Star photography
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Star trails
If you put your camera on a tripod on a dark clear night, set it to B (the "Bulb" setting) and open the shutter for several minutes you may get something like this:
Canon EOS5D 24-105mm lens @ 24mm 641s f/4 ISO200 2007:2:11 18:36:57
That is a wide-angle 10½-minute exposure of the constellations of Auriga and Gemini, with part of Orion below and Taurus on the right. The stars form trails because of the Earth's rotation and the fact that the camera is standing stationary on it. The Earth rotates by 15 degrees per hour so these trails are about 2½ degrees long.
Notice that the stars quite clearly have different colours. The human eye cannot see that so easily in the dark. Then notice that the colours are green and blue without much hint of red. That is sadly because of light pollution. The original image had a background of reddish fog due to suburban lighting which we have removed by processing with GRIP. Here is a scaled-down version, to fit in this page, of the original:
Canon EOS5D 24-105mm lens @ 24mm 641s f/4 ISO200 2007:02:11 18:36:57
Now you can see something else: the background is not uniform. This is partly due to a vignetting effect of the camera lens, perfectly normal but usually less obvious in daylight scenes, and partly because the street lighting does not produce a uniform reflection from dust in the atmosphere. GRIP can do background correction automatically and that was done to obtain the first picture above.
Here is a full-scale portion of a longer exposure:
Canon EOS5D 24-105mm lens @ 24mm 1817s f/8 ISO200 2007:02:11 18:47:55
The bright star here is the uppermost one in Auriga (see previous photo). This image shows another problem: camera noise. This is perfectly normal and occurs in all electronic devices. It is due to random thermal movements of electrons. The fact that it is thermal means that it gets worse at higher temperatures. Some types of astronomical cameras, based on CCD chips, can be cooled in liquid nitrogen to reduce the noise. Consumer cameras are not designed for that. The amount of noise also gets worse the longer we expose, so it is not a problem in the very much shorter exposures we use for daylight photos.
The good news is that thermal noise is random. So the speckled pattern we see here will be quite different from one exposure to the next. To get rid of noise we can therefore average several successive images, something which GRIP is designed to facilitate - see the batch menu or the table of images. However, for the kinds of photos we have considered so far the subject will have moved on. We need to move the camera to follow the stars rather than fixing it in relation to the Earth.
Avoiding trails
Before we look at that, consider that another approach is to take a much shorter exposure but increase the sensitivity of the camera and use the largest possible lens aperture. The next photo was taken in just 10 seconds but at f/1.8 and ISO1600. A fixed lens was used rather than a zoom in the hope of collecting more light. Zoom lenses generally have many more elements, so they contain more inter-component surfaces with the potential to reflect or scatter light. The author has not yet done any quantitative comparison of zoom and fixed lenses in this respect but he is hoping that GRIP will enable someone to do so and conclusions may be reported here.
Canon EOS5D 50mm lens 10s f/1.8 ISO1600 2006:12:06 17:54:11
At first sight this photo has nice sharp stars, but when we look at a portion of it at full scale some elongation of the star images due to the Earth's rotation becomes apparent. This would be worse at longer focal lengths:
Canon EOS5D 50mm lens 10s f/1.8 ISO1600 2006:12:06 17:54:11
So it depends what you want to do with the photo as to whether a short exposure at high sensitivity and wide aperture will suffice. This photo was also a lucky shot because there happened to be a meteor or perhaps an artificial satellite in the frame.
In subsequent pages we will look at how to make a longer exposure without trails.