Equatorial mount

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A large German equatorial mount on the Forststernwarte Jena 50cm Cassegrain reflector telescope. Forststernwarte Jena 50cm-Cassegrain 1.jpg
A large German equatorial mount on the Forststernwarte Jena 50cm Cassegrain reflector telescope.

An equatorial mount is a mount for instruments that compensates for Earth's rotation by having one rotational axis, called polar axis, parallel to the Earth's axis of rotation. [1] [2] This type of mount is used for astronomical telescopes and cameras. The advantage of an equatorial mount lies in its ability to allow the instrument attached to it to stay fixed on any celestial object with diurnal motion by driving one axis at a constant speed. Such an arrangement is called a sidereal drive or clock drive . Equatorial mounts achieve this by aligning their rotational axis with the Earth, a process known as polar alignment .

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Principle of operation and effect of an equatorial mount, assuming the subject is far enough that parallax is negligible Equatorial mount.svg
Principle of operation and effect of an equatorial mount, assuming the subject is far enough that parallax is negligible

Astronomical telescope mounts

In astronomical telescope mounts, the equatorial axis (the right ascension ) is paired with a second perpendicular axis of motion (known as the declination ). The equatorial axis of the mount is often equipped with a motorized " clock drive ", that rotates that axis one revolution every 23 hours and 56 minutes in exact sync with the apparent diurnal motion of the sky. [3] They may also be equipped with setting circles to allow for the location of objects by their celestial coordinates. Equatorial mounts differ from mechanically simpler altazimuth mounts, which require variable speed motion around both axes to track a fixed object in the sky. Also, for astrophotography, 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) a worm and ring gear system driven by servo or stepper 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

German equatorial mount Maksutov-Cassegrain Intes M703 mounted.jpg
German equatorial mount

In the German equatorial mount, [4] (sometimes called a "GEM" for short) the primary structure is a T-shape, where the lower bar is the right ascension axis (lower diagonal axis in image), and the upper bar is the declination axis (upper diagonal axis in image). The mount was developed by Joseph von Fraunhofer for the Great Dorpat Refractor [5] that was finished in 1824. The telescope is placed on one end of the declination axis (top left in image), and a suitable counterweight on other end of it (bottom right). The right ascension axis has bearings below the T-joint, that is, it is not supported above the declination axis.

Open fork mount

Open fork mount Warszawskie Obserwatorium Poludniowe Teleskop.jpg
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 catadioptric reflecting telescopes (200 mm or larger diameter) tend to be of this type. The mount resembles an Altazimuth mount, but with the azimuth axis tilted and lined up to match earth rotation axis with a piece of hardware usually called a "wedge". [6]

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

English mount on the Hooker telescope 100inchHooker.jpg
English mount on the Hooker telescope

The English mount or Yoke mount [7] has a frame or "yoke" with right ascension axis bearings at the top and the bottom ends, and a telescope attached inside the midpoint of the yoke allowing it to swing on the declination axis. The telescope is usually fitted entirely inside the fork, although there are exceptions such as the Mount Wilson 2.5 m reflector, and there are no counterweights as with the German mount.

The original English fork design is disadvantaged in that it does not allow the telescope to point too near the north or south celestial pole.

Horseshoe mount

Horseshoe mount on the Hale Telescope Palomar arp 600pix.jpg
Horseshoe mount on the Hale Telescope

The horseshoe mount overcomes the design disadvantage of English or Yoke mounts by replacing the polar bearing with an open "horseshoe" structure to allow the telescope to access Polaris and stars near it. The Hale Telescope is the most prominent example of a horseshoe mount in use. [8]

Cross-axis mount

Cross-axis mount. Zeiss di Merate - incrocio assi.jpg
Cross-axis mount.

The Cross-axis [8] or English cross axis mount is like a big "plus" sign (+). The right ascension axis is supported at both ends, and the declination axis is attached to it at approximately midpoint with the telescope on one end of the declination axis and a counter weight on the other.

Equatorial platform

An equatorial platform is a specially designed platform that allows any device sitting on it to track on an equatorial axis. [9] 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 mounted amateur astronomical telescopes, such as the common Dobsonian telescope type, to overcome that type of mount's inability to track the night sky.

See also

Related Research Articles

<span class="mw-page-title-main">Right ascension</span> Astronomical equivalent of longitude

Right ascension is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the point in question above the Earth. When paired with declination, these astronomical coordinates specify the location of a point on the celestial sphere in the equatorial coordinate system.

<span class="mw-page-title-main">Equatorial coordinate system</span> Celestial coordinate system used to specify the positions of celestial objects

The equatorial coordinate system is a celestial coordinate system widely used to specify the positions of celestial objects. It may be implemented in spherical or rectangular coordinates, both defined by an origin at the centre of Earth, a fundamental plane consisting of the projection of Earth's equator onto the celestial sphere, a primary direction towards the March equinox, and a right-handed convention.

<span class="mw-page-title-main">Proper motion</span> Measure of observed changes in the apparent locations of stars

Proper motion is the astrometric measure of the observed changes in the apparent places of stars or other celestial objects in the sky, as seen from the center of mass of the Solar System, compared to the abstract background of the more distant stars.

<span class="mw-page-title-main">Astrophotography</span> Imaging of astronomical objects

Astrophotography, also known as astronomical imaging, is the photography or imaging of astronomical objects, celestial events, or areas of the night sky. The first photograph of an astronomical object was taken in 1840, but it was not until the late 19th century that advances in technology allowed for detailed stellar photography. Besides being able to record the details of extended objects such as the Moon, Sun, and planets, modern astrophotography has the ability to image objects outside of the visible spectrum of the human eye such as dim stars, nebulae, and galaxies. This is accomplished through long time exposure as both film and digital cameras can accumulate and sum photons over long periods of time or using specialized optical filters which limit the photons to a certain wavelength.

<span class="mw-page-title-main">Dobsonian telescope</span> Type of Newtonian telescope popularized by John Dobson

A Dobsonian telescope is an altazimuth-mounted Newtonian telescope design popularized by John Dobson in 1965 and credited with vastly increasing the size of telescopes available to amateur astronomers. Dobson's telescopes featured a simplified mechanical design that was easy to manufacture from readily available components to create a large, portable, low-cost telescope. The design is optimized for observing faint, deep-sky objects such as nebulae and galaxies. This type of observation requires a large objective diameter of relatively short focal length and portability for travel to less light-polluted locations.

<span class="mw-page-title-main">Telescope mount</span> Mechanical structure which supports a telescope

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. Many sorts of mounts have been developed over the years, with the majority of effort being put into systems that can track the motion of the fixed stars as the Earth rotates.

Star hopping is a technique that amateur astronomers often use to locate astronomical objects in the night sky. It can be used instead of or in addition to setting circles or go-to/push-to systems.

<span class="mw-page-title-main">Altazimuth mount</span> Support mechanism with rotation about the horizontal and vertical axes

An altazimuth mount or alt-azimuth mount is a simple two-axis mount for supporting and rotating an instrument about two perpendicular axes – one vertical and the other horizontal. Rotation about the vertical axis varies the azimuth of the pointing direction of the instrument. Rotation about the horizontal axis varies the altitude angle of the pointing direction.

<span class="mw-page-title-main">Zenith telescope</span> Type of telescope that points straight up

A zenith telescope is a type of telescope that is designed to point straight up at or near the zenith. They are used for precision measurement of star positions, to simplify telescope construction, or both.

A 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. It works by having its slewing axis parallel, or almost parallel, to the Earth's polar axis so that the attached dish can follow, approximately, the geostationary orbit, which lies in the plane of the Earth's equator.

<span class="mw-page-title-main">Meridian circle</span> Astronomical instrument for timing of the passage of stars

The meridian circle is an instrument for timing of the passage of stars across the local meridian, an event known as a culmination, while at the same time measuring their angular distance from the nadir. These are special purpose telescopes mounted so as to allow pointing only in the meridian, the great circle through the north point of the horizon, the north celestial pole, the zenith, the south point of the horizon, the south celestial pole, and the nadir. Meridian telescopes rely on the rotation of the sky to bring objects into their field of view and are mounted on a fixed, horizontal, east–west axis.

<span class="mw-page-title-main">Orbital pole</span> Celestial coordinate system

An orbital pole is either point at the ends of the orbital normal, an imaginary line segment that runs through a focus of an orbit and is perpendicular to the orbital plane. Projected onto the celestial sphere, orbital poles are similar in concept to celestial poles, but are based on the body's orbit instead of its equator.

<span class="mw-page-title-main">Setting circles</span>

Setting circles are used on telescopes equipped with an equatorial mount to find celestial objects by their equatorial coordinates, often used in star charts and ephemerides.

<span class="mw-page-title-main">GoTo (telescopes)</span>

In amateur astronomy, "GoTo" refers to a type of telescope mount and related software that can automatically point a telescope at astronomical objects that the user selects. Both axes of a GoTo mount are driven by a motor and controlled by a computer. It may be either a microprocessor-based integrated controller or an external personal computer. This differs from the single-axis semi-automated tracking of a traditional clock-drive equatorial mount.

Polar alignment is the act of aligning the rotational axis of a telescope's equatorial mount or a sundial's gnomon with a celestial pole to parallel Earth's axis.

<span class="mw-page-title-main">Star position</span> Apparent angular position of any star in the sky; point on the celestial sphere

Star position is the apparent angular position of any given star in the sky, which seems fixed onto an arbitrary sphere centered on Earth. The location is defined by a pair of angular coordinates relative to the celestial equator: right ascension and declination. This pair based the equatorial coordinate system.

<span class="mw-page-title-main">Clock drive</span> Mechanism in a telescopes mount

In astronomy, a clock drive is a motor-controlled mechanism used to move an equatorial mounted telescope along one axis to keep the aim in exact sync with the apparent motion of the fixed stars on the celestial sphere.

<span class="mw-page-title-main">Equatorial platform</span>

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 small cameras up to entire observatory buildings. They are often used with altazimuth mounted telescopes, such as the common Dobsonian telescope type, to overcome that type of mount's inability to track the night sky. With careful polar alignment sub-arc second precision CCD imaging is entirely possible. Roeser Observatory, Luxembourg have contributed hundreds of astrometric measurements of Near Earth Asteroids to the Minor Planet Center using a home-built 20" Dobsonian telescope on an Osypowski equatorial platform.

References

  1. "LAS MONTURAS". Observatorio J. A. Soldevilla. Archived from the original on 2018-07-28. Retrieved 2008-09-20.
  2. "Observatorio ARVAL - Polar Alignment for Meade LXD55/75 Autostar telescopes". Observatorio ARVAL.
  3. Turn left at Orion: a hundred night sky objects to see in a small telescope ... By Guy Consolmagno, Dan M. Davis, Karen Kotash Sepp, Anne Drogin, Mary Lynn Skirvin, page 204
  4. "German and Fork Equatorial Mounts". 2002-2007 Mathis-Instruments. Archived from the original on 2009-01-02.
  5. Firefly Astronomy Dictionary . Firefly Books Ltd. 2003. p.  71.
  6. "Telescope Mount". Universe Today.
  7. "IMSS - Multimedia Catalogue - Glossary - Telescope mounts". 1995-2006 IMSS Piazza dei Giudici 1 50122 Florence ITALY. Archived from the original on 2010-08-08.
  8. 1 2 "Telescope Mountings". 2001, 2004 John J. G. Savard.
  9. Philip S. Harrington, Star Ware: The Amateur Astronomer's Guide to Choosing, Buying, and Using Telescopes and Accessories, page 168