Giving long exposures on a digital camera

Camera with cable release
Canon 40D with its special cable release

Many astronomical exposures require you to keep the shutter open for a long time – maybe seconds, or even minutes. This isn’t as straightforward as taking daytime pictures.

The cheaper compact cameras are usually limited to exposure times of around a second or less, because they are intended for snaps rather than creative photography. But the more advanced compacts and digital SLR cameras (DSLRs) usually allow timed exposures up to 30 seconds or more, plus a ‘B’ setting which holds the shutter open for as long as you want (or until the battery fails).

Keeping the shutter open
The way you hold the shutter open depends on your camera. Even if you’re using a tripod, pressing the shutter button manually can easily jog the camera, and even a slight jog shows up on the image. Some digital cameras take standard cable releases which screw into the shutter button, costing a few pounds in camera shops, but others, such as Canon DSLRs, require  electronic releases which cost quite a lot more.

One trick is to use the self-timer, the one that normally gives you time between pressing the button and racing round to be in the picture yourself. Then the exposure will start when you are not touching the camera. Many cameras allow you a choice of delay times. But this method won’t allow exposures on the ‘B’ setting, which means you’ll have to stack the exposures afterwards. With DSLRs you may be able to lock the mirror in the up position, which also reduces camera vibration.

Cable releases
Ordinary cable release (left) and Canon cable release

The alternative, which is easy to do with Canon DSLRs, is to link the camera to a computer using a cable supplied with the camera (mini-USB to USB). Canons include software which allows you to operate the camera,  and view the images, from the computer. This is by far the best way to do any astrophotography, if you can organise it, as you can view the shots in full resolution as they are taken and also store the images on the computer, so keeping your memory card free. Professional photographers refer to this method as ‘shooting tethered’. Viewing the images in full resolution means that you can check the focus precisely, whereas the viewing screens of many cameras interpolate the highest resolution so everything looks slightly fuzzy at the highest magnification.

Of course, it means you either need a laptop out there with you, or a lead coming through the window to your desktop computer. With this method you can photograph in comfort!

Powering the camera

Camera batteries have limited capacity, and can easily run down during the exposure, particularly in cold weather. Often there is a camera power input, which allows you to run the camera from AC power, which solves the problem but is not always convenient, and the combination of AC voltage and a dewy lawn is not a good one. You may be able to get a high capacity power pack for your particular camera, intended for photojournalists who strap it around them.

Focusing the camera

If you are using the camera’s lens for photography, for star trails or piggybacking on top of the telescope, focusing can be a big problem. Autofocus DSLRs are not nearly as good for astrophotography as the old manual-focus ones. With those, once you’d focused they pretty well stayed focused, barring extreme changes in temperature altering things. And there were focusing aids in the viewfinder which made the job easier. But autofocus cameras expect you to allow them to do the job for you, so the viewfinder is there just to help you see what you’re getting rather than focus carefully. The finders are often dimmer than they used to be, as well, so focusing on stars can be very tricky.

With non-autofocus lenses, all you have to do is to set the lens to the infinity end of its range. But autofocus lenses often don’t have such a mark, or if they do it is not well-defined and you can focus well beyond it. This is because autofocus works by hunting on either side of the focus point and aiming for the best contrast. The focusing mechanism is also much easier to turn, so that it can be operating by small motors. To focus manually, you must switch the lens off autofocus or you can easily damage the mechanism. Then you find that the exact focus position is easily disturbed.  So why not just let the autofocus do its job? It will, on a bright object such as the Moon or a planet, as long as the focus point can see it. But as soon as you turn away from the bright object to something fainter, it hunts wildly from side to side and refuses to focus or let you take the picture.

One method is to use the autofocus on the Moon or a bright planet, then carefully switch off the autofocus and hope that you don’t disturb the lens during your session. With long focal lengths, the focus can drift off or alter as a result of temperature changes or movement of the camera. You might try taping the barrel at the focus position and hoping it says put.

Just checking the correct focus position can also be tricky in the absence of focusing aids, whether you are using a lens or your telescope for imaging. The viewfinder itself is pretty difficult to use because of the lack of focusing aids. Many DSLR cameras these days have Live View, which allows you to see the image as picked up by the imaging chip, and on compacts this is standard. You can zoom in, so you would think you could focus critically. But many cameras, such as Canons we have tried, do not show you the true image when at full magnification, but average out several pixels at once. So images look fuzzy, even when they are sharp. You have to guess the point where the image is sharpest by going from side to side of the focus point.

However, when shooting tethered to the computer you can view the full resolution of the image, which helps greatly. Even if there is no bright object around you can see how things are going, and maybe tweak the focus slightly to correct it.

Robin Scagell