Southern Cross, defocused trails
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Southern Cross, defocused trails
Image and text © 1985-2002, Australian Astronomical Observatory, Photograph by David Malin.
Roll your mouse over the picture to see the Southern Cross stars
Star images are point sources of light of widely varying intensity, and normally no single photographic exposure on colour film or a digital detector can capture their subtle colours. With an in-focus photograph, those stars whose intensity is just right for their colour to be recorded seem tiny on the photograph. Anything brighter is overexposed and washed out. De-focusing the picture during the exposure as the star cross the sky solves this problem by expanding the star trails, reducing the effective exposure of each one. At some focus, depending on the star's brightness, the exposure is about right and the colours are recorded. This also produces an interesting abstract effect. Roll your mouse over the image above to see a representation of the Southern Cross stars created in Photoshop, based on Akira Fujii's photograph of the Southern Cross.

Here we see the stars of the Southern Cross and pointers (alpha and beta Cen) recorded in an exposure of about 40 minutes, during which the lens focus was moved from infinity to about 2m in six steps five minutes apart. For a fuller explanation see a similar image of the stars in Orion.

Related images
MISC 11. Orion star colours, step-focus technique
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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Updated by David Malin, 2012, March 17