APOD: Aurora over Icelandic Glacier (2015 Mar 10)

[APOD Robot]

Aurora over Icelandic Glacier

Explanation: Several key conditions came together to create this award-winning shot. These included a dark night, few clouds, an epic auroral display, and a body of water that was both calm enough and unfrozen enough to show reflected stars. The featured skyscape of activity and serenity appeared over Iceland's Vatnajökull Glacier a year ago January, with the Jökulsárlón Iceberg Lagoon captured in the foreground. Aurora filled skies continue to be common near Earth's poles as our Sun, near Solar Maximum, continues to expel energetic clouds of plasma into the Solar System.

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[Boomer12k]

That is just awesome.... Phenomenal work!!!!

:---[===] *

[chrisathena]

What a spectacularly beautiful photo!! Thanks.

[henrystar]

Gosh, I can almost resolve epsilon Lyrae in the water!

[billrausch]

One thing I learned taking photos over 40 years ago as a teenager is that with long enough time exposures all water is "calm". I took one 60 second exposure (middle of the night) with rolling waves, and I was totally surprised when I developed the film that the reflection of the lit bridge was just fine. I'd expected the bridge to be over blackness like it appeared to the naked eye.

[moontrail]

Water reflected stars seem more bright than directly seen in the sky. Is it just an optical illusion based in the contrast with the foreground or image post process?

[geckzilla]

Water reflected stars seem more bright than directly seen in the sky. Is it just an optical illusion based in the contrast with the foreground or image post process?

It's actually the dimmer stars which appear dimmer. The brighter stars are about the same because they manage to saturate the sensor. Everything in the water's reflection is dimmer. I've always liked reflection astro images because it's a natural way to see two brightness levels at once. In the sky, everything is overexposed and bright to bring out the dimmer stars. The reflection shows their individual brightnesses at more natural levels. Of course, the slight smearing that the moving water does helps to make the reflected stars appear larger, too.

[moontrail]

Thank you geckzilla for the explanation, if I understood it right there is an unintended sensor saturation effect that causes the difference with what would be expected.

[geckzilla]

Thank you geckzilla for the explanation, if I understood it right there is an unintended sensor saturation effect that causes the difference with what would be expected.

Maybe not unintended... but yeah, once every star is white, the only thing left to provide a sense of relative brightness is the size of the dots. If the dots don't spread much, then they all look about the same in brightness.

[Chris Peterson]

Thank you geckzilla for the explanation, if I understood it right there is an unintended sensor saturation effect that causes the difference with what would be expected.

Maybe not unintended... but yeah, once every star is white, the only thing left to provide a sense of relative brightness is the size of the dots. If the dots don't spread much, then they all look about the same in brightness.

Modern DLSR cameras have about 12 bits of dynamic range, meaning that they can capture about 9 stellar magnitudes between their noise level and their saturation level. That's muddied a little because of the way color is captured, but it's a reasonable approximation.

[Nitpicker]

Thank you geckzilla for the explanation, if I understood it right there is an unintended sensor saturation effect that causes the difference with what would be expected.

Maybe not unintended... but yeah, once every star is white, the only thing left to provide a sense of relative brightness is the size of the dots. If the dots don't spread much, then they all look about the same in brightness.

Modern DLSR cameras have about 12 bits of dynamic range, meaning that they can capture about 9 stellar magnitudes between their noise level and their saturation level. That's muddied a little because of the way color is captured, but it's a reasonable approximation.

Also, the dynamic range of DSLRs varies considerably with make and model and ISO setting. According to the internet, the Nikon D810 currently has one of the highest dynamic ranges on the market: almost 15 bits (or ~11 stellar magnitudes). But that is at ISO 50. Most deep sky DSLR astrophotography is done with ISOs of 800 or more, where the D810 gives a DR no better than about 11 bits (or ~8 stellar magnitudes). Also note that the dynamic range is not to be confused with the bit depth of the image. If I can trust the Exif data, this APOD was made with a Canon EOS 5D Mark III, at ISO 1000, which gives a DR of about 11 bits (pretty close to its maximum rating of almost 12 bits at ISO 100). Source: http://www.dxomark.com/Cameras/

(I have a Nikon D5100 with only a fixed, or an alt-az tracking mount, so I cannot make an individual exposure of more than about 30 seconds, and so I tend to use very high ISOs to compensate, and just accept a lot of noise. If I really push the ISO, my DR can be as low as 6 bits. In these cases, I don't really care if I saturate the brighter stars.)