Comet Machholz shows a tail - February 12

Fig. 9: Comet Machholz 14 February 21h 26m - 32m. A 4 min. mono exposure 270mm f/2.8 ISO 300, guided camera. Field 3 arc degree square approx. Shows stars to below 16 mag. despite presence of Moon (ph. 29%).

Fig. 8: Comet Maccholz imaged 21h 30m February 12, 2003. A 3 minute exposure 270mm f/2.8 ISO 500, guided camera (see text). In this 2.5° square field a faint tail is visible for about 2° in PA 80°. The brightest star in the field (top right) is mag. 4.84; stars to mag. 15 are visible.

The weather here in Orkney has been so appalling this winter that one is grateful for a few minutes glimpse of the stars, even if it means observing in a force 10 gale.

This was the case on February 12 and despite the presence of a crescent Moon, the comet was just visible to the unaided eye at 21h 30m.

Unfortunately here on Rousay the wind was gusting to force 10 from the NNW making it impossible to observe from the observatory site. A telescope was set up on a temporary site offering some shelter by which the photograph in fig. 8 was obtained.

It will be noted that the stars appear as short trails. This is due to rough equatorial alignment of the mounting for this 3 minute exposure. There was simply not time in the bad weather to be more precise. A thin, faint tail is visible for about 2° in PA 80°.

The comet has been receding from the Sun since January 25 so that one can only expect the tail to grow weaker.

JV 13/02/05

Imaging the Aurora Borealis with Digital Cameras

Fig 1 Looking S 2003 Nov 20 21h 21m. The bright object, bottom right, is Mars, mag. -0.6. Square of Pegasus above.

For auroral photography one requires a camera able to capture images over a large area of sky using the minimum exposure time.

Whereas there are now a fairly large number of digital cameras available with hefty zoom lenses, less attention is paid to wide angle compared to the other extreme - bigger is better!

Not so for our purposes. Anything more than 35 mm focal length (equivalent conventional film camera) will be restricting.

Fig 2 Looking NE 2003 Nov 20 21h 24m. Capella, Aldebaran and The Pleiades (near centre) are all clearly visible.

Some cameras, notably Sony's DSC V1, offer lens attachments which provide a much wider field. I have used this camera to obtain some fine images but the results vary in quality due to the camera's inability to focus at infinity, which then yields star images as blobs (Fig 1 & 7). (Unfortunately the attachment inhibits the use of manual focusing.)

This camera also offers speeds to 30 seconds, a good deal better than many digital cameras.

One will always go for maximum aperture when photographing aurora. Once again few digital cameras offer anything below f/2.8. The exposure time will have to be judged according to the brightness of the aurora.

Fig 3 The corona in the zenith 21h 25m. Perseus and Cassiopeia both visible in top left and centre respectively.

Where elements are fast moving (hence the term 'Merry Dancers') one has to aim for a short exposure and so the maximum ISO value should be used. Noise tends to become a feature in some cameras once ISO 800 or more is combined with long exposures, say 15 secs. or longer.

In any case, except for glows and other slow moving features, 15 secs. is too slow. Fortunately a great deal can be done at the computer to compensate for an underexposed image. (Fig 5)

Those with access to a SLR digital camera are in a better position.

Fig 4 Looking W 2003 Nov 20 at 21h 27m. The bright star is Deneb in Cygnus.

The Nikon D70 with its 18 - 70 (27 - 105 equiv.) is a handy tool despite the f/3.5 restriction. This camera offers speeds as low as 30 secs. as well as a "bulb" feature. Unfortunately the latter is not cable activated and I find the remote control fails frequently in cold weather.

However, this is not likely to worry auroral enthusiasts since rarely would there be any point in exposing above 30 secs even at f/3.5 - but see below. (Note: this camera offers ISO values up to 1600.)

Fig 5 Looking NE 2003 Nov 20 17h 29m. Capella middle left, Hamal (in Aries) middle right. The Pleiades low down left of centre.

The best lens combination with this camera would be the wide angle 20mm (30 equiv.) f/2.8.

The camera should be mounted on a firm tripod. Wind is so much a feature of Orkney life that one is seldom able to find protection from it.

Avoid camera shake when depressing the shutter button. A cable or remote control can help here. Also try the self-timer to avoid physical contact with the camera at the critical time.

Fig 6 The corona 2003 Nov 20 17h 31m. Deneb and Cygnus close to the zenith, Vega far right and Altair bottom mid-right.

If you work from indoors then make sure there are no internal reflections on the window pane.

The hallmark of a good photograph is one in which the features of the aurora show clearly against a background of stars whose images are as near to points of light as is possible with digital techniques. (Fig 3 & 4.)

Long exposures not only diffuse the detail in the aurora but the stars appear as trails when enlarged.

Fig 7 The corona, this time south of the zenith in Andromeda. 2003 Nov 20 21h 28m

The character of the aurora can change very rapidly, often within a few seconds. The choice of camera settings for image size (pixels) etc. is a matter of discretion.

The samples here have been reduced to a manageable size for transmission purposes.

Caution! Light pollution may fool the unwary. The presence of cloud and mist can scatter light from the ground so as to resemble aurora.

Finally, it is often profitable to photograph the region of active aurora even if the light is very low. Use longer exposures and experiment at the processing stage. You are very likely to reveal features that were quite invisible to the human eye.

The images shown here are all from the great aurora of 2003 November 20 taken on Rousay.

JV 28/01/05