The mount refers to both the legs and the mechanism that holds and points the telescope. The mount is critical for how you use and enjoy your telescope. In the old days mounts were just mechnical devices, but in these modern computerized times, mounts are becoming more intelligent. Mounts have computer guidance systems with built-in astronomical databases. There are many ways to mount and use a telescope, and thus many things to consider when choosing a telescope mount.
If all you want to do is look at the stars and planets then any mount that holds up your telescope and allows for easy aiming, steady viewing and comfortable use is a good mount. Dobsonian telescopes are a well known design with a mount configured for easy viewing. The Dobsonian is an example of the altazimuth mount combined with a Newtonian telescope optical tube assembly (OTA). An altazimuth mount is a design to hold the OTA and allow the user to guide the scope by using two motions. The up and down motion is call altitude, and the back and forth or round and round motion is call azimuth.
Between these two motions you can point a telescope to view any position above the horizon. It’s a simple system and easy to use and lends itself well to casual stargazing.
Because the earth spins, the stars and planets move in the heavens above us. Tracking a star while viewing it requires constant repositioning of the telescope to compensate for the motion of the earth. A second type of mount called the equatorial mount was design to make tracking match the apparent motion of the stars. The gimmick of this mount is the telescope is aligned along an axis that runs parallel to the earth’s spin.
To understand and use the equatorial mount requires a good deal more skill and conceptualization. This knowledge is not required to enjoy using a telescope, or even to use an equatorial telescope because such scopes can be used for casual viewing without mastering all the skills first. The technicque of properly setting up an equatorial mount is called polar alignment. Once configured the equatorial mounted telescope can be aimed with their setting circles using an astronomical coordinate system. Looking at the picture on the right try and imagine these concepts:
The tripod is positioned so it points along a line with celestrial north/south. The tripod can be placed pointing anywhere in a 360 degree circle, but you want the front of the mount aiming north/south. A polar alignment scope helps.
The first joint above the tripod allows adjusting the telescope to be tilted back the same number of degrees as the viewer’s latitude and will bring Polaris(north), the pole star into view in the polar finder scope. There are different kinds of polar alignment and various methods to perfect it including the drift alignment and iterative method. The goal is to get the scope polar alligned and then locked down for the night.
The next joint up allows for motion along the right ascension (RA) axis and allows the telescope to follow a star across the sky. If you have ever seen photos of stars making circles in the sky that should help you visualize trying to follow one star as it arcs across the sky. A motor along this axis will track a star. Visualize the celestrial globe turning above, this axis follows the spin of the globe.
The final joint, the one next to the optical tube assembly (OTA) is call declination and allows the telescope to move up and down the spinning celestrial globe to position the scope at the star’s altitude. A motor at this axis helps position the scope.
Motors along both right ascension and declination will allow a computer to position the scope and track a star. If the computer has a database of steller objects, you can request an object and the mount will look up the object’s position and move the telescope to show that object. The newest GOTO telescopes can do all of this without equatorial mounts and polar alignment because the computer can be programmed to know the position of the scope by using guide stars.
If you like learning how things work then buying an equatorial scope and using it properly will force you to learn the concepts of celestrial coordinate systems. If you don’t care, then consider buying a GOTO scope. Optional GPS units also available nowdays.
Equatorial telescopes come with setting circles which allow the user to manually position the telescope by using a coordinate system. It’s possible to position the scope using the angular markings on the circles, and then track a target by manually turning the RA knob. Motors on one axis or both axis will allow you to observe without readjusting the scope. This is important.