PART III: TELESCOPE MOUNTS
TYPES OF TELESCOPE MOUNTS
There are two basic types of mounts¡ªthe Alt-Az mount (moves in the vertical/altitude
direction, and the horizontal/azimuth direction) and the Equatorial mount
(has two axes of movement, one parallel to the Earth¡¯s axis, and one perpendicular
to it, resulting in N-S and E-W movement relative to the celestial sphere).
The EQ (equatorial) mount essentially has an axis parallel to the Earth¡¯s,
resulting in the movement around this axis being a true E-W movement.
An EQ mount can have this axis adjusted to that of the Earth¡¯s by means
of simple screws that allow one to adjust the angle of pointing. For visual
use, it is only necessary to point this axis at Polaris. Even though Polaris
isn¡¯t EXACTLY at the point the sky rotates around, it is close enough.
For photographic work, greater
accuracy can be had by using one of several techniques.
The primary advantage of this mount is that, because it turns on exactly
the same axis as the Earth, it can track an object in the sky with a motion
on only one axis (the E-W axis, known as the Polar Axis). Different objects
can be selected by moving the telescope on the other axis (the N-S axis,
known as the Declination Axis). This mount is also the easiest to use
photographically because of its simplicity of motion in tracking the object
as the Earth turns. It is the easiest mount on which to attach a tracking
motor that allows the telescope to turn at exactly the same rate the Earth
turns so the object stays centered in the field of view while you observe.
An example of an Equatorial mount.
The Alt-Az mount, on the other hand, is the most intuitive to use. Up-Down
and Left-Right are very easy motions to adapt to, and this makes pointing
somewhat easier for a lot of individuals. Objects do not rise straight
from the horizon, however, so motion in both axes is necessary to track
an object. Everyone is familiar with the most basic of this type of mount¡ªthe
photographer¡¯s tripod. This mount can also be used for binoculars and
small telescopes. Many variants of this mount exist, from
the simplest photo-tripod all the way up to a very-expensive mount with
a pier and high-end bearings on each axis.
An example of the most common type of Alt-Az scope¡ªthe Dobsonian
There is a modern variant of the Alt-Az mount, called the Computerized
Mount. After initial alignment of its on-board computer with some target
mount moves in two axes simultaneously, and tracks the objects found.
Further, on-board computer programs allow this type of mount to be used
to find objects in the sky. What a huge improvement for the astronomer-a
simplicity of motion combined with computer-aided finding of objects.
There is no doubt that because of these mounts more astronomers have seen
more objects than ever in the past. The computerized scope, with its computer-aided
alt-az mount, has revolutionized astronomy for the backyard stargazer.
It¡¯s a one-way street, for sure¡ªonce you¡¯ve experienced the ease and precision
of a computerized scope, you¡¯ll never want to go back to the simpler techniques
of finding objects without one.
An example of a computerized mount, carrying a Schmidt-Cassegrain catadioptric