The Isle of Man is blessed with lots of ’dark sky’ sites - sites with little light pollution and allowing unrivalled views of the night sky.

A recent article covered techniques for photographing the moon but stars require some rather different, more specialist techniques.

As with moon photography, however, essential kit includes a good tripod and a small torch or headlamp, together with ’manual’ camera settings, but thereafter the photographer has a greater choice of lenses - either wide angle for shots of the Milky Way, for example, or long lenses for shots of specific stars or galaxies.

However, other issues then need explanation.

The night sky is dark and the stars very faint pinpricks of light - and the camera requires light to register an image.

Balancing the three elements of an exposure - the ISO setting (the sensitivity of your sensor to light), the aperture (the size of the ’hole’ in your lens which allows light to hit the sensor) and the shutter speed (the length of time the aperture is open) - is a critical compromise.

The very low light levels mean you need to maximise the light gathering potential of each of these elements, so long exposures are inevitable even with the lens at its widest aperture (usually something like f2.8 or f4) together with a high ISO setting.

It helps that the latest camera sensors have become incredibly sophisticated as regards the ISO settings - allowing ISO settings as high as 10,000 or more (which maximises the light gathering potential) - which in turn allows for a faster shutter speed though usually at the expense of ’noise’ being generated (artefacts which deteriorate the image).

The next difficulty is that the earth rotates once every 23 hours and 56 minutes, which at the equator means a rotational speed of approximately 1000 miles per hour. With rotation however, long exposures result in individual stars blurring and becoming ’star trails’ which sometimes may be the photographer’s intention.

But blurring is exacerbated with long lenses as compared with wide angle due to the much narrower angle of view.

So there is a ’rule of thumb’ (really a guideline) used in astrophotography called the ’500 rule’.

This broadly suggests dividing 500 by the focal length of the lens you’re using to establish the longest shutter speed possible to achieve a reasonably sharp image without trails occurring.

A 20mm wide angle lens on a full frame camera would therefore have a longest shutter speed of 25 seconds (500/20 = 25) - but only 16 seconds on a crop frame body (because a 20mm lens on a crop frame is equivalent to 30-32mm focal length).

So, when shooting the Milky Way and using a wide angle lens of (say) 14mm, the following settings are suggested as a start point - an ISO of 3200, the lens wide open at f2.8 if possible, and the shutter set at 25 to 30 seconds.

For individual star or deep space shots, and with a necessity to use longer lenses (for example 500mm), the 500 rule would mean an exposure time of only 1 second - clearly insufficient to gather enough light for an image.

Many photographers therefore invest in ’star trackers’.

These mount on the tripod and move the camera in synchronisation with the speed of the earth’s rotation to allow the long exposure times necessary for a sharp image - typically 60 seconds or longer.

Sharpness will also be helped if you use the camera self-timer - this sets a delay of typically 2-12 seconds when you press the shutter button before the shutter releases and ensures a perfectly stable camera at the instant the shot is taken.

Whether shooting with wide angle or longer lenses, be prepared to experiment with the settings for best results - practice is necessary and there will be lots of unsatisfactory results.

But get it right, and you will be very pleased to have some wonderful images of the heavens.

Belonging to a camera club such as the Isle of Man Photographic Society will give opportunity to swap experiences with fellow members and to develop your skills. Have a look at our website - www.iomps.com or our Facebook page for lots of ideas to stimulate your interest.

Chris Blyth