Saddle Plate Rotation

Certain mounts (like the iOptron CEM40) might require their top plate to be rotated so the camera boxes face North when teh mount is set to its zero position.

Step 1: Find zero position

The Zero position (aka Home position) on an equatorial mount is the position at which the counterweight shaft points to the ground, the telescope is at the highest position with its axis parallel to the polar axis and the telescope is pointing to the celestial pole.

Plug in the 12V DC power supply to the DC12V power socket. Connect the hand controller to the HBX port on the mount side panel. Turn the mount power on.

Go to zero position in the hand controller and select search zero position. The mount will start to slew slowly to search and set the mount to the Zero position. When the mount has found the Zero position, the hand controller will ask if you want to calibrate the zero position. Press enter to confirm. Make a note of the North facing side of the mount top plate.

The way the camera box hardware is designed, the cameras would face 90 degrees away from the celestial pole when the mount is at the Zero position.

If your camera box is facing in the direction of the celestial pole at the zero position, you can skip doing the mount saddle plate rotation as described below.

The saddle plate of the mount needs to be mechanically rotated by 90 degrees (as shown below) so the cameras are facing North when the mount is at Zero position.

Step 2: Rotate saddle plate

Turn off the mount. Disconnect the power supply. Remove the screws in the order shown below, rotate the saddle plate, and reassemble the top plate.

Make a note of which cable corresponds to which port.

Only the four center screws need to be removed in order to remove the saddle.

The screws holding the saddle in place might be tight. It's best to have someone firmly grip the saddle while attempting to unscrew otherwise it's possible to rotate the saddle itself, which will cause the internal gears to grind against each other.

Be careful when re-attaching the screw on the underside (see image below). If you screw it in all the way it has the potential to grind against the underside of the saddle plate.

Put back all screws and reassemble the saddle plate.

The small box that has the wires can be re-attached on either side of the saddle plate.

Step 3: Test new Zero position

Plug in the 12V DC power supply to the DC12V power socket. Connect the hand controller to the HBX port on the mount side panel. Turn the mount power on.

Go to zero position in the hand controller and select search zero position. The mount will start to slew slowly to search and set the mount to the Zero position. When the mount has found the Zero position, the hand controller will ask if you want to calibrate the zero position. Press enter to confirm. Check to see if the new Zero position is rotated by 90 degrees compared to the Zero position seen in Step 1: Find zero position.