Until quite recently I wasn't really into astro photography. However, over the past couple of years it seems to have become very popular and many of my workshop clients are asking if we will get the chance to do some. Therefore, in order to add a little more variety to my future landscape photography workshops, I decided to spend some time learning how best to tackle this subject.
It is only really in recent years that modern digital cameras have enabled us to easily capture starry landscape images, but we still have to use the fastest possible lenses and high ISO settings to let enough light in to the camera for the stars to become visible. Things are improving all the time, but there are still many hurdles. One of them is finding a camera that provides acceptable results at very high ISO settings and up to 20 second exposure times. Another is finding a wide-angle lens that produces good results at maximum aperture without heavy vignetting, coma distortion or soft edges.
Personally I'm only interested in shooting very wide angle scenes that include large areas of sky as well as an interesting foreground. I always try to find a composition with a foreground that would work equally well in daylight. For wide-angle starry landscapes I had been keeping my exposure times below 15-20 seconds so that the stars still recorded at point sources of light rather than short streaks, usually necessitating the use of 6400 ISO on my camera and f2.4 on my Irix 15mm lens. That was until I read about sky trackers. Until fairly recently these devices tended to be very large and cumbersome as they were designed to use with longer focal length lenses and telescopes for deep space photography. However, there are now a few much smaller units available which are really only suitable for use with wider focal length lenses for so called "wide field" astro photography. I purchased an iOptron Sky tracker Pro (about £300).
This unit is mounted between the tripod and tripod head and includes a small telescope through which it is possible to align Polaris (the North Star). The iOptron Polar Scope app then shows you exactly were to position Polaris within the scope from your particular geographical position. Once alignment has been achieved and the tracker turned on (it is a rechargeable unit) it will precisely follow the movement of the stars. As your camera will be attached to a ball head fitted to the tracker you can now use much longer exposure times and the stars will remain as sharp point sources of light. This means that much lower ISO settings and slightly smaller apertures can be used to provide much improved image quality. As you can use smaller apertures it also means that you won't need to buy an expensive fast aperture wide-angle lens if you don't already have one.
The downside to this technique is obviously that the foreground of your image will be blurred by the movement of the tracker. Therefore it is necessary to first take a static exposure with the tracker turned off. I normally do this before it gets completely dark so I don't have to use a very high ISO setting. I then wait for it to get dark, check and adjust for the alignment of Polaris and take a second longer exposure with the tracker turned on. These two images then have to be blended in Photoshop using layers and selections. In many cases this is an easy task that can be completed in a couple of minutes, but this will depend upon what is included in the foreground of your image. I don't like to spend much time in front of the computer so I'm only shooting locations where the blending process will be easy! The small amount of extra effort required to blend the two exposures is definitely worth it for the huge increase in image quality this technique provides.
I don't intend to run any specific astro photography workshops, but wherever possible I will explain this technique during some of my normal landscape photography workshops in the future - for those clients who aren't already too tired to stay up all night as well!