There’s a wide spectrum of photography that could fall under the general description of “Night Sky photography”, and the techniques and approaches for (for example) taking an image of a landscape under a starry sky or Milky Way are very different from those for taking an image of a feint deep sky object. Some subjects may need very specialized equipment and techniques, while others are more accessible to the typical photographer with standard equipment used for daylight photography.
Subjects that can be tackled without specialized equipment include the Moon. It’s surprisingly bright so use the standard “Sunny 16” rule to decide on the exposure. for those not familiar with this, it’s a way to meter for correct exposure without using the camera’s exposure meter. So for example, if your ISO is 200 at f/16, then your shutter speed will be 1/200 secs. If your ISO is 100, then your shutter speed will be 1/100 secs. Other subjects are “starscapes” of a starry sky or the Milky Way over a landscape and star trails. There are also some brighter objects in the sky such as the Orion nebula, the Andromeda galaxy, and the Pleiades cluster, that can be tackled satisfactorily with only limited specialist equipment.
There are two main technical (and two more practical) challenges for the photographer wanting to take images of the night sky.
Practical
The more practical challenges for night-sky photography include the fact that you have to do it at night (duh), meaning it’s going to be dark, so you need to be comfortable with handling all your camera’s controls in the dark, and (in the UK at least) it’s going to be cold – dress accordingly.
The other big problem is light pollution – you need to get away from street lights, security lights, car headlights, and the general “glow” of the city or town as much as possible.
Technical
From a technical point of view, you need to take long exposures (30secs would be considered a very short exposure for most astronomy work), and don’t forget you are photographing a moving subject. Because of the rotation of the earth, in the time taken for the long exposures required the stars have moved enough to appear as streaks rather than points.
There are two ways to avoid the problem:
you can keep the exposure short enough so that it can be ignored
or
you can move the camera at exactly the right speed to counteract the problem (this is where the first bit of specialized equipment comes in).
For star-trail photography this is not a problem, of course, but for everything else it is.
If you want to avoid star movement without a specialist mount, then you can use the “rule of 600” to determine the maximum shutter speed you can use for a given focal length: 600/focal-length = exposure time.
Example using a 24mm lens, 600/24 gives 25 seconds, so you can expose for 25 seconds without the star movement being too obvious.
For a 50mm lens, the rule would suggest 12 seconds, and so on. Anything much wider than 50mm and the required exposure times start to get too short to be able to get a usable image.
25 seconds is long enough for a reasonable number of stars to show up and some of the Milky Way and this is a good starting point for “starscapes”. Experiment with the ISO setting – higher values will increase the sensitivity but also the noise, and too much noise can quickly ruin the image (but noise reduction is liable to remove stars too…).
If you have access to some form of telescope/camera tracking mount that can rotate the camera as you shoot, then the possibilities are much less limited. It’s possible to build your own “barn door” tracker for a few pound’s worth of hardware or to buy something like the AstroTrac, Vixen Polarie, or iOptron SkyTracker which will – once properly aligned – rotate your camera at exactly the same speed as the earth is rotating (but in the opposite direction) and keep the stars still. With a perfectly-aligned tracking mount, exposures of several minutes or even hours are possible. One caveat though – the stars will now be sharp but the landscape will be blurred. The solution here is to shoot multiple images – one untracked for the landscape, and one tracked for the stars – then combine them using Photoshop. An interesting imaging site is http://deepskystacker.free.fr/english/index.html.
So, you can expose for hours with a perfectly aligned tracker, but in practice you generally don’t want to do so – at least not all in one go. It’s usually better to take a series of shorter exposures – a few minutes at most – and then combine them later in the computer. There are several advantages to this, but three stand out.
- The alignment of the tracker is never perfect, meaning that eventually you will start to see noticeable movement of the stars despite the tracker.
- Things happen to ruin photos, and with a series of shorter frames you only have to throw away the ruined frames rather than the whole thing – things like clouds, passing aircraft, passing satellites, passing cars, etc.
- Light pollution may cause long exposures to be overexposed drowning out the detail. There are many programs that can be used for combining the images – probably the most popular is “Deep Sky Stacker”.
Hopefully that’s enough to get you started – just a couple of closing comments.
- if anyone wants to borrow a Vixen Polarie to try some of this, just ask me.
- if anyone gets seriously bitten by the bug, then consider joining the Bromsgrove Astronomical Society http://www.broms-astro.co.uk/.
Below are some examples of shots I have taken. I used a standard DSLR and lens, with a tracker mount. The two “starscape” ones were 30 second exposures I think, while the Large Magellanic Cloud was stacked from several dozen 30 second exposures.
Richard Chapman
January 2017
Click on image to enlarge
