Guiding an exposure involves looking through a separate telescope from the telescope or lens being used to take the picture. A telescope eyepiece with crosshairs embedded in it is used -- the photographer centers a star on the crosshairs, starts an exposure, and then carefully watches the star through the guidescope. When mechanical errors begin to move the star off the center of the crosshairs, the mount's drive controls can be used to put it back dead center. Keeping the guide star on the crosshairs during the exposure will ensure that tracking errors are minimized, and long exposures (up to several hours) can be made very accurately. Obviously, this can get quite tedious -- you must keep your eye glued to the guidescope for the entire exposure, and delaying too long in making corrections will ruin an image. Technology has offered a solution to manual guiding tedium -- a CCD guide camera. Commercial guide cameras are available, but even inexpensive digital Web cams can be easily modified to act as guide cameras. Attached to a computer, the camera takes an initial exposure, and notes the pixel position of a guide star (specified by the user). Then the shutter is opened on the imaging camera, and the guiding sequence started through the computer. It takes a new image every few seconds, and again notes the pixel position of the guide star. If it's moved from its original location, the computer sends signals to the equatorial mount to move it back to where it was, just as you would do manually. This "auto-guiding" can easily be more accurate than manual guiding, and doesn't require the photographer to sit at the guidescope for hours on end. The links section at the end of this article has resources for making your own auto-guider from an inexpensive Web cam.