AAO image reference MISC 11. « Previous || Next »
Image and text © 1985-2002, Australian Astronomical Observatory, Photograph by David Malin.
Star images produced by a camera lens are point sources of light of widely varying intensity scattered over the camera's digital detector, or in this case, film. Normally, no single photographic exposure can capture the star's subtle colours. Those stars whose intensity is just right for their colour to be recorded appear insignificant on the photograph, while anything brighter appears bigger, but is overexposed and appears washed out, colourless.
By changing the focus of the camera lens during a star trail exposure, star images are progressively spread out over the detector surface. This effectively dilutes the amount of light in each star image as well as making it bigger. The bigger it is, the lower the intensity of light on the detector. In this way the focus shifts alters the effective exposure of the pinpoint stars. If the focus shifts are done in a series of steps, as here, sooner or later all the star images will be recorded with an exposure that will reveal the star colours. The motion is provided by the earth's rotation -- this is a simple star trail photograph with a stationary camera -- with a twist.
Here we see the stars of the constellation of Orion recorded in an exposure of about 30 minutes, during which time the lens focus was moved from infinity to about 1 metre in a series of 10 steps about three minutes apart. The effect is to reveal the star colours and that of the vivid red Orion nebula. Any camera where the shutter can be left open for long periods will work well. Equipment that thinks for itself does not know how to do this, so an old fashioned, mechanical camera like the Hasselblad used here works best, but digital cameras can also be persuaded to do this. The lens here was the standard, 80mm, F/2.8 lens for this camera and was used with a normal clear-glass UV filter. It was set to about F/4 and 400 ISO transparency film was used. None of these settings, equipment or materials is critical, but the smaller the F-number the bigger the out of focus images become.
MISC 16. Southern Cross and Pointers, star colours - step-focus technique, long trails
MISC 19. Sunset 'star' trail, the track of the setting sun
MISC 23. Southern Cross and Pointers, star colours - step-focus technique, short trails
MISC 32. Antares and Jupiter, defocused star trails
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