It is often assumed that because the stars only come out at night, taking photographs of them must be difficult and require very specialised and expensive equipment. In fact, this is not the case and with advances in technology even relatively ordinary cameras can produce images that only a decade or two ago would have required much more complex gear.
Firstly, it's important to understand that a photograph is produced by exposing a light sensitive medium to a certain amount of light for a certain amount of time. In the past that medium was film, and it was relatively insensitive to light, the degree of which was decided at the time of purchase. The single biggest change in photography in recent years is that sensors are now digital, they are considerably more sensitive, and that sensitivity is adjustable.
This gives us many opportunities when photographing the night sky and its many wonders, which we'll consider through some examples taken by snap-happy stargazers. To start at a simple place we’ll assume that you have a camera with at least some adjustment of aperture, shutter speed and sensitivity (ISO). Most high end compacts and virtually all bridge cameras, compact system cameras and digital SLRs will have these controls.
The main accessory that you may not already have is a reasonably sturdy tripod. The length of exposures used in night time photography is such that hand-holding cameras is impractical. When an exposure can be several seconds long then good support is essential. A cable release is useful as this allows the shutter to be pressed without risk of moving the camera, though the self-timer can also be used for this if your camera has no remote socket.
A refinement on the cable release is an intervalometer – this is a timer that takes control of the camera and can be set up to take many exposures over a period of time. This setup can be used to take single shot exposures of several seconds, or even tens of seconds, and also can take a sequence of many images over a period of time.
Getting the focus right is crucial to getting good pictures and is made more difficult by the fact that autofocus doesn’t work in the dark. The standard trick is to switch to manual after having set the focus using distant lights, or, if it is available, the moon. In practice, any source of light more than a mile or so away is far enough to bring the night sky into focus.
The Earth's Rotation
It might be assumed that because the Earth takes almost 24 hours to rotate, we don’t need to worry about it with camera exposures of only a few seconds. And we’d be wrong – the effects of the Earth’s movements become very apparent quite quickly. There is a rule of thumb called 600/f which defines the longest exposure that can be taken with a lens of a focal length (35mm equivalent) of f. So with the example of the commonly used 50mm lens the longest possible exposure without star trailing is 600/50 = 12 seconds.
A telescope may have a focal length of 1200mm, so using it the maximum exposure would be 0.5 sec. The solution to this is to attach everything to an equatorial mount and this is more or less essential for any photography involving a telescope. Many telescopes have a custom mount which enables a camera to be fitted to them via an intermediate adaptor called a T-mount.
Camera images taken at night will generally benefit from some additional processing. Photoshop in its various editions is the standard tool for the job but there are also free tools such as Paint.net and Gimp which can be adequate enough stand-ins. There are also specialised tools such as Deep Sky Stacker, which adds together lots of images to reduce noise, Startrails.de, used to stack images taken over a period of time to mimic a much longer exposure and others.
Although this phenomena has been somewhat over-hyped in the press, it has been helpful in raising interest in astronomy and to that extent it has been a good thing. In fact the full moon is only slightly larger than usual at this time and there are three of them every year, so not as rare as they're often made out to be.
The full moon is not the most photogenic of moons either – I would suggest earlier phases such as the first quarter would make a stronger photo. The moon is a small subject, but it is very bright in astronomical terms, being the second brightest object in the sky after the sun. It is small however and getting surface detail requires a lot of magnification. Without using a telescope, a camera with a long telephoto lens, such as a bridge camera, is best for this.
These are images that can be taken by anyone with a camera and standard equipment such as a tripod and remote shutter release or by using the self-timer feature on your camera to avoid blur.
The Northern Lights, or Aurora Borealis, are generally thought of as being an Arctic Circle phenomena, but actually when the circumstances are right they can be seen as far south as Northern Ireland. Some displays can be very bright with red, green and purple colours visible to the naked eye. More often though only the green is visible to the eye with other colours appearing as a white fog because the display is not bright enough to activate the colour sensors in the eye which are less sensitive than the luminance receptors. The camera will show a different view entirely and this is called a 'Photographic Aurora' for that reason.
The setup here involves having the camera on a tripod. The displays are large so a wide-angle lens is best, but it’s also faint so long shutter speeds and high ISO will be used.
This was taken with a Lumix G3 mirrorless camera with a 20mm (40mm equivalent) f1.7 lens – 10 secs f1.7 ISO 1250. Had I been using a compact camera or a bridge camera I would have used a longer shutter speed and lower ISO as the small sensors in these cameras tend to be noisier. Since aurora displays move, sometimes quite quickly, shorter exposures capture more detail and this is one area where an expensive full-frame camera has an advantage.
This is a rare example where, because the subjects were quite bright, it was possible to use a small telescope on a simple tripod – no motors required. This happens occasionally – because the solar system is fairly flat sometimes objects pass each other in the sky. The sunlit part of the crescent moon is overexposed here, but the earthshine – the rest of the moon lit by light reflected from the earth - has exposed well and as well as Jupiter we see the four Galilean moons.
Constellation or Starfields
This is a wide field area of the sky – in this case the constellation of Ursa Major which can be viewed on any clear night of the year from our latitudes. The Plough is an ‘asterism’ or group of stars which are part of the constellation. Cameras with standard 50mm lenses are very suitable for taking wide field images such as this.
Startrails with Aurora Borealis
This one uses a combination of techniques. The original photos were taken over a period of 20 minutes – there were 50 of them each 20 seconds long, all taken with a mirrorless camera with 7.5mm fisheye lens on a tripod. The original intention was to create an animation showing the motion of the aurora and this was done as well, but as an experiment the images were fed into Startrails.de software which is designed to stack a number of images together to create star trails as we see. The end part of the processing used a piece of software called PTLens which straightens out fisheye images.
The Milky Way
Being a faint diffuse object, the most important thing with getting a good Milky Way image is to find a good dark sky site. Fortunately Northern Ireland has many good locations such as the Antrim Hills, the Sperrins and the North Coast from which it is possible to get good images. Martin used a wide angle lens with a wide aperture, and a fairly long exposure.
An example of a meteor or shooting star. Taken with a camera mounted on a tripod with a standard 50mm lens. This image taken by Pat O’Neill from his back garden in Belfast was snapped during the Perseid meteor shower in August 2015 with a 10 second exposure at ISO 1600.
During the months of June, July and August in the Northern Hemisphere it is possible to see displays of noctilucent or ‘night-shining’ clouds (NLCs). Again these can be photographed with a camera on a tripod with a standard 50mm lens. Typically the shutter can be left open for 15 - 20 seconds and the ISO set to 400. This picture was taken over Belfast Lough from Carnalea by Andy McCrea in 2014.
Pat O’Neill captured this image of a fine crescent moon with a passing aircraft over a church spire. A tripod mounted camera with standard 50mm lens was used.
Professor Alan Fitzsimmons captured this fantastic photograph of moonset from La Palma in the Canary Islands. This mountain peak is the setting for some of Europe’s largest telescopes and the sky is amazingly clear.
A Sun Halo is caused by refraction of light by ice crystals in the atmosphere. Image David Stewart, taken at St John’s Lighthouse Killough. Camera Sony KSC, ISO 125, 1/1600 sec, f9
Beginners will have the opportunity to try some of the settings and techniques outlined in this article with some upcoming conjunctions. From 7.30 - 8.00am on January 19 the moon and Jupiter will be close together in southeastern sky. Then on January 31 the moon, Venus and Mars will be in conjunction just after sunset. Look around 5.50pm onwards in the southeastern sky. These conjunctions can be seen ahead of then using the free planatarium programme, Stellarium.
The Irish Astronomical Association has over 200 members across the UK and Ireland and it holds regular lecture meetings in Belfast every two weeks from September through to March. Meetings, outreach activities and observing sessions are open to amateur astronomers and members of the public and more information is available on www.irishastro.org.
The IAA has recently staged a number of photographic exhibitions across Northern Ireland, starting in the Linenhall Library in Belfast, moving to the Clotworthy House in Antrim and currently the exhibition of members’ photographs is available for viewing in the Lisburn Island Arts Centre until February 9. To find out more visit www.irishastro.org.