Sep 29, 2008

Barnard's Star

(voir ici pour une traduction automatique de cette page en français)

This is the star with the highest proper motion of the Heavens: more than 10" every year. See the Wikipedia page on this star here.

I took this picture yesterday, with my Canon 350D DSLR piggy-backed on the C8, with the 85mm f/1.8 lens. Single 30 seconds exposure, no processing except contrast enhancement, and double size for clarity. It is possible to see stars of magnitude 14 on this picture...

The bright star on the left is 66 Oph, magnitude 4.8. It is highly overexposed, and appears less bright than it should be. This is due to the limited dynamic range of the Canon EOS 350D captor . Its design range is 12 bits (4096 values) per pixel. 1 bit is lost in the electrical noise (seen as the sigma value of a dark frame), and 2 more in the suburban sky background. So only 9 bits remain, giving a dynamic range of 512 values, which corresponds to log2.5(512) = 7 magnitude range. As stars of magnitude 14 can be detected, everything above magnitude 7 is overexposed, and appears dimmer than it should be... This is not a problem, since only 66 Oph is brighter than mag 7 here (next bright star is 7.6).

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