The NexStar User's Guide II - Excerpt from Chapter 12
Objects Drift Out of the Field of View While Tracking
First, note that you must generally complete an alignment to begin tracking. Next, review the additional tips for each of the alignment methods in Chapter 4 to determine reasonable expectations for each. For example, the Solar System Alignment method does not provide extremely accurate tracking, particularly if the tripod is not level.
One additional factor that can affect tracking in all models is the backlash compensation setting. If backlash compensation is inaccurate, an object can drift out of the field of view right after you manually center it. For example, in the Northern Hemisphere most areas of the sky are moving to the right in the sky. If you manually center an object with a final movement to the left, backlash compensation should rewind the motor to the right after you release the left arrow button. This re-engages the gears to allow tracking to start immediately. If your backlash compensation settings are too low, the motor is not rewound far enough and tracking does not start immediately causing the object to drift out of the eyepiece. Refer to Chapters 5 and 6 for details on adjusting backlash compensation.