Image Credit & Copyright: Miguel Claro.
What an amazing view captured by Miguel Claro of the Milky Way over the great lake Alqueva, Portugal. This region belongs to the Dark Sky Alqueva Reserve and by the looks of this image it’s someplace I’d love to see for myself.
One of the amazing things about this image is the beautiful natural color of the stars reflected on the water. With the very limited experience that I have shooting the sky over calm water I’m always fascinated by the effect that water has to deliver star color. Also seen in this image is the beautiful color of some Milky Way objects entangled in the plane of material that laces our home star city.
Below you will find an annotated version of this image and I thank Miguel very much because I absolutely love when night sky imagers deliver an annotated rendering because it really brings the image home, especially for those who may not have the night committed to memory yet.
Airglow makes a colorful appearance in this image as well. Let’s take a moment and discover what we’re talking about because this phenomenon is often confused with light pollution and sometimes noctilucent clouds and they’re actually very different.
First of all, light pollution is why you can’t see the Milky Way in the city or why I can’t face south very effectively when I image the night sky. Even in New Hampshire, Boston is my personal night light.
Noctilucent clouds on the other hand are actually pretty tough to catch but are primarily high latitude, summer treats. They’re believed to be created by dust and ice crystals of about 100 nm in diameter at elevations of 75-85 km (46-53 mi.) that reflect sunlight.
Airglow (Atmospheric Chemiluminescence): The most popular color of airglow if you will is the beautiful green (558 nm) that you can see in many images of Earth from the ISS. Just check out the cover of astronaut Ron Garan’s book “Orbital Perspective.” Green airglow occurs between 90-100 km (56-62 mi.) high and it’s caused by excited oxygen atoms. The yellow in this image is airglow occurring at an elevation of 92 km (57 mi.) and it’s caused by the excitation of sodium atoms. Not seen in this image is also red airglow and that occurs above the green layer (150-300 km or 93-186 mi.) and is caused by the excitation of atomic oxygen. Since this phenomenon is caused by ultraviolet sunlight, airglow is 1,000 times brighter during the day we just can’t see it because of, well, the Sun.
Airglow Basics: http://www.atoptics.co.uk/highsky/airglow2.htm