Astronomy Picture of the Day
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2002 July 29 
Posted on 07/28/2002 10:26:49 PM PDT by petuniasevan
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2002 July 29
Explanation: The Sun appears to move on the sky because the Earth rotates. The extreme brightness of the Sun, however, makes it difficult to capture a sun-trail -- the path the Sun traces on the sky. To capture the above picture, a very dark filter covered the camera lens for most of the time, allowing only a trifle of light from the bright Sun to peak through. Just after the Sun had dipped below the horizon but before it was completely dark, the thick filter was removed and the pretty foreground scene was captured. Slight flares appeared when the Sun went behind thin clouds. Star-trails and planet-trails are much easier to image, and a similar Moon trail has also recently been imaged.
You can try this at home, IF you have the right equipment. You will need a tripod, camera with manual exposure settings, manual film advance, and a GOOD QUALITY solar filter. Use low-to-medium film (ISO 50 to 100).
About this photo (quote from David Malin):
The trusty Hasselblad was set on a tripod with the 80mm Planar lens set to f/16. It was loaded with 200 ISO Ektachrome because that's all I had. It was also fitted with a neutral density filter with an attenuation factor of 10 million (equivalent to ND 7), made simply by mounting Wratten ND 3 and ND 4 filter gels together in a screw-on filter mount.
The shutter was opened before the sun appeared in the frame and was left open all afternoon, probably five hours. Once the sun had set, the shutter was closed, the ND filter removed and, without winding-on the film, a short exposure was made of the after-sunset horizon, probably 1/25 at f5.6 or f/8. The flares around the sun trail are where it passed behind high cloud and they are greenish because the Wratten filters are not truly neutral and even the slightest colour cast would be evident with this amount of ND over the lens.
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Moonset in the Warrumbungles
Image and text © Anglo-Australian Observatory, Photograph by David Malin.
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