The mount refers to both the legs and the
mechanism that holds and points the telescope. The mount is critical
for how you use and enjoy your telescope. In the old days mounts were
just mechnical devices, but in these modern computerized times, mounts
are becoming more intelligent. Mounts have computer guidance systems
with built-in astronomical databases. There are many ways to mount and
use a telescope, and thus many things to consider when choosing a
telescope mount.
Altazimuth Mounts
If
all you want to do is look at the stars and planets then any mount that
holds up your telescope and allows for easy aiming, steady viewing and
comfortable use is a good mount. Dobsonian telescopes
are a well known design with a mount configured for easy viewing. The
Dobsonian is an example of the altazimuth mount combined with a
Newtonian telescope optical tube assembly (OTA). An altazimuth mount is
a design to hold the OTA and allow the user to guide the scope by using
two motions. The up and down motion is call altitude, and the back and
forth or round and round motion is call azimuth.
Between these two
motions you can point a telescope to view any position above the
horizon. It's a simple system and easy to use and lends itself well to
casual stargazing.
Equatorial Mounts
Because
the earth spins, the stars and planets move in the heavens above us.
Tracking a star while viewing it requires constant repositioning of the
telescope to compensate for the motion of the earth. A second type of
mount called the equatorial mount was design to make tracking match the
apparent motion of the stars. The gimmick of this mount is the
telescope is aligned along an axis that runs parallel to the earth's
spin.
To understand and use the equatorial mount requires a good
deal more skill and conceptualization. This knowledge is not required
to enjoy using a telescope, or even to use an equatorial telescope
because such scopes can be used for casual viewing without mastering
all the skills first. The technicque of properly setting up an
equatorial mount is called polar alignment. Once configured the
equatorial mounted telescope can be aimed with their setting circles
using an astronomical coordinate system. Looking at the picture on the
right try and imagine these concepts:
The tripod is
positioned so it points along a line with celestrial north/south. The
tripod can be placed pointing anywhere in a 360 degree circle, but you
want the front of the mount aiming north/south. A polar alignment scope
helps.
The first joint above the tripod allows adjusting the
telescope to be tilted back the same number of degrees as the viewer's
latitude and will bring Polaris(north), the pole star into view in the
polar finder scope. There are different kinds of polar alignment and
various methods to perfect it including the drift alignment and
iterative method. The goal is to get the scope polar alligned and then
locked down for the night.
The next joint up allows for motion
along the right ascension (RA) axis and allows the telescope to follow
a star across the sky. If you have ever seen photos of stars making
circles in the sky that should help you visualize trying to follow one
star as it arcs across the sky. A motor along this axis will track a
star. Visualize the celestrial globe turning above, this axis follows
the spin of the globe.
The final joint, the one next to the
optical tube assembly (OTA) is call declination and allows the
telescope to move up and down the spinning celestrial globe to position
the scope at the star's altitude. A motor at this axis helps position
the scope.
Motors along both right ascension and declination will
allow a computer to position the scope and track a star. If the
computer has a database of steller objects, you can request an object
and the mount will look up the object's position and move the telescope
to show that object. The newest GOTO telescopes can do all of this
without equatorial mounts and polar alignment because the computer can
be programmed to know the position of the scope by using guide stars.
If
you like learning how things work then buying an equatorial scope and
using it properly will force you to learn the concepts of celestrial
coordinate systems. If you don't care, then consider buying a GOTO
scope. Optional GPS units also available nowdays.
Equatorial
telescopes come with setting circles which allow the user to manually
position the telescope by using a coordinate system. It's possible to
position the scope using the angular markings on the circles, and then
track a target by manually turning the RA knob. Motors on one axis or
both axis will allow you to observe without readjusting the scope. This
is important.
