GUIDING OUT ERRORS
When shooting with short focal length lenses (under 135 mm), you can usually turn on the drive, open the shutter, and walk away. Longer lenses, however, are more demanding, and slight irregularities in the speed of the drive must be guided out.
To do this, you monitor a star through the main telescope during the exposure. An illuminated reticle—an eyepiece with cross hairs illuminated by a dim red light—can help you detect any wandering of the guide star.
The mount should have drive motors on both axes, each with a push-button speed control. If the star wanders one way along the cross hairs, press the appropriate button to jog it back. Taking a long exposure demands constant vigilance at the eyepiece, or the use of an automatic CCD guider.