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Imaging the Aurora (Merry Dancers)

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Fig. 1:The appearance of off-focus star images. The stars shown are the tail of Cygnus (Deneb the brightest star in the constellation) and the bright star Vega - lower right. Image taken November 22 2003 at 19h 35m UT.
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I have had a number of requests to explain the appearance of some of the auroral images in the Site’s Archive Section.

More specifically, why are some of the stars seen as blobs rather than dots?

(Fig. 1) This feature appears in some early images when I was using a small digital camera equipped with a Zeiss wide-angle lens. This lens was not a standard feature for the camera body in question and so it was not possible to employ manual focus. One took pot-luck whether or not the camera’s sensor could locate the faint images of stars as “points” of light. (Fig. 2); sometimes it did, on other occasions it did not. Hence, the blobs are out-of-focus star images.

Another question I am frequently asked is what type of digital camera is best suited to this type of work? Without question a single-lens reflex camera provides greatest flexibility. What is required is a wide-angle, fast lens together with control over aperture and shutter speed.

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Fig. 2 A lens focussed for infinity and therefore giving sharp star images. The stars of The Plough may be seen at the centre of the photograph. November 22 2003 at 23h 17m UT. Note: this display went on for over five hours.
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There are now available on the market a number of “old” manual lenses at reasonable prices. Remember that the effective focal length when such lenses are used with a digital reflex camera will be 1.5 longer than that specified for a film camera. For example, a 24mm focal length lens will function as a 36mm lens. Ideally one requires something shooter than this to achieve a truly wide-angle.

The next consideration is speed. Fast, short-focus lenses tend to be rather expensive. A dedicated 18mm f/2.8 would be ideal. For auroral work. I use a 18mm f/3.5 zoom lens on my D70 and D100 digital cameras. To compensate for the relatively small aperture of f/3.5 I set the ISO to between 400 and 800, depending on the brightness of the aurora.

For really bright displays it is possible to use even lower ISO values. In addition I employ long exposure compensation to reduce “noise” since some exposures may be as long as 15 seconds. Noise can be a factor when using high ISOs.

Ideally an exposure should be a fast as possible. Auroral patterns can change very quickly requiring exposures of no more than 2 seconds, even faster in some cases. For an auroral glow I sometimes use speeds as slow as 25 seconds. Star images may then appear slightly elongated, but this is hardly noticeable with wide-angle, short-focus lenses.

It is essential to have the camera mounted rigidly on a tripod. Never use a hand-held camera if you can possibly avoid it. Few can hold a camera firmly enough to enable exposures of longer than 1/25 seconds to be used effectively and auroral photography, in general, requires much longer exposures than that.

One further point to consider when using a digital single-lens reflex camera is the appearance of fragments on the protector plate. Most manufacturers now fit a “self-cleaning” device to the camera but early models, particularly by Nikon, require this tricky operation to be done by hand. Always avoid changing lenses with the camera pointing in the vertical position.

When best to look out for an aurora.

Another frequently asked question is: when to look out for an aurora? (I phrase the question almost exactly as it is asked.) The answer is simple – from high latitudes, any dark, clear night! This is not to say you will be guaranteed an aurora for no one can be certain on that score.

Because the phenomenon of the aurora is linked to solar activity one can gain a great deal from examining the Sun’s behaviour. The appearance of sunspots may not be indicative of aurora in itself but must be taken in conjunction with activity observable in other, discrete wavelengths (H-alpha for example) and from magnetic observations via the magnetometer.

The Sun has been relatively passive now for over a year. There have been weeks at a stretch when nothing has appeared in visible light on the disc. (Visible light = that region of the spectrum to which the human eye is sensitive.) In my fifty or so years of observing I have not known such a dearth of spots close to sunspot minimum.

JV04/12/08

 

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