Meridian Flip

A lot of folks who have switched from forked mounted telescopes to a German Equatorial Mounts (GEM) are baffled by the meridian flip.  I was one of those people who could not quite picture the concept of the meridian flip in my mind's eye until I saw it for the first time. Fork mount scopes configured in Az/Alt or mounted on a wedge in equatorial mode can track continuously from one side of the sky to the other as shown in this SLEW  MOVIE (approx 6.5 megs). While there are some,  most GEMs can not slew from one side of the sky to the other because thier design incorporates mechanical stops. These stops are intended to prevent the Optical Tube Assembly (OTA) from impacting the pier or tripod and to also prevent the cables attached the OTA from wrapping around the mount. Because of this limitation the mount has to flip is position somewhere near the meridian in order to continue it's tracking across the sky. 

In the first image the GEM mounted telescope is pointed at the star SAO 39331. The location of the star is marked by the crosshairs.  The star is located on the east (left) side of the meridian. The meridian is depicted by the division line of the purple and red shaded areas.

I the case of the Paramount it will only track about 5 degrees past the meridian (the area shown in red).
 


This  image shows the actual position of the mount and OTA when pointed at star SAO 39331. A dashed white line has been added to the image to indicate the meridian. Notice the OTA is on the West side of the meridian but pointing East toward the star.

 

 

 

In the image on the right, Galaxy NGC-1039 (crosshairs) is on the west side of the meridian, while the scope shown below is on the eastern side but pointed west. Again the dashed white line depicts the meridian
Clicking on the image to the  right will show a 3.5 meg AVI file* of a meridian flip.

The video starts with the beginning of a meridian flip. The scope was positioned on the western side of the meridian while tracking an object on the eastern side. In order help visualize the scope's movement in its westward tracking in this video, the silver counter weights on the left side would have been moving upward before the scope stopped to execute the flip. After the flip is completed the counter weights will be on the right side and would now be moving downward during tracking.

 

The other important item to mention about the mederian flip which is a concern to astrophotographers, is the camera attached the scope is rotated 180 degrees after the flip occurs. This means all the images after a meridian flip will be flipped top to bottom and left to right as shown by the two images on the right of the Flame Nebula (NGC 2025) and its bright companion star Alnitak which also illuminates the nebula Herschel H28-5.  One way to avoid the flipping of the image is to use a camera rotator to re-orientate the camera before the image is aquired, or manipulae the image during post processing.

*The file is very slow and choppy. It does not accurately represent the speed and smooth movement of the Paramount ME.

 

     
                 Pre-Flip                              Post-Flip

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