Star Tracker Calculator is a tool to help you build and work a barn door star tracker.

Want to build a barn door tracker? Don't worry about precision measurement and buying specific screws. Just build what you want, input the pitch of your drive screw the distance from the hinge to the screw, and you're all set! Star Tracker Calculator will tell you how fast to turn the drive screw and provide a helpful vibration and gentle screen flash to time it correctly, no matter what your setup.

Want to know how far from the hinge to put your drive screw for exactly 1 rpm? Again, no problem. Just type in the pitch of your screw and this app will work it out for you.

Night vision friendly colours.

Set it up and keep it in your pocket. Your phone will buzz at the correct rate to time your drive wheel turns, so you can keep your eye on your equipment and the sky!

Note - this app does not correct for tangent error, which can be an issue for very long exposures. It will however provide the correct values for you to input into Barn Door Clock, which does.

Includes a tilt meter to help with polar alignment, a night vision friendly torch / flashlight, help file, and a gallery to show what is possible tracking the night sky in a light polluted town.

This app is not compatible with Android 4.4. An earlier version is. Please send an email if you would like this earlier version.

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What is a barn door tracker?
Long exposures are needed to take pictures of the night sky. As a rough guide, 500 divided by the focal length of your lens will tell you the maximum exposure before you will see star trails because of the rotation of the sky. So for a typical 28mm wide angle lens, stars will appear elongated after about 18 seconds, and for a 100mm lens, after only 5 seconds. A barn door tracker is essentially a big hinge with your tripod on the bottom and your camera on top, aligned with the pole star and opened at the same rate as the sky turns (about 1 degree every 4 minutes). I use a variation on this one: http://garyseronik.com/build-a-hinge-tracker-for-astrophotography/ but there are many others (just search for barn door star tracker). Many people who post their designs have done the maths for you, but that means using exactly the same sized equipment as them. This app lets you build with whatever you have handy, does the maths for you, and then provides beeps or a visual signal to time it correctly.