Perhaps some of the best value for money that I’ve had from any piece of camera kit that I’ve purchased over the last decade, is the few hundred dollars that I spent earlier this year on a star tracker – a small device that sits between the top of ones tripod and camera, and that allows your camera to automatically follow the regular progression of the stars across the night-sky. The value I’ve got out of this little device – and the frustrations that it caused me – have been such that I thought that it was time that I got back to my blog and shared my experience of learning how to get it working well…
A number of small star trackers are available on the market, but I purchased an IOptron SkyTracker Pro. I’m not advocating for this one in particular; it just happens to be readily available in New Zealand (via Astronz), and I’d read good reports of its performance. One advantage that it seemed to offer was its simple operation, including its ability to operate without an external power source or cell-phone connection, an important consideration for me, given my liking of taking night photos in out of the way places.
When my tracker arrived, I unpacked it with a sense of anticipation. Its main components are clearly visible in the image above – the tracker itself (right), an adjustable wedge that sits between the tripod head and the tracker (left), and a small alignment scope (centre). The accompanying manual indicates that this is used to align the tracker to the Pole Star if one is in the northern hemisphere, or a more obscure star by the name of Sigma Octantis in the southern hemisphere – these lie close to the northern and southern celestial poles, respectively, and aligning the tracker to point to the hemisphere-relevant one of these is fundamental to accurate tracking. I charged the internal battery, watched the weather forecasts eagerly to see when a dark, cloud-free night might arrive, and finally got to set up in the courtyard, ready to align my tracker with this little star that marks the position of the Southern Celestial Pole – that is when all my problems began!
Where is Sigma Octantis?
Prior to the arrival of this first, cloud-free night, I had scoured the internet to find instructions on how to locate Sigma Octantis, and the theory of it all sounded very achievable – the practice proved to be quite different! I carefully followed the instructions in the manual, setting the angle of the wedge to the latitude of my home, and then pointed my tripod and tracker in the general direction of said star. I then looked through the scope, only to find that all I could see were a few very faint objects, none of which corresponded in layout to the page that I had printed from the internet, at least in part because the image it showed was upside down. Try as I might, I could not identify anything even remotely like what I was supposed to see.
Eventually, feeling somewhat discouraged, I put my camera onto the tracker, turned it on, and took a 30-second exposure on my Sony A7RII with a standard 50 mm lens. I was blown away. Previously any exposure longer than about 4 seconds with this lens had shown trailing of the stars – this first image with the tracker showed stars that were almost perfectly round. Somehow I had managed by accident to point the tracker in the right direction.
Finding a solution
After a few more trial shots, I retreated back inside to review my procedure. I cleaned and adjusted the focus on the the alignment scope, and read more about how to find Sigma Octantis, but the next clear night I suffered the same indignity – the tracker worked beautifully if I could get it pointing in the roughly right direction, but this was a hit-and-miss process, for which the scope gave no assistance at all. Then the breakthrough came…
In pursuing my new interest in photographing the night sky, I had downloaded an app onto my Android phone called Sky Map, which can be pointed at any direction in the night sky and it will show the stars that are on view. By chance, I happened to notice that it also showed the position of the Southern Celestial Pole – could I use this to align my tracker? Somewhat tentatively, I held the bottom of the phone face flat against the surface of the tracker that faces the SCP, adjusted the position of the tracker until the SCP as shown on Sky Map was centered in the screen, and bingo – I had alignment good enough to take 30 second exposures with an 85 mm lens!
Over the next few nights I experimented further – the outcomes were a bit erratic to start with, but I learned gradually that I could improve the consistency of my alignment by: (1) ensuring that the tripod was set as level as possible; (2) setting the app option to get its position coordinates from the GPS rather than the nearest cell-phone tower location, (3) using an app option that allows refreshing of the phone’s compass calibration, and (4) holding the phone as high up on the front of the tracker as possible and removing the ball head from the tracker, the metal in which I presume interferes with the compass in the phone. Following this approach, I found that I could generally get good enough alignment for 30 second exposures with my 85 mm lens within just a few minutes of setting up my tracker – a major advance.
Adding drift alignment
Since then, I’ve refined my alignment process a little further, mostly by learning how to drift align. In brief, if an overhead star is drifting towards the north between successive thirty second exposures, the orientation of the tracker needs to be adjusted westwards; move it eastwards if stars are tracking southwards. Similarly, if a star on the eastern horizon is drifting northwards between successive exposures, then the elevation of the wedge needs to be adjusted upwards; if they are drifting southwards, the elevation needs to be reduced. Using this approach, I’ve at times been able to get good enough alignment to take 30 second exposures with my Canon FD 300 F/4 L, way beyond the published specs of the tracker.
And where is Sigma Octantis?
Once I got a good pair of binoculars on the job, I gradually become more confident in my ability to find this feature in the night sky, mostly by stepping across towards the Southern Cross from beta-Hydrus, a prominent star just on the poleward side of the Small Magellanic Cloud. Its relatively easy on a clear night with minimal light pollution, but is still a challenge if trying to do it in a city environment. I’ll stick to my cell phone for now…