The Basic Astronomical Telescope Mounting Designs
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
telescope if you want to view wide star fields, or need a portable instrument,
or both. You might choose a long telescope if you want to look at high
resolution targets like planets, double stars, and the moon, through
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. You can check out the Refractor Tutorial for
more information about refractor telescopes.
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.
Tutorial provides more information about Newtonian telescopes.
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. You can check out
the Cassegrain Tutorial to see how this trick is done.
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
will also show you where to point your altazimuth or equatorial mounted
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
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
SmartStar-E 8500G Computerized AltAz Telescope Mount (Terra Green) for example. And you could always mount something
like that to a pipe-fitting base for stability.
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 is a mount of this type -- relatively inexpensive
and easy to use.
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
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 have entered the hobby in a big way, particularly
because of their convenience and their surprising affordability.
Schrodingers Cat Small Poster
The Dobsonian Mount
If you examine a Dobsonian telescope mount, you will see that it 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
10015 StarBlast 4.5 Astro Reflector Telescope, or husky enough to easily handle something like the
8945 SkyQuest XT8 Classic Dobsonian Telescope. In fact, Dobsonian telescope models up to 15 inch
are commercially available, and ATM designs handle up to 30 inch
The image at left 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 Dobsonian
Mount. For amazingly low prices, you can buy Dobsonian telescopes ready to
use, like the Orion
8944 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. I purchased the telescope shown from Discovery
Telescopes. They no longer sell that model, but the Celestron
Omni XLT 150mm Telescope Newtonian Reflector 31057, much like my purchase, is still
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
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
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
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
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 model. You can still see what looks like a fork
mount on the new ETX 90, but it operates in the altazimuth orientation.
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
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
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.
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
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.