Star Spy: Art and science above

FREIDL HALE - TEKAPO STARLIGHT
Last updated 10:44 26/02/2013

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A SKY FOR ASTROPHYSICISTS

When doing scientific research on distant objects through optical telescopes, such as that done by various universities at Mt John Observatory, you want a perfect dark sky. 

You usually don’t get perfect, but you want no moisture in the atmosphere, no smoke, or smog, or dust; no turbulent layers of air with different temperatures, or different wind speeds in different directions.

You don’t want light pollution from towns shining up through the atmosphere as it can brighten the whole sky, diminishing the light you want to capture using your telescope. 

You want a pure, undisturbed, stream of photons streaming into your instrument. 

Ideally, what you want is a telescope situated above the atmosphere, like the Hubble Space Telescope. But in reality you have to make do with what you have available, both the sky and the telescope.

A SKY FOR ASTROPHOTOGRAPHERS

A photographer, on the other hand, can be more forgiving and use some of the scientist’s bad sky features to good effect, even clouds.

The beautiful picture above, of the nearly full Moon rising in clouds, is a lovely example of the astrophotographer as artist supreme. 

Fraser Gunn has many excellent photos sharply illustrating the details of craters, ridges, smooth plains, impact rays, and other geologic formations on the Moon.

But the image above is romantic - special, precisely because of the clouds that appear to be swirling around the Moon, clouds glowing with its light.

It is also special because of the deep golden colours. These are taken from the atmosphere, from its density as viewed low on the horizon, and perhaps from pollution from some near or far event, like smoke from a forest fire in Australia.

Also in Fraser’s time-lapse video of an aurora (below), the passing of clouds adds interest, drama and a sense of the passage of time to what we see.

What about those nights when all the stars are twinkling madly? 

They brighten and dim rapidly and even change colour because of turbulence in the upper atmosphere, or moisture in the air that bends the starlight.

What about high thin cloud or haze everywhere?

Some of Fraser’s wide-angle images of constellations, and the zoomed in views that I have cropped out of them, have benefitted from being taken on just such nights. 

Haze can give a slightly fuzzed appearance to the brighter objects, making them appear larger. Twinkling can also cause this effect during a long exposure.

At the same time that bright objects are made bigger, dimmer objects are obscured leaving just the brighter ones.

This can make it easier to identify the main stars in a constellation or objects of interest, by isolating them and enhancing their light. 

Also, in a clear dark sky most stars appear white, while many are actually more red or blue in colour than they appear. 

That same moisture in the air that gives the fuzzy appearance can also capture more of a star’s colour, making for not just a more colourful image, but a more informative one as well. 

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In the second example above, from last year, you have bright blue star Spica upper left, and golden Saturn above centre, both with their size and colour enhanced by cloud, while Mars, below left, is not in the cloud.

Then there can be the occasional night when you observe a remarkable ring around the Moon, formed from the moonlight on ice crystals high in the atmosphere.

The lesson here is don’t necessarily put your gear away just because the sky isn’t sharp and clear.

- Stuff

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