Friday, March 21, 2008

Aurora borealis photography - No lens filter!

Filters on a lens can cause concentric rings to appear in the center of an image (this is a crop) be sure to remove the filter when photographing the aurora.

When photographing the aurora borealis, or northern lights, it is important to remove the filter from your lens. Why? Look at the photo and you will see a series of concentric rings, which appear at the center of the image. This can be a disheartening discovery after a night of shooting the aurora, since the rings are very difficult to remove, with even the best photoshop geek on the job. What causes the rings? Charles Deehr, a professor emeritus in physics at the University of Alaska Geophysical Institute (the one who is responsible for this aurora forecast site), has been quoted by Dick Hutchinson on his website as saying:
"These are interference fringes due to the parallel faces of the filter and to the narrow spectral emission at 5577 Angstroms in the aurora. That green, atomic oxygen emission line is the strongest emission in the aurora near our film and eye peak sensitivity, so it shows up first when there is any device in the optical path which sorts out the spectral emissions."
Harry Manos, a physics teacher from California who gave a lecture on aurora photography (and consulted Charles Deehr for material review) describes it this way:
"A haze filter in front of the lens acts as a Fabry-Perot interferometer on the 1S auroral green emission line of oxygen, creating green concentric circles"
So what does that mean exactly? This is a good interpretation: just take your filter off!

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