Telescope Mounts and Celestial Tracking Tutorial

Telescope Mount Introduction

This webpage provides an introduction to telescope mounts and tracking astronomical objects. The purpose is to provide a basic understanding of the different types of telescope mounts and how they are constructed. Understanding equatorial telescope mount construction requires a basic knowledge of how celestial objects move relative to the Earth; it is recommended for beginners to first read the Celestial Motion Tutorial webpage before this webpage.

Alt-Az and Dobsonian Mounts

Most beginner telescopes come with simple altitude-azimuth (Alt-Az) fork mounts (below diagram, left image). an Alt-Az mount is a telescope that moves in two perpendicular planes: one plane is parallel to the ground (azimuth) and allows back-and-forth motion, and the other plane is perpendicular to the ground (altitude) and allows up-and-down motion.  

Probably the most common type of homebuilt telescope mount is the Dobsonian mount. The Dobsonian mount was invented by John Dobson, and is a very sturdy Alt-Az mount (below diagram, right image).  A basic Dobsonian mount is simply a 3 sided plywood box (pink) that sits and rotates on some type of Dobsonian bearings (usually Teflon pressed against metal or plastic). The telescope sits inside the telescope box (light blue), which rests on top of the 3 sided box. Dobsonian mounts are very inexpensive and easy to build.  These mounts are very sturdy with a smooth motion, are rugged, easy to set up and use, and well suited to large aperture telescopes. Dobsonian mounts are not only limited to homebuilt telescopes, but are commercially available on many medium to large aperture reflector telescopes. The only major disadvantages are that objects at the zenith are difficult to track, and that the Dobsonian suffers from the disadvantages common to all Alt-Az mounts (described in the next section).



 Alt-Az mounts function very well as terrestrial spotting scopes and camera supports, but the azimuth rotational axis causes celestial images to rotate during extended tracking; this is called field rotation and is the subject of the next section.  

Field Rotation

As the Earth rotates, celestial objects will appear to move in an arc around the celestial pole. If an Alt-Az mounted telescope (old fashioned up and down mount) tracks a celestial object, it will remain centered in the eyepiece, but will rotate with respect to time; this is called field rotation and is illustrated in the following two diagrams. The below diagram shows an Alt-Az telescope tracking a constellation. As we progress from left to right in the diagram, time increases. It is clear that the constellation follows an arc around the celestial pole. The Alt-Az telescope tracks the constellation, but notice the orientation of the small finder scope on the telescope (the finder scope is always on top). This is because the telescope swivels up-and-down and side-to-side, but does not rotate with the constellation.  This means that as the constellation rotates with time, it will also rotate in the telescopes field of view.

 

Field rotation generally doesn't present a problem for visual observation, but is a problem for long exposure astrophotography. The below diagram illustrates how field rotation affects astrophotography. If we take pictures (at different times) through the telescope in the above diagram and lay them side by side, they look as in the below diagram (top figure). If these 5 images are combined, they will give a picture similar to the bottom figure in the below diagram. Since ccd imaging combines multiple images to a single composite, field rotation will cause stars in the final composite image to appear as arcing trails.

 

There are several methods to correct for field rotation on an Alt-Az mount. One method is to individually rotate each image before combining them to a final image. Another solution is to use a field derotator to rotate the camera at the same rate that the image rotates in the telescope.  The below diagram shows a field derotator (red) rotating a camera (grey) attached to an Alt-Az telescope tracking a constellation. Since the rate of field rotation is dependent upon the telescopes orientation, field derotators are expensive accessories that integrate into a GoTo telescope's computer tracking system. 


Equatorial Tracking

A far simpler method than field derotation is to construct a telescope mount that rotates the entire telescope around the celestial sphere's polar axis; this is what an equatorial mount does. The telescope is inside the celestial sphere with one telescope axis orientated parallel to the Earth's polar axis (below diagram). Revolving the telescope about this polar axis (dashed white line) tracks the star as it moves (yellow arrow).  It is only necessary to move an equatorially mounted telescope in a single direction, requiring a single motor, to track a celestial object. Since the motors on all astronomical telescope mounts introduce a small amount of tracking error, using only a single motor minimizes this error (compared with a two motor Alt-Az GoTo telescope).



As the equatorial mount tracks, the entire telescope rotates at the same rate as the tracked object, eliminating field rotation. The following two diagrams illustrates how an equatorial mount negates field rotation. The below diagram shows a circumpolar star revolving around the Earth's celestial pole (marked with a small +). The equatorial telescope is mounted to revolve around an axis parallel to the Earth's polar axis (the straight dashed line).  The below diagram shows how the entire telescope revolves and hence rotates as it tracks the star.



The below diagram further illustrates the lack of field rotation in an equatorially mounted telescope. As the telescope tracks a constellation, the entire telescope rotates (note the position of the small finder telescopes on top of the large telescopes). The top of the telescope (marked with the finder scope) always points at the top of the constellation and rotates at the same rate as the constellation, negating field rotation.


Equatorial Fork Mounts

One of the simplest equatorial mounts is the equatorial fork mount. This is an Alt-Az fork mount that is placed on an inclined plane (wedge). The wedge tilts the azimuth axis to be parallel with the Earth's polar axis. The below diagram illustrates an Alt-Az mount and an equatorial fork mount (left and right, respectively).



The equatorial fork mount is orientated with the azimuth axis inclined at the same angle as the observers latitude. The below diagram shows the azimuth axis (dashed yellow line) inclined at an angle equal to local latitude, tilting the azimuth axis to be parallel with the polar axis.  The 5 stars in the below diagram all sit on the celestial equator (dashed green line) and have equal declinations= zero. Once the equatorial fork mount is aligned on the polar axis and the declination is set to zero, it is only necessary to rotate the telescope in right ascension to find any of the 5 stars or track any one of the stars. 



The below two diagrams further illustrate equatorial fork mount tracking. As the star at the intersection of the right ascension line (red) and declination line (pink) moves across the sky, it is only necessary to rotate the telescope in right ascension to track the star.




Split Ring and Yoke Mounts

The split ring mount (below diagram, left) holds the telescope inside a horseshoe shaped ring. The telescope pivots about the declination axis (pink) inside the split ring. The right ascension axis (yellow) is normal to the face of the split ring and parallel to the Earth's polar axis.

The yoke mount (below diagram, right) is very similar to the split ring mount. The telescope is suspended inside an inclined fork, supported at both ends, and forming a right ascension axis (yellow) parallel to the Earth's axis. The telescope pivots about the declination axis (pink) inside two parallel forks. 



English Cross Axis and Hemisphere Mounts

The English cross axis mount (below diagram, left) is a variation on the yoke mount. The telescope is mounted to a declination axis with counterweight that is perpendicular to the right ascension axis. The right ascension axis is supported at both ends and is parallel to the Earth's axis. This is a very solid, but unportable mount, and is best suited to fixed locations and observatories.

The
hemispherical mount (below diagram, right) is a telescope attached to a large sphere. The sphere is supported in a cup or similar device that allows it to rotate in all directions. Some telescope builders have used old bowling balls for the hemisphere.


German Equatorial Mounts (GEM)

The German equatorial mount (GEM) is an English cross axis mount with a RA axis supported on only one end. The below left diagram shows a comparison of the English cross axis and GEM mounts. Note how the GEM can be formed by removing the green sections of the English cross axis mount. The GEM (below right diagram) has a central right ascension axis (yellow) inclined parallel to the Earth's polar axis. The declination axis (pink) is perpendicular to the right ascension axis. The telescope is mounted off center around the right ascension axis and requires a counterweight to balance the associated torque. The telescope and counterweight both rotate around the right ascension axis.
 

The GEM is orientated with the right ascension axis inclined at the same angle as the observers latitude. The below diagram shows the right ascension axis (dashed yellow line) inclined at an angle equal to local latitude and pointing at the polar axis.



The below two diagrams further illustrate GEM tracking. As the star on the pink declination line moves across the sky, it is only necessary to rotate the telescope about one axis (right ascension axis) to track the star. Both the telescope and the counterweight rotate around the right ascension axis (dashed orange line) during tracking.


                                                                                                                                                                  

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