Computerised German equatorial mounts (GEM) are used to track the night sky. They do this by moving on an axis, parallel to the axis of the rotation of the earth. Being able to track targets of the night sky is essential for producing quality astrophotography images because of the ability to expose for longer without star trailing.
The longer an astrophotographer can expose, the more photons that can be collected by the sensor meaning more image detail and a better signal-to-noise ratio. Computerised GEM's have the added advantage, over manual mounts, to slew (go-to) your desired target, plate solve (determine where in the night sky your telescope is pointing) and automate polar alignment. While not the only computerised or tracking mount available.
GEMs are the preferred option for astrophotography because they are designed for use with larger instruments and tend to be more stable with less vibration.
Some key features and functions to look at when purchasing a computerised equatorial mount are:
a quality tripod
payload capacity
mount weight
tracking accuracy
built-in USB / Cable control hub.
Equatorial mount diagram
Parts of a motorised equatorial mount
(1) Azimuth adjustment bolts - rotate these during polar alignment to move the position of the telescope left or right until it points true north or south depending on your hemisphere.
(2) Altitude adjustment bolts - rotate these to adjust the mount's altitude angle. The angle needs to be set to the latitude of your location for proper polar alignment.
(3) Latitude display - shows the mount's current altitude.
(4) RA lock lever - locks the right ascension axis.
(5) Declination lock lever - locks the declination axis.
(6) Dovetail saddle - uses a dovetail bar to mount your telescope to the GEM.
(7) Counterweight - Counterbalances the weight of the telescope and other accessories.
(8) Counterweight lock lever - tightens the counterweight bar to prevent movement (not imaged, approximate location provided).
(9) Polar alignment scope - illuminated wide field scope used to aid polar alignment.
(10) On/off switch - powers the mount
(11) Hand controller port - Connection point for the mount's hand controller.
(12) Tripod - Supports the equatorial mount.
(13) Mount power port - used to provide power to the mount
(14) Auto-guider port - to be used for on-mount guiding from a guide camera.
Setting up a motorised GEM
First, use a compass or phone app to get a rough idea of true north or south (depending on your hemisphere) and place your tripod on the ground in that direction.
Before adding the GEM, ensure the tripod is level. Most tripods will include a built-in bubble level that is not always accurate. The tripod feet will have lock levers, lengthen or shorten each leg until the tripod is level.
Place the GEM onto the tripod and secure it by rotating the bolt underneath the tripod. The bolt can also be a rod used to hold an accessories tray.
Now that the mount is secured, you can check your position relative to true north or south again before adding more weight and adjusting if required.
Before adding a telescope, it is important to add the weights first to avoid the telescope colliding with the mount.
Once the weights are on, lock the weights and counterweight rod in place. Now add the telescope to the mount via the dovetail saddle. Secure using the two bolts on the dovetail saddle. Ensure the telescope is facing north or south and lock the declination lever to prevent the telescope from turning.
At this point, it is time to add a camera, filter wheel and other accessories.
Next, it is time to balance the mount. Starting with the RA axis, loosen the RA lock lever and rotate about 90 degrees. If the RA axis moves, then it is not balanced. Adjust the weights on the counterbalance rod up and down until the RA axis remains in a 90-degree position. Then rotate the RA axis back to the starting position and lock the RA axis in place using the RA lock lever.
The telescope also needs to be balanced. Unlock the declination lever and rotate the telescope 90 degrees. If the telescope continues to rotate in either direction, the telescope is not balanced. The position of the telescope's dovetail bar on the dovetail saddle will need to be adjusted back or forward until the scope no longer rotates. Tighten the saddle bolts. Once this is balanced, bring the telescope back to its starting position and lock the declination lever firmly in place. Check the RA axis is still balanced by repeating the above step.
Make sure all levers and bolts are secure. Now that the mount has been levelled and balanced, the hand controller and cables can be connected to the equatorial mount, camera and other accessories.
Now that setup is complete, camera focusing and polar alignment can begin.
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