When taking long exposures on astronomical telescopes, tracking is required to compensate for the Earth’s rotation. Typical astronomical telescope mounts are not accurate enough for long exposure imaging without some form of guiding.

Tracking errors are usually sufficiently small on short duration images (15 to 90 seconds depending on the mount) that guiding is not required. Therefore a series of short exposure images can be taken and combined together later using the Stacker. However, in many cases a single long exposure is often superior due to the elimination of extra readout noise and the residual blurring caused in realigning the images.

Although telescopes can be guided manually, many observers are now using CCD or CMOS autoguiders. In some cases these are stand-alone units that are interfaced directly to the telescope. However, some units can or must be controlled from software. MaxIm DL provides support for autoguiders,and can run them in parallel with the main camera.

Instructions for installation of the autoguider are included with the unit. Generally speaking, the autoguider connects electrically to the telescope’s guide controller using a standard RJ-11 jack. In some cases, special connections are required. Optically, the autoguider is connected either to a guide telescope attached mechanically to the main telescope, or to an off-axis guider. The off-axis guider includes a pick-off mirror that diverts light from the telescope into the autoguider. Normally the pick-off mirror is positioned outside of the main camera’s field of view.

Some cameras include two autoguider sensors, either in the same package or not, with both operating through the same interface. Most SBIG camera models work this way, as do some Starlight Xpress cameras. If both sensors are looking through the same instrument, this ensures that mechanical flexure is not an issue, although limited field-of-view and losses due to filters can be a problem.