Autoguider
Encyclopedia
An autoguider is an electronic tool used in astrophotography
to keep the celestial object being photographed from drifting across the field of view.
(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 telescope
s do not eliminate the need for manual tracking because astrophotography demands an extremely high level of precision that these devices cannot achieve.
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. 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".
Astrophotography
Astrophotography is a specialized type of photography that entails recording images of astronomical objects and large areas of the night sky. The first photographs of an astronomical object were taken in the 1840s, but it was not until the late 19th century that advances in technology allowed for...
to keep the celestial object being photographed from drifting across the field of view.
Background
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 mountEquatorial mount
An equatorial mount is a mount for instruments that follows the rotation of the sky by having one rotational axis parallel to the Earth's axis of rotation. This type of mount is used for astronomical telescopes and cameras...
(an altazimuth mount
Altazimuth mount
An altazimuth or alt-azimuth mount is a simple two-axis mount for supporting and rotating an instrument about two mutually perpendicular axes; one vertical and the other horizontal. Rotation about the vertical axis varies the azimuth of the pointing direction of the instrument...
would rotate the field) to hold either the target itself or a nearby guide star
Guide star
In astronomy, a guide star is a reference star used to accurately maintain the tracking by a telescope of a heavenly body, whose motion across the sky is primarily due to the rotation of the Earth....
at a certain position, but such a method is tiresome and time-consuming. Even computer-tracked mounts and GoTo telescope
GoTo (telescopes)
In amateur astronomy, "GoTo" refers to a type of telescope mount and related software which can automatically point a telescope to astronomical objects that the user selects...
s do not eliminate the need for manual tracking because astrophotography demands an extremely high level of precision that these devices cannot achieve.
Operation
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. 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
Astronomical seeing
Astronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by turbulent mixing in the Earth's atmosphere varying the optical refractive index...
, 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".