It's a difficult task to shoot the Northern Lights. You never really know when and where they will show up. When you finally find a good spot, you wait ... and wait. Nothing happens. It's cold and dark. And then suddenly, out of nowhere, a small green spot appears on the distant horizon. It grows quickly and forms a green band over the sky. The band starts to move. It takes on more colors, red and violet. More bands start appearing. They get more structure. They move extremely fast, And then, a corona appears, right upon your head. It's coming straight down on you, and now the whole sky is filled with the aurora, dancing wildly and moving around in all colors. There is so much light that it casts shadows on the ground. And after five minutes, it's all gone again, and it's dark once more.

This comes from Bernt Olsen, one of the photographers featured in yesterday's gallery of photographs of the northern lights. One of the great pleasures of putting that piece together was Flickr-messaging with the people who had taken the shots, all of whom were excited to get to share their work -- and the Norwegian skies they showcased -- with a broader audience. Olsen offered to fill me in on the technique that goes into getting great shots of the lights. Here's what he said, edited a bit for clarity.

That's how it was on my latest aurora hunt last week, when I took the Ørnflya and Corona pictures. They were taken late at night at Sommarøya, on a small top* called Ørnflya. It was windy and cold, and after fours hours of waiting the aurora finally arrived and I got my shot exactly where I wanted it, with the small islands to the left, the main island in reflection in the sea. I only wished that the moon had been there to lighten up the scene a bit. These photos have not been photoshopped. They are from the camera, as is.

The two from Grøtfjorden were taken late Tuesday night, with my two other aurora-hunter friends, Per Wollen and Tor Even Mathisen. We were out the whole evening and got several shots. Both places are just an hour of driving from the main city of Tromsø.

Another challenge for shooting the aurora is that you never know for sure when it's going to show up, nor where and in which direction. NASA and can help you know when to expect an incoming Coronal Mass Ejection, but it's never 100 percent certain. I've seen great aurora arrive completely unexpected, and ones that were predicted have not shown up at all.

You have to choose the right shutter, ISO, and focus, all from one second to 25-30 seconds of exposure. There is no template that you can use. You must set it up all manually and adjust the camera to the intensity and movement of the aurora. And of course the aurora can form in all kinds of forms and shapes. It's a stunning experience.

More of Bernt Olsen's work can be found on his Flickr page.

* I believe he means hilltop.

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