In May 2013, I published an article called “How to Photograph Star Trails” which explained amongst other things, the one fundamental issue that astronomers have to deal with when taking photos of the heavens – the fact that the Earth rotates, which makes everything appear to move across the sky. That article encouraged you to embrace the fact that the stars move, and make a feature of it in your pictures. However, once you have done that a few times, you will want to start taking pictures where the stars are NOT trailed in to lines.
Basically, in order to allow you to do long exposures of the night sky (around 5 to 10 minutes for example) you need to mount your camera on a device that rotates about the same axis as the Earth, at the same speed as the Earth, but in the opposite direction, so that the field of view in the camera remains the same throughout the exposure.
The first York AS meeting of 2014 was an informal evening, where members step up to do a 5 minute presentation on a subject of their own choice. I decided to talk to the audience about the Haig mount, and how you can build your own. A few people have asked for further details so please read on, dear reader.
The “Barn Door Tracker” as it was then called, was first described in a Sky & Telescope article in 1975 by George Haig, hence the name “Haig Mount”. It is a surprisingly simple piece of apparatus that can be made in a few hours with minimal equipment and only basic DIY tools. The idea is that you have two pieces of wood that are hinged together. The camera is mounted on the top piece and the centre pin of the hinge is pointed towards the Pole Star. When in operation, the two pieces of wood are forced apart by a rotating bolt, or threaded rod, causing the camera mounted on it to slowly rotate at the Earth’s rotational speed. There are many variations of the basic design, each improving the accuracy of the drive, but I will describe here the very simplest version of the design.
- Cut two matching pieces of flat wood, approximately 300mm x 150mm x 15mm in size. These will form the two halves of your “Barn Door”. Join them along the short edge with a low-profile hinge (piano hinge is good if you have a piece), so that they open like a book.
- You will eventually need to drill a hole in the lower piece of wood, and thread it. You can do this by inserting a spiked nut (pictured here) in to the wood itself. Before drilling the hole, it is essential that you know exactly where to drill it, and that location is determined by the pitch of the thread of the bolt that you are using. Here comes the maths bit:
- We are going to want to drive the two barn doors apart by rotating the bolt once every one minute.
- We will assume for this description that the bolt being used is a metric M6 bolt, which has a pitch of 1mm.
- We know that the Earth rotates through 360 degrees in 24 hours (1440 minutes). This means that in just 1 minute, it rotates by 0.25 degrees. This is the rate at which we want the barn doors to open.
- Using trigonometry, we apply the formula A = O / tanθ, where “O” is the pitch of the thread (1mm), “θ” is the angle of opening (0.25 degrees) and “A” is the distance from the hingepin to the location of the centre of the bolt, and therefore where you should drill the hole. So, A = 1 / tan0.25, which is 1 / 0.00436, which equals 229.18mm. You now have your drilling measurement.
- Drill the hole half way down the board (75mm from each edge) at a distance of 229.18mm from the centre of the hinge pin, and mount your M6 spiked nut, to make the hole threaded.
- The M6 bolt you are using needs to be about 100mm in length. Cut a piece of wood or metal about 80mm x 20mm and drill a 6mm hole at one end of it. Slide that on to your bolt and secure it at the head end using an M6 nut. Your “driveshaft” now has a turning handle!
- The Barn Door mount is now almost complete, but in order to make it usable as a camera platform there are two last things you need to do:
- You will need to put a small aluminium plate on the underside of the bottom barn door, and that plate needs to have a 1/4 inch Whitworth screw threaded hole in it. This is the universal standard tripod screw size. You can buy whitworth taps for a few pounds from eBay etc, or ask around your friends – somebody might have one and make it up for you.
- Lastly you will need a ball and socket tripod head. This is the only “proper” piece of camera equipment you will need to buy. They can cost anything from £10 upwards depending on how beefy you want it to be. This needs to be mounted on the top barn door, and will allow your camera to point anywhere in the sky once the mount is set up.
- Insert the M6 bolt “driving handle” in to the underside of the bottom barn door and wind it through until it just starts to open the barn doors.
- Attach the whole mount on to the top of a sturdy tripod.
- Take it out to your garden, and point the hinge pin towards Polaris, the Pole Star. (Approximate eye judgement is fine)
- Mount your camera on the top of the ball and socket mount and point it towards your chosen subject.
- Make your exposure and start winding the handle round at a rate of one revolution per minute. To minimise the amount that you actually touch the mount during exposure, it is acceptable to wind the handle one quarter-turn every 15 seconds. You can time this period either with a watch and a torch, or one neat way to do it in the dark is to get the quartz movement out of an old clock and hold it to your ear. Every 15 ticks, move the handle around by a quarter.
This design of mount is by no means perfect, but it will allow you to guide your camera accurately for about 10 minutes, which under UK skies is probably plenty of time. Here is an example of a photo taken with this type of mount. As you can see, the stars themselves are sharp and untrailed, and it is the surrounding foreground that has trailed instead. Sadly this shot was NOT taken under UK skies, but in Australia instead, so sorry to build your hopes up!
Keep checking back to this article in the coming few weeks, as I am intending to make a short film, demonstrating all of the above. In the mean time if you have any questions, contact me at email@example.com