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When is a comet not a comet?

 

This cropped extract from a photo of Aquila contains a blue blur which looks rather comet-like. I can find it in no atlases, nor listed in the web as a current comet. Could it be a new one?

Canon EOS 5DMkII 24-105mm@105mm f/4.0 25x30s ISO 3200 2010-08-05 21:56:46-22:11:25 UT
From Rookhope 54.8N 2.1W 330m asl. Rural, almost no light pollution (3 Bortles)

Here is a more detailed view of the "comet" itself, at 1:1 scale (1 pixel on screen = 1 pixel in camera):

Canon EOS 5DMkII 24-105mm@105mm f/4.0 25x30s ISO 3200 2010-08-05 21:56:46-22:11:25 UT

Unfortunately the blue blur is a ghost reflection of the bright star Altair, reflected from the multiple glass surfaces inside the lens. A clue is given by the fact that on the original full-sized image the blur is diagonally opposite to Altair and equidistant from the centre of the image. The lens used is an L-series Canon lens so even the very best quality lenses are not immune from this problem. It is a zoom lens and zoom lenses have a greater tendency for this effect than fixed lenses because they generally have more glass surfaces inside them.

We can only be sure about a "new" comet by taking more than one photo and checking that it moves sensibly against the stars.

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