An autoguider is an electronic tool used in astrophotography to keep the celestial object being photographed from drifting across the field of view.
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When imaging dim targets, usually deep sky objects, hours-long exposure times are often necessary. For the resulting image to be clear, the target must be held at a certain position within the telescope's field of view; any apparent motion would cause point sources of light to appear as streaks. This could be accomplished by manually adjusting the orientation of an equatorial mount (an altazimuth mount would rotate the field) to hold either the target itself or a nearby guide star at a certain position, but such a method is tiresome and time-consuming. Even computer-tracked mounts and GoTo telescopes do not eliminate the need for manual tracking because astrophotography demands an extremely high level of precision that these devices cannot achieve.
An autoguider is usually attached to either a guidescope, a telescope oriented in the same direction as the main telescope, or an off-axis guider, which diverts some of the light originally headed towards the eyepiece.
The device has a CCD sensor that regularly makes short exposures of an area of sky near the object. After each image is captured, its computer measures the apparent motion of one or more stars within and issues the appropriate corrections to the telescope's (computerized) mount.
An autoguider does not need to be an independent unit; astronomical video cameras or webcams can serve as CCDs and programs such as Guidedog, or general-purpose astronomical software such as MaxDSLR, run on a personal computer as data processors.[1] However, these setups are not as sensitive as specialized units.
Since star images take up more than one pixel due to lens imperfections and other effects, autoguiders use the amount of light falling on each pixel to calculate where the star should actually be located. As a result, most autoguiders have subpixel accuracy. In other words, the star can be tracked to an accuracy better than the angular size represented by one CCD pixel. However, atmospheric effects, called astronomical seeing, typically limit accuracy to one arcsecond. To prevent the telescope from moving in response to changes in the guide star's apparent position caused by seeing, the user can usually adjust a setting called "aggressiveness".[1]