Barndoor Camera Mount (Star Tracker)

My brother is interested in both Astronomy and photography, but one of the challenges when taking pictures of the stars is the Earth's rotation. Because the stars are so dim in the night sky, a rather long exposure time is required, and also a high degree of magnification. The combination of the two greatly exaggerate the subtle effect of our continuous rotation, and the result is very visible trails in the picture you would have taken, which are left by the stars. The stars themselves simply become streaks in the sky, instead of clear points of light. The solution to this is quite simple, rotate your camera along the earth's axis, and at the exact same but opposite angular velocity. The two angular velocities will then cancel each-other out, and the camera will remain pointed at the same point in space while the Earth spins. The earth rotates along an axis which by sheer luck intersects an easy to locate star on the northern hemisphere, Polaris (The North Star), and it is this star which the star tracker must be aligned with.

Comparison gif. 30s exposure with a 200mm lens showing significant star trails.

Several different star tracker mechanisms have been designed over the years, but I chose one with the least mechanical difficulty, simply due to my limited access to specialized hardware components. As an additional bonus, the required motor control is also simplified. The design utilizes two planks held together by a hinge on one end, and with a curved drive rod fasten to one of the planks, and allowed to move freely through the other. The curvature of the rod must be exactly equal to the radius of the star tracker, i.e. the distance from the hinge to the middle of the rod. Getting the curvature correct is the only difficult part in constructing the star tracker, and even that is easy if you use a compass to draw the required bow on some paper to use as a guide. A constant movement of the rod corresponds to a constant angular velocity. An easy way to do this is to use threaded rod, with a nut which is spun a constant velocity. Older designs would utilize a hand-crank, and require the user to turn a set fraction of a rotation every X seconds. However with the price of electronics nowadays it would be foolish not to automate this, using an electric motor run at a set speed. I opted to use a scrap stepper motor from a printer in my star tracker, and some 3D printed gears to increase the torque and resolution of the motor. I have provided a spreadsheet document with all of the needed calculations to adapt the mount to any type or size of motor. The only requirement of the motor is that it's angular velocity can be set exactly. For drive electronics I used an ATmega board from another project, and an EasyStepper board to control the motor. Pretty much any scrap electronics will suffice here, they just need to run a stepper motor at a single defined speed.

To align the star tracker with the North Star the axis of the hinge must point directly towards Polaris. Doing this in the dark is near impossible, so I purchased a "Celestron star pointer finderscope" red dot sight used for large telescopes which makes it incredibly easy to sight in the star. A mount was 3D printed to support the sight. When choosing the thickness and curvature of the rod keep in mind that the nut must be able to move without binding, and also that relative measurement error decreases with length. Each diameter of threaded rod has it's own minimum bend radius, so stay above this. I used a 8mm (M8) brass rod which was easy enough to bend accurately to a radius of 27cm. As the hinge will bear all the weight of the camera, the thickness of the rod isn't too critical. An M4 or M5 rod will probably work just as well. The .stl and OpenSCAD files for the 3D models, and ATmega firmware can be downloaded here. When placing the ball joint used to hold the camera, and the star sight, it may seem necessary to have them located on the hinge so they only rotate, and not move as is the case when placed some distance away from the hinge. This is not a problem, as the enormous distance to the stars means any spacing between the sight/camera and the hinge is negligible compared to the spacing between the sight/camera and the distant stars. If this is not intuitive, think of a triangle with the hinge in the orthogonal corner, where one side is the distance to the stars, and the other is the distance to the sight or camera. It should then be clear that this triangle can be approximated with a straight line for large distances, and that the hinge, camera and sight are all in a single point.

Disclaimer: I do not take responsibility for any injury, death, hurt ego, or other forms of personal damage which may result from recreating these experiments. Projects are merely presented as a source of inspiration, and should only be conducted by responsible individuals, or under the supervision of responsible individuals. It is your own life, so proceed at your own risk! All projects are for noncommercial use only.

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.

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