Equatorial mount
Encyclopedia
Celestial sphere
In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the...
) by having one rotational axis parallel to the Earth's axis of rotation. This type of mount is used for astronomical telescopes and camera
Camera
A camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...
s. The advantage of an equatorial mount lies in its ability to allow the instrument attached to it to stay fixed on any object in the sky that has a diurnal motion
Diurnal motion
Diurnal motion is an astronomical term referring to the apparent daily motion of stars around the Earth, or more precisely around the two celestial poles. It is caused by the Earth's rotation on its axis, so every star apparently moves on a circle, that is called the diurnal circle. The time for...
by driving one axis at a constant speed. Such an arrangement is called a sidereal drive.
Astronomical telescope mounts
In astronomical telescope mountTelescope mount
A telescope mount is a mechanical structure which supports a telescope. Telescope mounts are designed to support the mass of the telescope and allow for accurate pointing of the instrument...
s, the equatorial axis (the right ascension
Right ascension
Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:...
) is paired with a second perpendicular
Perpendicular
In geometry, two lines or planes are considered perpendicular to each other if they form congruent adjacent angles . The term may be used as a noun or adjective...
axis of motion (known as the declination
Declination
In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...
). The equatorial axis of the mount is often equipped with a motorized "clock drive
Clock drive
In astronomy a Clock drive is a regulatory mechanism used to move an equatorial mounted telescope along one axis to keep the telescope in exact sync with the apparent motion of the celestial sky...
", that rotates that axis one revolution every 23 hours and 56 minutes in exact sync with the apparent diurnal motion of the sky. They may also be equipped with setting circles
Setting circles
Setting circles are used on telescopes equipped with an equatorial mount to find astronomical objects in the sky by their equatorial coordinates often used in star charts or ephemeris.-Description:...
to allow for the location of objects by their celestial coordinates. Equatorial mounts differ from mechanically simpler 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...
s, which require variable speed motion around both axes to track a fixed object in the sky. Also, for astrophotography
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...
, the image does not rotate in the focal plane, as occurs with altazimuth mounts when they are guided to track the target's motion, unless a rotating erector prism or other field-derotator is installed.
Equatorial telescope mounts come in many designs. In the last twenty years motorized tracking has increasingly been supplemented with computerized object location. There are two main types. Digital setting circles take a small computer with an object database that is attached to encoders. The computer monitors the telescope's position in the sky. The operator must push the telescope. Go-to systems use (in most cases) servo motors and the operator need not touch the instrument at all to change its position in the sky. The computers in these systems are typically either hand-held in a control "paddle" or supplied through an adjacent laptop computer which is also used to capture images from an electronic camera. The electronics of modern telescope systems often include a port for autoguiding. A special instrument tracks a star and makes adjustment in the telescope's position while photographing the sky. To do so the autoguider must be able to issue commands through the telescope's control system. These commands can compensate for very slight errors in the tracking performance, such as periodic error caused by the worm drive that makes the telescope move.
In new observatory designs, equatorial mounts have been out of favor for decades in large-scale professional applications. Massive new instruments are most stable when mounted in an alt-azimuth (up down, side-to-side) configuration. Computerized tracking and field-derotation are not difficult to implement at the professional level. At the amateur level, however, equatorial mounts remain popular, particularly for astrophotography.
German equatorial mount
Open fork mount
The Open Fork mount has a Fork attached to a right ascension axis at its base. The telescope is attached to two pivot points at the other end of the fork so it can swing in declination. Most modern mass-produced catadioptricCatadioptric
A catadioptric optical system is one where refraction and reflection are combined in an optical system, usually via lenses and curved mirrors . Catadioptric combinations are used in focusing systems such as search lights, headlamps, early lighthouse focusing systems, optical telescopes,...
reflecting telescopes (200 mm or larger diameter) tend to be of this type. The mount resembles 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...
, but with the azimuth axis tilted and lined up to match earth rotation axis with a piece of hardware usually called a "wedge."
Many mid-size professional telescopes also have equatorial forks, these are usually in range of 0.5-2.0 meter diameter.
English or Yoke mount
Mount Wilson Observatory
The Mount Wilson Observatory is an astronomical observatory in Los Angeles County, California, United States. The MWO is located on Mount Wilson, a 5,715 foot peak in the San Gabriel Mountains near Pasadena, northeast of Los Angeles...
, and there are no counterweights like German mount has.
The original English fork design has the disadvantage of not allowing the telescope to point too near the north or south celestial pole.
Horseshoe mount
Hale telescope
The Hale Telescope is a , 3.3 reflecting telescope at the Palomar Observatory in California, named after astronomer George Ellery Hale. With funding from the Rockefeller Foundation, he orchestrated the planning, design, and construction of the observatory, but did not live to see its commissioning...
is the most prominent example of a Horseshoe mount in use.
Cross-axis mount
Equatorial platform
An equatorial platformEquatorial platform
An equatorial platform or equatorial table is an equatorial telescope mount in the form of a specially designed platform that allows any device sitting on it to track astronomical objects in the sky on an equatorial axis. They are used to give equatorial tracking to any device sitting on them, from...
is a specially designed platform that allows any device sitting on it to track on an equatorial axis. It achieves this by having a surface that pivots about a "virtual polar axis". This gives equatorial tracking to anything sitting on the platform, from small cameras up to entire observatory buildings. These platforms are often used with 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...
ed amateur astronomical
Amateur astronomy
Amateur astronomy, also called backyard astronomy and stargazing, is a hobby whose participants enjoy watching the night sky , and the plethora of objects found in it, mainly with portable telescopes and binoculars...
telescopes, such as the common Dobsonian telescope
Dobsonian telescope
A Dobsonian telescope is an alt-azimuth mounted newtonian telescope design popularized by the amateur astronomer John Dobson starting in the 1960s. Dobson's telescopes featured a simplified mechanical design that was easy to manufacture from readily available components to create a large, portable,...
type, to over come that type of mount's inability to track the night sky.
See also
- Altazimuth mountAltazimuth mountAn 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...
- Barn door trackerBarn door trackerA barn door tracker, also known as a Haig or Scotch mount, is a device used to cancel out the diurnal motion of the Earth for the observation or photography of astronomical objects. It is a simple alternative to attaching a camera to a motorized equatorial mount.-History:The barn door tracker was...
- Equatorial roomEquatorial roomAn equatorial room, in astronomical observatories, is the room which contains an equatorial mounted telescope. It is usually referred to in observatory buildings that contain more than one type of instrument: for example buildings with an "equatorial room" containing an equatorial telescope and a...
- Hexapod-TelescopeHexapod-TelescopeThe Hexapod-Telescope is a design of telescope mounting developed by engineers of the company Vertex in collaboration with astronomers of the Astronomy Department of the Ruhr-Universität Bochum in Germany....
- List of telescope parts and construction
- List of telescope types
- Parallactic angleParallactic angleIn spherical astronomy, the parallactic angle is the angle between the great circle through a celestial object and the zenith, and the hour circle of the object. It is usually denoted q. In the triangle zenith—object—celestial pole, the parallactic angle will be the angle at the celestial object....
- Polar alignmentPolar AlignmentPolar alignment is the act of aligning the rotational axis of a telescope's equatorial mount in parallel with that of the Earth. There are various ways to achieve this.- Alignment Methods :...
- Polar mountPolar mountA polar mount is a movable mount for satellite dishes that allows the dish to be pointed at many geostationary satellites by slewing around one axis...
- a similar mount used with satellite dishes - Poncet PlatformPoncet PlatformA Poncet Platform or Poncet mount is a type of equatorial platform that uses a simple polar pivot and an inclined plane. The motion of the mount allows any device sitting on that platform to track the apparent motion of the stars in the night sky...