibit diffraction spikes. These most commonly appear as four relatively faint projections radiating outward from the star arranged 90 degrees from each other, but can also appear as two projections on opposite sides of the star. Diffraction spikes are caused by light diffracting around the support vanes of the telescope's secondary mirror.

Vignetting

This is the result of an uneven distribution of light, with more light concentrated in the center of the image than the outer regions. It often appears as a round "halo". In the image below, the effect has been exaggerated to show it clearly. The left and right edges of a circle are vaguely visible; note how the background is brighter inside of this circle than outside. In this particular image, it is tempting to interpret this as part of the galaxy's "halo" of matter, but this is not the case.

Initially, most of our images suffered from vignetting. This problem is in the process of being corrected.


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