How can I use GoTo Approach to minimize backlash with my mount?
October 5, 2018
To minimize backlash with your mount, use your hand control and select Menu – Scope Setup – GoTo Approach. This will enable you to choose the direction the scope will use when approaching an object during the final slewing – the ideal way to minimize backlash.
The directions can be either positive or negative and are defined as seen from the back of the scope in the north position. For altitude or declination, positive moves the front of the tube up, negative down. For azimuth or RA, positive moves the front of the scope to the right, negative to the left.
Always set the direction of approach in the opposite direction from the load pulling on the scope. This minimizes effects of backlash in the gear train. The load in altitude is usually because of gravity acting on the scope’s imbalance from accessories (eyepieces, cameras, etc). The load in azimuth is due to tracking and is opposite (in reaction) to the tracking direction, so the approach direction will be the same direction as tracking.
Whether to set positive or negative in altitude or azimuth depends on your scope, the type of mount, its orientation, Southern or Northern Hemisphere location and the load from attached accessories.
In a typical situation with Schmidt-Cassegrain (SCT) and refractor telescopes, the tube will be back heavy, because accessories are attached. (For Newtonians, attaching accessories may result in a front-heavy situation.) The load acts to pull the front of the tube up (positive), so the opposite direction (negative) should be chosen for the GoTo Approach. For azimuth, the scope's tracking determines the load. When the telescope tracks to the west (positive), the GoTo Approach should be set to close in from the same direction, moving west (positive).
Many users use the default setting, which is altitude negative, azimuth positive. For the SCT with a star diagonal, GoTo slews will be seen as moving to the up and right with these settings.
For German equatorial mounts, when you flip the scope to the opposite side of the mount when tracking across the meridian, you must change the GoTo Direction in declination (altitude) to maintain minimal backlash.
In the Southern Hemisphere (as seen from behind the scope), the west is to the left, reversing the GoTo Approach settings for azimuth.
Note: when aligning the mount, use the same approach directions to center your alignment stars as you are using for GoTo Approach for the best GoTo performance.