How to build a tabletop tracking mount

This project is a simple tabletop mount that can track the apparent movement of the stars, which allows you to increase exposure times and capture more light with your camera.
Written by Mark Parish.
Image Credit: 
Mark Parish

Download useful drawings, printable templates and plans for this project here.
Astrophotography takes a lifetime to perfect, but at every stage a little know-how and specialist equipment helps you to progress. This project is a simple tabletop mount that can track the apparent movement of the stars, which allows you to increase exposure times and capture more light with your camera. Once the mount is correctly aligned, you simply rotate a drive disc, which keeps your camera pointing at the same spot in the sky.

Tools & materials

Tools - Coping saw, drill with bits to suit the screws and rod, sandpaper, ruler, pencil, screwdriver, 10mm spanner or pliers.
Materials - Plywood (12mm thick, fine quality) for the arms and the base, scrap wood for the drive disc gauge.
Sundries - M6 studding (minimum length of 300mm), hinge, six M5x20 bolts and nuts for hinge, seven M6 
nuts, two M6x40 bolts, three rubber tap washers. 

Finish - Exterior-quality gloss paint or spray paints for a nice finish.

This well-known design (sometimes called a Scotch mount or a barn door tracker) can be made using materials found in most DIY stores and requires only minimal woodworking skills. In this section, we are going to focus on the construction of the main elements of the mount and the drive rod – you can download printable templates for these here. In the section below, we’ll cover the drive mechanism and describe how to operate it.

Building on the basics

The principle behind the mount is simple. Earth turns one full revolution every 23 hours 56 minutes and four seconds. This period is called a sidereal day. To an observer on Earth the stars appear to revolve slowly around the polar axis – which for the northern hemisphere is very close to the star Polaris in Ursa Minor.
The mount has a hinged upper arm on which the camera is mounted, and the hinge’s axis is arranged to point towards Polaris. There is a threaded, curved M6 rod between the arms, and this has the drive disc on it. Turning this disc causes the upper arm (and the camera) to rotate at the sidereal rate. The clever bit is the maths. One full rotation of the disc on the rod causes the arm to move by 1mm. For accurate tracking the centre line of the rod must be at the correct distance from the axis of the hinge – in this case 228.5mm.
If this all sounds a bit technical or tricky to achieve, don’t worry, we have also provided a template, which you can use to bend your rod to the right shape, and the design is quite forgiving of a few inaccuracies. We found that it is easier to bend a whole length of rod by hand, and then choose and cut the best section (you need approximately 200mm) for your mount.
The main parts of the mount are made from strips of 75x12mm plywood, which is easy to cut. Once you have printed out the A4 templates, lightly stick them to the plywood and cut around each shape. The angled tops to the base arms need to correspond to your latitude, which will make polar alignment easier to achieve. We made ours for 51°N; if you are in the UK you will be somewhere in the region of 50°-60°N.
The hinge needs to move smoothly and not have too much ‘play’. We found a good quality and inexpensive stainless steel one on eBay, but a reclaimed one from an old door could be a good candidate. While you are shopping you also need a ball joint tripod head, which you will fix to the tracker. This allows you to aim your camera in any direction. These heads require a ¼-20 tpi screw (not M6 – even though the size might look similar) so you can fix them to the mount from below.
In the section below we will finish assembly and show you how to add a simple motor drive, but the beauty of a barn door tracker is that it works just as well when hand-driven, and there is something very satisfying about the interaction involved. The drive wheel doesn’t need to be turned constantly – just keep the average rate at one turn per minute.

Step 1 - Use the templates available online to mark out and cut the timber to size. You could even ask your local timber merchant to cut the plywood into 75mm-wide strips for you. All you would then need to do is cut the profiles on each section.

Step 2 - Check the template hole spacing for the hinges and adjust if necessary to suit yours. Use a nail or punch to tap marks into the plywood to aid accurate drilling. We taped both arms together and drilled some holes at the same time. 
Step 3 - It is worth spending some time smoothing the parts with fine sandpaper before proceeding. You can also temporarily fit the hinges and enlarge the holes slightly if necessary until the screws fit. After carefully sanding, apply a nice finish. We used spray paints.
Step 4 - Bend the M6 drive rod. We stuck our rod template on a piece of scrap wood and cut this to make a gauge. Bend the rod a little at a time around the gauge. Don’t hammer it or the thread will be damaged. Make small corrections until you have enough length.
Step 5 - The rod and hinge are fixed on using M6 and M5 bolts and nuts. The base and arms are joined to lower arm are joined using small wood screws. Countersink the screw heads so they sit flush with the surface, and make pilot hole in the base arms to avoid splitting the wood.
Step 6 - Fix the levelling feet, which are made from M6 bolts passed through holes in the base cross bar. Glue M6 nuts above and below the hole (but be careful not to get glue on the thread) and push a domed rubber tap washer onto the end.

In the first part of this ‘how to’ we described how to make the main elements. Here, we describe the drive mechanism, optional motorisation and how to operate the mount.

Tools & materials

Finish - Paint or sticky labels for marking the drive disc.
Electronics - Small 6V DC motor, push-to-make switch, double pole-double throw slide switch, battery holder and connector for two AA cells, clock mechanism. 
Materials and components - An offcut of 6mm plywood or similar for the drive disc.
Sundries - Two CDs, an M6 nut, short length of 15mm copper tube, wire and solder for electronics, epoxy resin glue, elastic band for drive belt, small offcut of felt for bearing surface, 
small bead to fit motor spindle.
Tools - Coping saw, drill with bits to suit an M6 nut (11mm), sandpaper, ruler, pencil, soldering iron.

The drive disc is made from a sandwich of two CDs or DVDs glued either side of a wooden disc with a slightly smaller diameter, forming a pulley. The disc has an M6 nut glued into its central hole, which runs up and down the curved drive rod in operation. The upper arm of the mount sits on the CD surface so that as the disc turns clockwise, the arm (and the camera mounted upon it) slowly descends.

We stuck a felt pad to the underside of our upper arm to provide a low friction bearing where it touches the CD and the resulting movement required is very light and smooth. With some combinations of cameras and lenses, the balance point might make it necessary to fasten an elastic band between the upper and lower arms to maintain contact between the upper arm and drive disc. A small hook or screw in the end of each arm would facilitate this.

Simplicity in motion

The beauty of this mount is that it works very effectively when hand-driven, and there is something very satisfying about the simplicity and interaction involved in moving it. To add a little sophistication you can motorise the mount, for which you need a small DC motor. We used a 6V one, powered by two AA cells (3V) so it turns more slowly. Experiment with different rubber bands running between your motor spindle and the drive disc until it runs smoothly. The electronics couldn’t be simpler – we’ve included a wiring diagram online to help. The circuit consists of a push-to-make button switch, with which you send short pulses to move the motor, and a slide switch, which is used to reverse the motor for ‘rewinding’ the mount back to its starting position. A soldering iron is needed to join the components and wires together. We used a simple plastic box to house the switches and batteries. The drive disc doesn’t need to be turned constantly – just keep the average rate at one turn per minute. We mounted a cheap clock mechanism on the upper arm, with only its second hand fitted, as a visual reference.
Providing you keep within the following range of movements, your image should not show any trails:
- A wide-angle lens (30mm or less): half a turn every 30 seconds.
- A standard lens (30-70mm): one-quarter of a turn every 15 seconds. 
- A telephoto lens (70mm or more): 1/12th of a turn every five seconds.
Put a mark on the disc; if you can keep that marking within the same quarter of the clock face as the second hand, you should be successful. The mount is quite easy to set up. Place it on a suitable table (or skip the base part and mount it directly onto a tripod). Turn it so it is facing north and close to the table edge so you can look up through the alignment tube. Use the screw adjuster feet to sight and centre Polaris through the tube. Set up your camera, taking care not to knock the mount out of alignment. Turn the disc and start taking images! You might consider perfecting your technique on shorter exposures but it won’t be long before you are trying minutes rather than seconds. Seeing conditions, camera noise, focus and exposure settings will become the major factors then… but that’s the astrophotography learning curve you are now embarking on.

Step 1 - Cut a 110mm diameter disc from some scrap wood approximately 6mm thick. Drill an 11mm hole and press an M6 nut into it. Apply some glue (avoid the threads) and use a spare screw to check it is straight. Glue a CD to either side to complete.

Step 2 - Wrap the motor with electrical tape so it fits tightly in its hole. Fit a suitable elastic band; a small bead glued to the motor spindle will prevent it riding off. After Step 3, check it runs smoothly in each direction, adjusting the motor position in the hole if necessary.
Step 3 - Using the wiring diagram as a guide, solder short lengths of wire between the switches and batteries. A longer pair of wires extends to the motor. Mount the switches and batteries in a suitable sturdy box. Glue or fix the switches in place.
Step 4 - Carefully remove the hands from a standard quartz clock mechanism. Stick a thin paper shape (we used a hole reinforcer) onto the end of the second hand and refit it. Use double sided tape to secure the mechanism after installing a battery.
Step 5 - Carefully glue the alignment tube between the hinge and end of the upper arm – make sure that you only apply glue to the parts of the hinge that remain still with respect to the arm. Confirm the tube is aligned with the axis and tape it in position before the glue sets.
Step 6 - After checking that everything works smoothly, fix your camera mount to the upper arm, high up and close to the hinge. Check the balancing with your camera pointing in all directions. If the upper arm lifts up, a rubber band between the ends of the arms will help.
Mark Parrish is a craftsman who loves making astro accessories

This How to originally appeared in the April and May 2015 issues of BBC Sky at Night Magazine.


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