Home | Aq Track

To take pictures of the night sky

If you ever tried to take pictures of the stars with a camera on a tripod, you certainly realized that even with a wide angle lens, your exposure time is often limited to less than 20 seconds before stars start making trails on your image. That's simply because Earth's rotation is making the stars circle the sky in 24 hours. It may seem very slow but during a two minute exposure the stars will have moved a distance equivalent to a full Moon diameter. A star tracker will simply rotate your camera at the same speed to compensate.

This website looks at how to build such a tracker that will be able to drive lenses from 16 to 135 mm for about 80$. Skill levels needed are minimal. I'm not a woodworker myself but have learned to use the tools because my projects required me to. After drawing different designs I settled on one you see pictured here and called it the aQ-Track as in "accurate tracker". Unlike barndoors, this design has a tangent drive rod so the motor can turn at a constant rate.

In order to get accurate tracking I realized that an equatorial mount needs three things: a smooth equatorial movement, a precise drive train and a precise polar alignment.

Building the mount

DETAILS

The clock drive

DETAILS

Precise polar

alignment

DETAILS

The summer Milky Way is full of nebulosities invisible to the naked eye, but a stack of 2 min exposures with a 35 mm lens reveals them easily. (Cygnus region with Pentax K-70, Sigma Art 35mm F1.4 @ f/2.0, 20 X 120 s iso 400)

The constellation Orion, a beautiful part of the sky. (Pentax K-70, Sigma Art 35mm F1.4, 30 X 120 s iso 400)

Other images (click for better view)

Building the mount

Recommended tools

- Bench saw

- Router with circle cutting tool

- Press drill

The project is simple enough that a jigsaw and drill will do the job as well

Construction

The aQ-Track mount is built arount a polar axis that slowly turns to compensate Earth's rotation The camera sits on a hub plate at the top end of the axis. The rest of the mount serve to support the axis and make it rotate at just the right speed. The design I chose uses three arms to do this, a static arm that hold the polar axis and attaches it solidly to a photographic tripod. A drive arm is attached to the hub plate and, as the name says, is used to transmit the movement from the motor to the polar axis. A third arm, tangent to the drive arm, carries the motor and worm that make the drive arm turn.

The polar axis has to be fairly stiff so it doesn't bend under your camera and lens weight. The one I used is made of a 3/4" steel shaft with a 3" hub plate soldered at one end. It was part of a snowblower impeller and found at a scrap dealer. It is fairly strong and will not bend under any load I may subject it to.

To allow the shaft to turn very smoothly and without friction I use two taper bearings to hold it. The figure to the right shows how everything is held together using 3/8" baltic birch plywood.

Note that the part of the static arm holding the polar axis is 1.25" thick (2 X 3/8"+ 1 X 0.5") and the part receving the mounting tripod head is 3/4" thick, for increased rigidity.

The drive arm is placed below the hub plate to keep the gap between the two arms small. Wood screws go from the top of the drive arm and through the hub plate into the mounting disk to hold everthing firmly. The lower end of the drive arm was cut into a circular section with a radius of 11.5" using a router equiped with a circle cutting tool.

The aQ-Track mount

Drive arm

Static arm

Polar axis

Tee-nut (1/4-20) for mounting the aQ-Track on a tripod head

Two taper bearings

Tee-nut (3/8-16) with threaded rod (glued with epoxy) to receive a tripod head base

Tripod head to mount the camera.

Stepper motor on tangent arm (not shown)

The polar axis

The tapered bearings

The tangent arm

The end of the drive arm has 1/4"-20 teeth made by setting an 1/4"-20 rod into curing epoxy placed on a section of wood veneer. Vaseline was used as a mold release and within one hour the piece of veneer was glued to the end of the drive arm using ordinary white wood glue. At that time the epoxy is still flexible enough to adopt the round shape of the drive arm. Even though the bending spreads the teeth apart from each other, there is no play between the teeth and the strait rod. The picture also shows the tangent arm that holds the 1/4-20 rod and the stepper motor. The rod sits in two small ball bearings to minimize friction. It is important to note that a threaded rod is not the best choice for this application. After a few trials I realized that such rods carry important periodical errors limiting the maximum usable focal length to about 135 mm. Probably a lead screw would be better suited for the job.

Construction details of the drive arm teeth.

The idea of using epoxy to make teeth comes from an article I read in the Sky and Telescope issue of November 1987. David A. Harbour mentioned the idea but never built an epoxy sector. He prefered leaving the edge of the drive arm bare, and by pulling the rod onto it with strong springs, he made the contact strong enough to drive the mount without slipping! I prefered his idea of epoxy mold and built at the time a small fork mount. The epoxy teeth were still working 30 years after being made, so I know that this new set of teeth is going to last. To make them, I placed a mold made out of a piece of cardboard and waxed paper around a piece of wood veneer cut to 3/8" wide. Epoxy glue was poured on the piece of wood and the rodset in place resting on two identical washers, so the epoxy thickness remains constant. The first picture shows the setup right after setting the rod into epoxy. The second picture shows the rod ready to be released.