Digital photography has revolutionized celestial renderings

Imaging the sky

Thanks to the steady flow of images from the Hubble Space Telescope and the James Webb Space Telescope onto the Internet and into the media, we are getting really used to stunning images of the cosmos.

Even amateurs, using backyard telescopes, can achieve images of a quality undreamed of even a decade or so ago. It is easy to forget that achieving these images is the end of a very hard road, over centuries.

When Galileo first pointed the telescope he made at the sky back in the early 17th century, he had to draw what he observed. Maybe surprisingly, this process of "imaging by hand" remained main stream in astronomy well into the 20th century.

Some astronomers do that today, spending hours at the telescope with a pencil and paper.

If photography became widely available in the 19th century, why did it not immediately take over? If you look at photographs of planets in astronomy books published as recently as the 1960s, you will see why. The photographs are usually blurry. There are two causes for this—the turbulence in our atmosphere and the long exposures needed to collect enough light to record the images.

Our turbulent and inhomogeneous atmosphere distorts the light waves from objects in space. This makes stars twinkle romantically. However, when we observe a planet, such as Mars, Jupiter or Saturn, we need to apply enough magnification for us to see surface details. Unfortunately this magnifies the atmospheric problems by more or less the same amount, giving us an image that shimmers or shakes, changing dramatically in fractions of a second.

Even with the light-collecting powers of telescopes, most cosmic objects are fairly faint, so exposures ranging from seconds to, in some cases, many hours are needed to collect enough light to build an image.

Unless the atmosphere is very steady, the result is a blurry image. That is because the camera recorded a superposition of many distorted images. Some astronomical objects appear quite large in the sky, they are just faint. For example the Andromeda Galaxy covers a patch of sky about twelve times the size of the Full Moon.

With little need of magnification, the atmosphere is not quite as much of a problem when imaging such objects. Atmospheric distortion can be partially avoided by putting our telescopes on the tops of mountains, above a good chunk of the atmosphere.

If we look through a telescope at a planet, we see that for occasional moments, the atmosphere steadies, the image stops jumping around and we see the details we seek. Then the dancing starts again. If we have a pencil and paper we can record the details we observed and wait patiently for the atmosphere to stabilize again, so we can grab some more details, gradually building up our image of the planet.

This requires great patience, but it does work, which is why this manual process of imaging has continued to be used, even in the age of photography. Then, in the last decade or so, digital cameras came to astronomy.

Imagine we are looking at an object where our telescope will need to collect light for an hour to build up a usable image. A continuous, long exposure will probably give us a useless blur. However, instead we can take lots of short exposures, over maybe several hours. Then afterwards we can select only the exposures that caught those rare moments of good seeing conditions. Then we combine them to build up the long exposure image we want. The result has been a revolution in astronomical imaging, for both professional and backyard astronomers. Just look in any astronomy magazine.

The other solution, available to those with the budget, is to avoid the whole atmosphere problem by putting our telescopes above that troublesome atmosphere.


• Saturn rises around 9 p.m., Jupiter around 10 p.m., Venus around 4 a.m. and Mercury, low in the dawn glow, at 6 a.m. The Moon will be full on the Sept. 29.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory near Penticton.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.

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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]

The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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