Telescopes used for astronomy, whether Newtonian, Dobsonian, Cassegrain, or
Refractor, have two basic types of mounts. The simplest one is called an
Altazimuth mount, and the more complex one is called the equatorial mount.
The
Altazimuth mount is certainly simplest, allowing the telescope to be pointed up
and down, and around (azimuth). The various equatorial designs are made to
facilitate easier tracking of celestial objects. 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.
It is common today for modern telescopes to include not
only clock drive mounts, but computerized clock drives that allow the user
to simply select objects via computer or hand-held controller. The telescope
computer and drive then do the work and locate objects for you.
The Altazimuth Mount
Pictured here is a simple configuration of the classic Altazimuth mount.
This 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.
As shown with the small refractor in this picture (a 50mm telescope), such a
mount often doesn't even need a counter weight. If you happen to be observing
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 in 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 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 straight up requires
mostly changes in azimuth.
This 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 and a bit of valve grinding
compound for about $50. A description of such a mount is at Inexpensive Tripod.
If you examine a Dobsonian 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.
Mouse Over this image to see 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. Dobsonian telescopes up to 15 inches in aperture
are available at surprisingly affordable prices due to this simple design.
Slight nudges are all that's needed to move the telescope around the azimuth
axis.
Elevation
Mouse Over this image to see an illustration of changes in
elevation. The elevation axis bearings usually sit on a couple of
Teflon pads, again making for a simple, stable, and smooth bearings. The larger
the telescope, the larger in diameter the larger 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 are it's simplicity and ability to handle
large telescopes. The model pictured above has about the look a typical f/5.5
8 inch Dob would have. If you can us a saw, you can easily make a fine
Dobsonian mount to complete any reflector telescope project. Check out the
plans at Making A
Dobsonian Mount.
The Equatorial Mount
Shown at the left 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 adjustable axis called the polar axis. For any given location on the
Earth, the polar axis is adjusted to align with the Earth's rotational axis,
thus properly compensating for Earth rotation at the observer's Latitude. With
this adjustment, the polar axis can be aligned with the Earth's spin axis,
regardless of observer location (unlike the Altazimuth mount). Having this axis
tipped to the proper angle necessitates the use of counter weights to keep the
telescope in any given position.
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 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.
One simply puts a single motor on the polar axis to rotate in the opposite
direction of the Earth's spin at the Earth rotation rate (once per sidereal
day).
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).
If you think about it, you'll realize that the star grid appears to move
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 grid coordinates of a star are essentially constant, the star grid itself
appears to rotate 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. It is
particularly well suited for the shorter telescope designs.
In this configuration, the fork mount is sitting in the Altazimuth mode.
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 clock drive on one axis. Newer versions of fork mount
made by Meade, Celestron, and others have computerized mounts with motor
drives in both axes.
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 is in an 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. For simple
observing, aligning with Polaris is quite sufficient.
The small black box with the red button is a modification I added to the
telescope to give a fast/slow motion slewing control.
The equatorial mode was common for the 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 calculate drive rate
include an extensive almanac database of thousands of
objects. With these instruments, the user must put the telescope through an
alignment sequence, then he or she can simply select objects from the database
and the telescope automatically slews to the object. As a long time user
of telescopes before such a modern convenience, I can tell you the
computerized instruments can save a lot of time. You can spend much more
time observing, and a lot less time hunting for elusive targets.
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 an equatorial mounted telescope. For viewing purposes this is hardly
a problem. If you intend to do long-exposure astrophotography, however, you
need to have an equatorial mount. The good news is that the two motor,
computer driven mounts can generally be ran in an equatorial mode.
Purists will also point out that if you start out and continue to use
computer driven telescopes, 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 equatorials
were home made, and the others 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 were flimsy also, but the one shown in these
pictures (with the 6 inch f/5 Newtonian telescope) is actually quite smooth and
sturdy. 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 mount to get the best performance, but in the end I'm
very happy with the unit. At the time I purchased, Discovery sold the 6 inch
f/5 and an 8 inch f/5 with the same mount. I opted for the 6 inch, assuming
that if the mount could even remotely handle an 8 inch, it should be great with
the 6 inch. It was a good choice.
I don't think Discovery sells that particular telescope anymore, but other
vendors do have much the same model. Use the astro-customized search engine to
shop for an EQ Newtonian.
Custom Search
As to Altazimuth mounts, most of the ones I had were home made. I
constructed a couple of Dobsonians and a couple of pipe fitting 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.
