In astrophotography, a meridian flip is the process of rotating your telescope or camera on its mount to the other side of the mount when the object being observed or photographed passes through the meridian, an imaginary line in the sky that runs from due north to due south and passes through the zenith.
Because the Earth rotates on its axis, objects in the sky appear to move from east to west over time. An object will eventually reach the meridian as it moves across the sky. If you don’t perform a meridian flip, your telescope will continue to track the object as it moves toward the western horizon, potentially causing your equipment to collide with the mount or tripod.
Performing a meridian flip ensures that your telescope or camera continues to track the object as it moves toward the eastern horizon, allowing you to capture images or observe the object for a longer period. You can perform a meridian flip manually by selecting your target again in your hand controller after it passes the meridian, as the mount is smart enough to know that it should do the flip. However, it will not do this automatically while tracking. Many astrophotographers program their imaging software to automatically initiate a meridian flip when the object passes through the meridian, which can save time and prevent the need for manual intervention.