The Basic Astronomical Telescope Mounts
There are many different types of telescopes used for astronomy. But for amateur astronomy, these can broadly be divided into refractor telescopes and reflecting telescopes. Within each of these categories are short versions (short focal length or catadioptric), and long versions. You choose a short focus telescope if you want to view wide star fields, or need a portable instrument, or both. You might choose a long focus telescope if you want to look at high resolution targets like planets, double stars, and the moon, though there are short reflector designs that can do this well.
As to refractor telescopes, the short ones now available use newer types of glass in the objectives that allow short focal lengths with much less chromatic (color) distortion than older designs. The long refractor telescopes are still made pretty much with crown and flint glass objectives, as these work quite well in long focus designs.
As to reflector telescopes, they can be further divided into Cassegrain (catadioptric) or Newtonian styles. The oldest design is the Newtonian, invented by -- you guessed it -- Isaac Newton. It uses a parabolic mirror at the bottom end of the telescope as the main light collector and image maker, and a small diagonal mirror at the observing end of the telescope that reflects the image out to the eyepiece. The Newtonian telescope, like the refractor telescope, can be made as either short focus or long focus. However, the performance characteristics vary quite a bit with the different focal ratios.
Interestingly, the Cassegrain type reflecting telescopes, though very
short in physical design, provide more the performance of a long telescope
in that they actually have long effective focal lengths.Having a convex secondary mirror on back of the corrector plate considerable reduces the focusing convergence angle, effectively extending the focal length.
The reason for considering the style of telescope you might most enjoy is that telescope design can have a lot to do with the optimal mounting type for the telescope you choose. As there are different telescope types, there are also different types of telescope mounts. This web page will take you through some of the most common telescope mount types used in amateur astronomy.
Telescopes used for astronomy, whether Newtonian, Dobsonian, Cassegrain, or Refractor, have two basic types of mounts, with variations of each. The simplest telescope mount is called an altazimuth mount, and the more complex one is called the equatorial mount.
The altazimuth telescope mount is certainly the simplest, allowing the telescope to be pointed up and down (elevation), and around (azimuth). This telescope mount type is easy to make, can be very sturdy, and works nicely for visual work.
The more complicated equatorial telescope mount designs are made to facilitate easier tracking of celestial objects, especially for motorized tracking. Each of these come with variations to allow for telescope size and weight, and slow motion or motor control. I'll show here a few of the most common configurations. Incidentally, if you have an altazimuth or equatorial mount with setting circles, the freely available web page utility Star Pointer will also show you where to point your altazimuth or equatorial mounted telescope.
It is common today for modern telescopes to include not only motor driven mounts, but computerized motor drives that allow you to simply select objects via computer or hand-held controller. After selection, the telescope's computer moves the telescope to point at your selected target. These smart altazimuth mounts also allow automatic tracking, something that could only be done with the more complicated equatorial telescope mounts that were more common before the invention of computerized telescope mounts.
These modern telescopes use the computer and drive do the work and locate
objects and track them for you. You can buy some pretty incredible computerized
mounts for even small telescopes in today's market. Look at this
iOptron 9502B-A SmartStar-R80 Computerized Telescope - Astro Blue with Carry Bag
If you happen to have, or are only interested in, non-computer driven telescopes, this site has something to offer you. As long as you have setting circles, either in altazimuth or equatorial modes (or are willing to add setting circles), then you can use the Star Pointer utility. Star Pointer is a web page utility designed to list all objects that are above 25 degrees in elevation at your observing location (if you will allow it to keep your location in a cookie). If you want to make some setting circles, you can download the Setting Circle PDF file for a template. Just use a graphic program or copy machine to adjust the size.
Star Pointer presents any one of three popular target catalogs, and provides a periodically updating coordinate table for the objects visible in your location, with their current pointing angles for either altazimuth or equatorial type mounts. Available catalogs include the Messier, the Caldwell, and the Herschel 400. It might really save you time in finding objects, as it tells you precisely where to point your North-aligned telescope. I use it with my Android smart phone and its web browser.
The Altazimuth Mount
Pictured here is a simple configuration of the classic altazimuth telescope
mount. The
Celestron Heavy-Duty Altazimuth Tripod
The altazimuth type telescope mount has a vertical axis (Labeled Az) that is perpendicular to the ground, and a horizontal axis (Labeled Elev) that is parallel to the ground. Movement of the telescope in the elevation axis points the telescope up or down, with a zero angle being level with the ground. Rotation in the azimuth direction moves the telescope around between the cardinal directions, with zero being North.
As shown with the 50mm refractor in this picture, such a mount in combination with a small telescope often doesn't even need a counter weight. If you happened to observe from the North or South Pole, the vertical axis would be aligned with the Earth's spin axis. The nice thing about that would be that when you found an object to observe, rotation around only the vertical axis would be needed to keep the object in the field of view. Rotating at the Earth's spin rate in the opposite direction as the Earth's rotation would keep and object motionless in the eyepiece.
However, for observing from any other latitude on the planet, the vertical axis is not aligned with the Earth's spin axis. This means that to keep an object in the field of view requires motion in both axes. The motion rates will change over time as the elevation angle changes. Tracking objects near the horizon requires mostly changes in elevation, and tracking objects more North or South requires mostly changes in azimuth.
The altazimuth telescope mount is the simplest mount to build, and inexpensive telescopes often come with some variation of this type of mount. If you happen to have a telescope that doesn't have a mount, or one with an inadequate mount, you can build a substantial altazimuth mount out of pipe fittings, polishing the threads on the two axes with a bit of valve grinding compound. The total cost can be as low as about $75. A description of such a mount is at Inexpensive DIY Tripod.
Since the increasing integration of computers into the astronomy hobby, the
altazimuth mount is getting more frequent use. By putting a drive motor on both
axes of the altazimuth telescope mount and using a computer to calculate the
correct drive rate on each motor for any given target, smart altazimuth
telescopes like the
Celestron NexStar 5 SE Telescope
The Dobsonian Mount
The Dobsonian mount is a common mount for Newtonian telescopes. The Dobsonian mount is just
another configuration of the altazimuth mount. It has a vertical axis
perpendicular to the ground, and an elevation axis that is parallel to the
ground. The Dobsonian design can be as compact as on this
Orion StarBlast 4.5 Astro Reflector Telescope
The image above is an illustration of changes in azimuth. The Dobsonian telescope base usually sits on 3 Teflon pads, making a smooth bearing with a very big diameter. This gives good support for large Newtonian telescopes. Slight nudges are all that's needed to move a Dobsonian mounted telescope around the azimuth axis.
This image is an illustration of changes in elevation. The elevation axis bearings usually sit on a couple of Teflon pads, again making for simple, stable, and smooth bearings. The larger the telescope, the larger in diameter are the elevation axis shafts. As with azimuth, simple nudges to a properly made Dobsonian telescope are all that's needed to move it smoothly in elevation.
The advantages of the Dobsonian telescope mount are it's simplicity, low
cost, and ability to handle large telescopes. If you can use a saw, you
can likely make a fine Dobsonian mount to complete any reflector telescope
project. Check out the plans at Making A DobsonianMount. For amazingly low prices, you can buy Dobsonian telescopes ready to
use, like the
Orion SkyQuest XT6 Classic Dobsonian Telescope
The Equatorial Mount
Shown above is a Newtonian reflector telescope on an equatorial mount --
specifically a German equatorial mount. You can see that it looks more
complicated than the altazimuth mount. What makes it more complicated is that
it has an axis with an adjustable tilt. That adjustable axis is called the
polar axis. The
Celestron Omni XLT 150 Telescope
The equatorial mount shown with this telescope is popular because it simplifies the tracking of celestial objects. For any given location on the Earth, the polar axis can be adjusted to align with the Earth's rotational axis, thus properly compensating for Earth rotation at the observer's Latitude. Having this axis tipped to the proper angle necessitates the use of counter weights to keep the telescope in any given position.
Many equatorial mounted telescopes have an alignment telescope mounted within the polar axis, making alignment easy. The above image shows some of the parts of a typical equatorial mount, including the built in Polar alignment telescope. You merely look through the polar axis telescope and center Polaris (for North hemisphere observers) in the field of view, then lock down the adjustment.
Why tip one axis, you might ask? I could go into all the geometry, but it stands to reason that if the Earth's rotation about its spin axis is what makes stars move across the night sky, something aligned with the Earth's spin axis would provide a means of compensating for the motion of the Earth around that axis. A mount with one axis aligned with the Earth's spin axis is much easier to motorize. A single motor on the polar axis that rotates in the opposite direction of the Earth's spin at the Earth rotation rate (once per sidereal day) will do the trick. No computer is necessary, in that the motor rate is constant.
The R A in the diagram near the Polar Axis label stands for right ascension. If you look at a star chart, you will see a grid of lines that look much like the latitude and longitude lines on Earth maps. Star coordinates are mapped onto a two dimensional grid much like the grid used to signify Earth object coordinates. The star coordinates have different names, those being right ascension (similar to longitude) and declination (similar to latitude).
The star grid moves with respect to the Earth grid because of Earth's rotation with respect to the stars. In an evening you'll see the position of any particular star or pattern of stars move though the sky (at 15 degrees per hour as it happens). So while the star grid coordinates of a star are constant, the star grid itself rotates with respect to the Earth system.
The Fork Mount - Altazimuth Mode
This image illustrates the popular fork mount. Cassegrain telescopes often use this type of mount because of their short tube length. The fork telescope mount is particularly well suited for the shorter telescope designs.
In this configuration, the fork mount is sitting in the altazimuth mode, a configuration not useful for astronomical viewing, but handy for viewing daylight targets. Note that like the refractor and Dobsonian illustrations, the telescope shown can move around a vertical axis (azimuth) and a horizontal axis (elevation).
The telescope shown is my ETX 90M Meade telescope. It is an older model, and
only has a drive motor on one axis. Newer versions of fork mount made by Meade,
Celestron, and others have computerized mounts with motor drives in both axes,
and are most often used in the altazimuth configuration. The modern ETX 90,
for example, no longer uses just the single motor drive like my old model, but
is modernized into the Meade
Instruments ETX90 Observer
With these computerized instruments, the altazimuth mode is a fully functional star tracking configuration, with the computer adjusting the speed of the two motors to keep the telescope pointed at a particular object.
The Fork Mount - Equatorial Mode
This fork mounted telescope image shows my old ETX 90 model in the equatorial configuration. Note that what was the vertical axis is now tipped to the observer's latitude angle. With the tipped (Polar) axis aligned with the Earth's spin axis, the single motor drive of the telescope is sufficient for tracking targets.
The small black box with the red button that you see in the image is a
modification I added to the telescope to give a fast/slow motion slewing
control. By pushing the red button, part of the drive circuitry is bypassed
which speeds up the motor, providing a slow slewing motion. Pushing the black
button stops the motor, allowing Earth's rotation to catch up
.
The equatorial mode fork mount was common on older Cassegrains. It is still a good mode for even the newer ones for long exposure astrophotography.
Personal Notes
The telescopes with a computer on board to provide altazimuth tracking include an extensive database of thousands of objects. Before taking advantage of the computerized mount, you must put the telescope through an alignment sequence, then you can simply select objects from the database and the telescope automatically slews to the selections.
To get the most efficiency in observing sessions with my computer controlled telescope, I still make use of the Star Pointer web utility. Even though the telecope's computer already knows where the star targets are, I have to tell it which ones I want to observe. If I'm unprepared and happen to pick objects scattered around the sky, I'll spend a lot of time waiting as the motors whir as they move from target to target.
But the Star Pointer utility makes a list of all targets that are above the horizon, and arranges them in azimuth order. So if I select targets from the browser displayed list to enter into my computerized telescope, the telescope will have to travel very little to get from one target to the next. That saves a lot of time.
The down side of the two-motor, computer driven altazimuth mount is that the field of view through the eyepiece rotates as the telescope tracks. This isn't true for equatorial mounted telescopes. For viewing purposes this slow field rotation is hardly a problem. But if you intend to do long-exposure astrophotography, you need to have a motorized equatorial mount to facilitate tracking without field rotation.
The good news is that the two motor, computer
driven mounts can generally be operated in an equatorial mode as shown
in the Nexstar 5SE image above. In fact, the NexStar 5SE has a built in
wedge
as it is called, that allows use in equatorial mode. Some
other such telescopes don't have the wedge built in, but have it available
as an accessory.
Purists will also point out that if you start out with a computer driven telescope, you won't learn nearly as much about the night sky. There is something enjoyable about having the skill to find objects without the aid of a computer.
I've used both equatorial and altazimuth mounts. A couple of equatorial mounts were home made, and a couple were commercial. I can tell you that the home made ones were heavy and clumsy, and even at their hefty size were inadequate for the 8 inch and 10 inch telescopes I attempted to mount on them.
Most of the commercial ones are a bit flimsy also, but the one shown in these pictures (with the 6 inch f/5 Newtonian telescope) is actually quite smooth and sturdy. When I bought my Newtonian telescope from Discovery Telescopes, they admitted to me that the telescope and mount were actually imported (from China I suspect), but the optics were made by Discovery. I've had to do a few tweaks on the instrument and tripod mount to get the best performance, but in the end I'm very happy with the unit.
As to altazimuth mounts, most of the ones I've used are home made. I constructed a couple of Dobsonian telescope mounts and a couple of pipe fitting telescope mounts. In each case, these mounts performed admirably. I guess the point of the story is that altazimuth mounts are easier to make and use, supporting the fact that Dobs are the most often home constructed telescope. But if photography is your goal, then as some point you'll likely end up with an equatorial mounted telescope.
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