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About star trails


The important things

The quality of the sky is the single most important ingredient in successful star trails photography, and that depends a lot on where you live. The next most important thing is to experiment and take notes of your exposures and camera settings as you go. The third most important thing is to be patient -- exposures can last for hours ... you will have plenty of time for your notes.

These notes refer mainly to work using colour film between 1976 and around 1996, a period when digital cameras were a distant dream. Of course such cameras are now much superior to traditional photography for most work involving faint light, though very long exposures are (as used here) are not practicable. And for star trails they don't capture the colours of the stars as well as photography did. With film, the image is captured in one exposure, but with a digital camera many relatively short exposures are made and combined in software. However, the basic ideas below are applicable to all star trail imaging.

Some background - the airglow

Australia has about the same land area as the USA but only 8 percent of its population, and most Australians live in the large cities on the south-eastern seaboard. In addition, the southern hemisphere is relatively unpopulated and only lightly industrialised, so there is little air pollution. What little there is cleansed by the vast expanses of ocean that encircle the globe at southern latitudes. Away from the cities the sky of the outback is transparent and very dark at night - if there are no clouds. This is not the case in most of the USA or Europe, indeed over most of the northern hemisphere. Even when a truly dark sky is found, airborne particulate pollution from distant industrial activity is carried around the world and can affect night-time photography. Happily, it does not cross the equator.

In the southern hemisphere, the velvet blackness allows long photographic exposures during the night without any signs of the sky fogging from artificial light. However, the night sky is never completely dark even in the middle of Australia. The air above is feebly luminous at night, mainly from the natural 'airglow' which arises as the atoms and molecules of the air lose the solar energy they have absorbed by day. When the sun is very active the night sky is brighter, even to the point of being obvious to the eye, as in aurorae.

Rarely, the sky just appears milky to the eye as it does when the moon is up, but there is no moon. Long exposure photographs show it to be coloured, and examples are here and here. Other effects such as scattered starlight, interplanetary dust (zodiacal light and gegenschein) and distant, unresolved stars and galaxies are always there and add a little to the airglow. Of course, long-exposure star trail photographs are compromised when the moon is in the sky, and exposures are relatively short, as here. But sometimes a sliver of moon and a little cloud can make an interesting picture.

Siding Spring Mountain, in the Warrumbungle Range, is about 500 km by road from Sydney in eastern Australia and was chosen as an astronomical observatory for its clarity and darkness. It is the site of the Anglo-Australian Telescope (AAT) a 3.9m reflecting telescope built in the mid-1970s as a joint Australian-British venture. The telescope itself can be used as a giant camera, when it functions as a 12.7 m focal length 'lens', working at f/3.3. The observatory also operates the UK Schmidt telescope (UKST) which is effectively a 3000 mm catadioptric 'lens' working at f/2.5.

About star trails

Star trail pictures are made when the camera is fixed (best to a tripod) and the shutter is left open for long periods at night. Not all cameras can do this, especially not digital cameras. Long exposures with digital cameras produce 'noisy' images. This has improved as digital cameras improve and some cameras have noise reduction technology built in. It can also be useful to make a long exposure 'dark frame' to be subtracted from each image exposure. Even so, at the time of writing, exposures longer than a few remain difficult for most digital cameras, and exposures longer than an hour (like those mentioned below) involve combining many separate, noise-subtracted frames in software. This is not to detract from digital cameras; they can make fine star trails and beautiful time-lapse videos of the night sky, but the images referred to here were all made on film.

Digital cameras are generally much more sensitive to faint light than film cameras, so for many low-light or twilight images they are excellent, however, working at the time I did, I used film, and at the time of writing it's still available. Modern, battery-operated film cameras can be made to take very long exposures, but usually at the cost of battery life. The best sort of camera for this work using film is a simple, wholly mechanical camera, that can be picked up easily for modest cost in many used camera stores.

When I use 35 mm film for star trails I use an ancient Pentax Spotmatic I have had for 40 years. It is best to use a fixed-focus (non zoom) lens if possible, partly because zoom lenses used near full aperture show serious vignetting -- light fall-off towards the corners of the frame. Zoom lenses generally have poorer image quality than an equivalent fixed focal length lens and have larger F numbers, so they are slower, photographically speaking. Extreme wide angle lenses produce interesting star trail effects but a standard lens (55mm on 35mm film) is perfectly useful. Of course, long exposures can be made with a tracking camera -- but that's a different topic.

Most of the star trail images referred to below were made with a mechanical camera, a Hasselblad 500C/M. The 6 x 6cm format produces better quality than 35mm but a fancy camera like this is not necessary for star trails. However, it was useful when many of these pictures were made, in the 1970s and early 1980s. In those days, fast films were quite grainy, so small formats were not as good for star trails. This is no longer true, so a 35mm camera loaded with 200-400 ISO slide film works perfectly well. Colour negative (print) film is OK, but it is always grainier for a given speed and most labs don't know how to print star trails pictures. Of course negatives can be scanned for good results, but in general, slide film is better.

Most of the exposures were made on 200 ISO Ektachrome, some on similar Fujichrome and usually the exposure was started about 90 minutes after sunset, when the sky is completely dark except for the low western horizon. Under a dark sky, exposures can last up to 10 hours, using a lens set to F/5.6-F/8. For many of the pictures on these web pages the film was push-processed by one stop, which gives useful extra contrast as well as speed.

The picture AAT 5 shows the trails of stars setting behind the AAT dome. The motion is the Earth's rotation of course -- the stars don't move. There's also some artificial light from houses on the mountain top and the signs of a night assistant (telescope systems operator) lighting his pipe as he wanders around the dome catwalk.

The radius of the curved star trails decreases towards the left of the picture which is towards the south. If the camera was pointed due south you would see the stars apparently circling a point on the sky, the south celestial pole. This is the projection of the earth's axis of rotation in the sky and is seen above the dome in the companion picture AAT 6. More detail on what the star trails show is on MISC 36.

Of course from the northern hemisphere you would have to point your camera to the north to achieve the same circular effect.

Extended caption for AAT 6

The rotation of the Earth makes the stars follow gigantic arcs across the sky. These arcs appear to circle a point which is, in the southern hemisphere, the south celestial pole, the projection of the Earth's axis of rotation on the sky. At Siding Spring, where this picture was taken, this point is 31 degrees 16 minutes above the horizon, the exact geographical latitude of the site of the AAT. If a similar picture was taken in Antarctica or the Arctic (90 degrees south or north) these circles would be directly overhead, while from the equator they would appear as semicircles on the northern and southern horizons.

The stars have traced out an angle of about 160 degrees, indicating an exposure time of over 10 1/2 hours. If you measure the angular extent of the star trails yourself, don't forget you are measuring 'sidereal time' not clock time. The difference is negligible for these purposes, but it is instructive to think of why the two time systems should be different. There's more about sidereal time on the caption for MISC 5.

The trails reveal the colours of the stars, which in turn tell us of their surface temperatures. Blue trails are left by stars which are much hotter than the Sun, while cooler stars leave orange or yellow trails. This exposure was so long that the feeble glow of the night sky itself was recorded and during the long winter night observers using AAT occasionally peered outside to inspect the weather. As they walked around the dome in darkness, their lights produced the irregular lines at the catwalk level.

Long exposure star trails pictures on this site

AAT 5.     Star trails southwest of the AAT dome
AAT 6.     Star trails around the S celestial pole
MISC 4.   The AAT dome by the light of the full Moon
MISC 5.   Dawn and evening twilights reflected in the AAT dome
MISC 6.   Moonset into cloud over the Warrumbungle Range
MISC 7.   Pinatubo sunset and star trails around the AAT dome
MISC 8.   Aurora Australis from Siding Spring
MISC 11. Orion star colours, step-focus technique
MISC 12. Orion's belt rising over the lights of Coonabarabran
MISC 13. North celestial pole star trails
MISC 14. South celestial pole star trails
MISC 15. North and South celestial poles star trails
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
MISC 22. The AAT dome from the Director's Cottage.
MISC 23. Southern Cross and Pointers, star colours - step-focus technique, short trails
MISC 29. The UK Schmidt Telescope under a milky sky
MISC 32. Antares and Jupiter, defocused trails
MISC 36. Southern Cross star trails around the AAT dome

Other beautiful examples of star trails are seen on the pictures by David Miller and Akira Fujii

David Malin, March 2012.

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Updated 2012 March 23   dfm@aao.gov.au