The Basic Mounting Types
Telescopes used for astronomy have two basic types of mounts. The simplest
one is called an altazimuth mount, and the more complex one is called the
equatorial mount. Each of these come with variations to allow for telescope
size and weight, and slow motion or motor control. I'll show here two of the
most common configurations.
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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.
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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 would keep and object motionless in the eyepiece.
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However, for any other place on the planet, you'll find that 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. It's something to keep in mind (a tutorial is
coming on that).
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.
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The Dobsonian Mount
As the pictures of the model Dobsonian illustrate, the Dobsonian mounted
telescope rotates around the base (azimuth axis), and up and down around the
elevation axis. It looks different, but has motions the same as the refractor
pictured above.
The advantage of the Dobsonian is it's simplicity and ability to handle
large telescopes. The model pictured above has about the look as typical
f/5.5 8" Dob would have. Note the diameter of the azimuth axis base. Having
Teflon pads near the periphery of the large base allows the Dobsonian mount
to handle large telescopes adequately.
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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 compensate for 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). Tipping this
axis to the proper angle necessitates the use of counter weights to keep the
telescope in 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. 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 rate rotation .
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).
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If you think about it, you'll realize that the star grid appears to move
with respect to the Earth grid, caused by Earth's rotation. 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 rotates with respect
to the Earth system.
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The Fork Mount - Altazimuth Mode
This image illustrates the popular fork mount. The Cassegrain
telescopes often use this type of mount. 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. But newer versions of fork mount
made by Meade, Celestron, and others have computerized mounts with motor
drives in both axes.
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With these 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.
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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 Polaris and thus 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 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.
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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.
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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" and 10" 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" f/5 Newtonian telescope) is actually quite smooth and
sturdy. I bought that 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 package to get the best out of it, but in the end I'm very happy with the
unit. At that time, Discovery sold the 6" f/5 and an 8" f/5 on the same mount.
I opted for the 6", assuming that if the mount could even remotely handle an
8", it should be great with the 6". It was a good choice.
I don't think Discovery sells that particular telescope anymore, but
Celestron and others do make pretty much the same model. I think you can find
something very similar at SHOP.COM  .
Celestron makes both 4.5" and 5" Newtonians now with fully computerized
mounts. I've seen them at telescopes
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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, supporting the fact that Dobs
are the most often home constructed telescope.
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