Backlash and How to Set Anti-backlash Compensation
All gears, including the ones used to move a NexStar telescope, have a characteristic known as backlash. This is simply the play in the gears that shows itself as a delay when using the arrow buttons to manual slew the scope. The NexStar GPS, CPC, CGEM, CGE, CGE Pro models use precision worm-driven gears, which inherently exhibit minimal backlash. The AS-GT models use worm-driven gears, but of lower precision and thus have moderate backlash. All other models use spur gears, which have a good deal of backlash.
The delay caused by backlash is noticeable when using the arrow buttons to move in the direction opposite of tracking. For example, if you are in the Northern Hemisphere and viewing an object towards the south, pressing the left arrow button will require compensation for backlash otherwise you will notice a delay before the scope actually starts moving. The NexStar scopes employ backlash compensation, or Anti-backlash as Celestron calls it, to rewind the motor and take up the slack so that the scope starts moving immediately after pressing an arrow button. After you release the arrow button, compensation winds the motor back the other direction to reengage the gears and resume tracking. Without this compensation, scopes with significant backlash will exhibit a delay before the scope actually starts moving, allowing the object of interest to drift out of the field of view.
Since no two sets of gears are exactly the same, we can fine-tune the settings for the altitude and azimuth axis on each scope.
NexStar 5/8 and Original GT
On the original NexStar 5/8 and the original GT control for the little NexStars, there is just a single setting for each axis. In other words, you can specify a value for azimuth compensation and a value for altitude compensation.
To correctly set backlash compensation, start by aligning the scope. Point the scope towards the southeast, up at about 45 degrees from the horizon. Set the RATE value to the one you use move often when manually slewing the scope - generally 4, 5 or 6. Set the Azm Backlash to 0 and the Alt Backlash to 0. Azm Backlash corresponds to the left/right buttons, while Alt Backlash corresponds to the up/down button. While viewing an object in the eyepiece, observe the responsiveness of each of the four arrow buttons. Note which directions exhibit a pause, for example, right and down. Working one axis at a time, adjust the backlash setting high enough to cause immediate movement without resulting in a pronounced jump when pressing or releasing the button.
NexStar GPS, 'i' series, New GT, CGE, CGE Pro, CGEM and AS-GT
On the newer models, there are two values for each axis, positive and negative. Positive is the amount applied when compensation is required in a clockwise (azimuth axis) or upward (altitude axis) direction while negative is the amount applied when compensation is required in a counterclockwise (azimuth axis) or downward (altitude axis) direction. Normally the positive and negative values on either axis should be set the same. In other words, you might find the azimuth axis requires the positive and negative values to be set to 70 while the altitude axis requires the positive and negative values to be set to 30.
To correctly set backlash compensation, start by aligning the scope. Point the scope towards the southeast, up at about 45 degrees from the horizon. Set the RATE value to the one you use move often when manually slewing the scope - generally 4, 5 or 6. Next, press the Menu button, and select Utilities on the NexStar GT or Scope Setup on all newer models. Scroll to Anti-Backlash and press Enter. Set the Azm Positive and Negative to 0 and the Alt Positive and Negative to 0. Azm values correspond to the left/right buttons, while Alt values correspond to the up/down button. While viewing an object in the eyepiece, observe the responsiveness of each of the four arrow buttons. Note which directions exhibits a pause, for example, right and down. Working one axis at a time, adjust both the positive and negative backlash settings high enough to cause immediate movement without resulting in a pronounced jump when pressing or releasing the button. If you note a jump when releasing the button, but setting both values lower results in a slight pause when pressing the button, generally the lower value will be best for overall performance.
Note - when mounted equatorially (as are any of the German EQ mounts or when using a wedge with the fork-mounted models), the declination axis does not employ compensation when you release the up/down arrow buttons as there is no tracking direction associated with the declination axis. Thus there will be no jump when you release the button even if you have the compensation set too high. Instead, compensation is only applied when the direction of movement is opposite the last movement in declination. In other words, if your last movement was with the down arrow and now you press the up arrow, positive compensation is applied when you press the arrow button. Conversely, if the last movement was up and you now press the down arrow, negative compensation is applied only when you press the arrow button. (This orientation can vary depending upon the rate you are using and the current setting of DirectionButton on the Scope Setup menu.) In this case, keep the positive and negative values the same, and set them to the lowest value which provides no noticeable delay when you alternate between up and down movements.