AAO image reference MISC 23. « Previous || Next »
Image and text © 1985-2002, Australian Astronomical Observatory, Photograph by David Malin.
Star images are point sources of light of widely varying intensity. Normally, no single photographic exposure on colour film can capture their subtle colours. Those stars whose intensity is just right for their colour to be recorded appear insignificant on the photograph, while anything brighter is overexposed and washed out.
By changing the focus of the lens during a star trail exposure, star images are spread out over the film surface. This effectively changes the amount of light in each star image as well as making it bigger. If this is 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.
Here we see the stars of the Southern Cross and pointers (alpha and beta Cen) recorded in an exposure of about 30 minutes, during which the lens focus was moved from infinity to about 1m in about 15 equally spaced steps. For a fuller explanation see a similar image of the stars in Orion.
Related images
AAT 5. Star trails southwest of the AAT dome
AAT 6. Star trails around the south celestial pole
MISC 5. Dawn and evening twilights reflected in the AAT dome
MISC 6. Moonset into cloud over the Warrumbungle Range
MISC 7. Star trails around the AAT dome, after Pinatubo sunset
MISC 8. Aurorae Australis
MISC 12. Orion's belt rising over the lights of Coonabarabran
MISC 14. South celestial pole star trails
MISC 13. North celestial pole star trails
MISC 15. North and South celestial poles star trails
MISC 11. Orion star colours, step-focus technique
MISC 22 The AAT dome from the Director's Cottage.
MISC 16. Southern Cross and Pointers, star colours - step-focus technique, long trails
MISC 18. The view to the north from Siding Spring
MISC 19. Sunset 'star' trail, the track of the setting sun