A barn door tracker is a (usually self made) mount for a camera, which moves the camera in sync with earth's rotation. The camera could be used for longer exposures without creating the typical lines of moving stars on a photo. This only works, when the axis of the barn door tracker is parallel with the axis of the earth. This PolePointer app is used to calibrate your barn door tracker by using your iPhone to exactly point to the celestial pole, either the celestial North pole or South pole, depending on your exact position on earth. The app uses only red light to protect your night vision.
After you used the app 5 times, and you like it, you are asked to purchase the app via In-App Purchase (IAP).