Starguide - A Practical Guide to Astro-Photography

Mount operations known as Meridian Flips are important when using a German Equatorial Mount (GEM). The meridian is an imaginary line that divides the sky into east and west halves. It starts at 180 degrees (south) and passes directly overhead to 0 degrees (north). It is static and does not move with the sky. Imaging an object typically begins when it is in the east half of the sky. As the night progresses, the object will approach the meridian, cross it, and then be in the west half of the sky.

When a GEM RA axis approaches the meridian, with the telescope on the west side of the mount and looking east, a "flip" must be performed to swap the side of the mount that the telescope is on. This is to avoid the mount tracking past the meridian. Otherwise the counterweights will be higher than the telescope (an undesirable situation on some mounts) and the telescope (or some part of it) contacting the pier or tripod legs. Some mounts and equipment configurations are more tolerant than others to these conditions. Some mounts can track for hours after passing the meridian in a counterweight-up condition. Some telescopes are both short enough in length and height that they will not crash into the pier or tripod legs. Every situation is different, so it is up to the user to know when a meridian flip should be commanded.

Many sequencers and mounts support automatic meridian flip, including the ASIAIR guider,  N.I.N.A and  Sequence Generator Pro.

From and more on: Automated Meridian Flip - Nighttime Imaging 'N' Astronomy

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