Astronomer Book
actually quite small. The simplest mount is called an alt-az mount. The term
"alt-az" is short for altitude-azimuth and refers to the fact that the mount
moves from side to side (azimuth) and up and down in altitude. A standard
photographic tripod is an example of an alt-az mount.
The problem with a basic manual alt-az mount is that it does not naturally
follow the stars across the heavens. If you think about it, placing a small
telescope on a photographic tripod allows you to move the telescope
horizontally and vertically. The stars do not move horizontally parallel to the
horizon unless you happen to be observing from the North or South Pole.
If you watch the stars on a clear night, they move in circular arcs centred
on a stationary point in the sky known as a celestial pole. In the northern
hemisphere the stationary point is known as the North Celestial Pole (NCP)
and is located in the sky very close to the star Polaris. In the course of a day,
Polaris describes a very small circle around the NCP.
If you take an alt-az mount and tilt it so that the azimuth axis points
towards the NPC, a telescope fitted to the mount will, moving in what is now
tilted azimuth, follow the curved arcs the stars naturally follow across the
sky. This is the principle of the equatorial mount. If you add a motor drive on
the "tilted azimuth" axis which turns the axis at the same rate as the Earth
rotates but in the opposite direction, the mount is able to compensate for the
rotation of the Earth and anything in the eyepiece of an attached scope will
stay in view and not drift out of sight.
So to recap, so far we’ve described an undriven alt-az mount, a tilted
version of the alt-az mount known as an equatorial mount, and a driven
version of the equatorial mount known as a driven equatorial mount.
Putting a drive on the azimuth axist of an an alt-az mount isn’t that useful,
as the mount does not emulate the natural motion of the stars unless, as
stated, you happen to be observing from one of the Earth’s poles. However,[9] A small Newtonian telescope on a basic equatorial mount.
[10] Single-arm, computer-controlled mount with GOTO computer control.
[11] The Sun imaged through a 40mm Coronado PST hydrogen-alpha telescope.putting a drive on both the azimuth and altitude axes allows an alt-az
mounted scope to follow the stars if a computer is involved to emulate their
actual motion.
Modern computerized mounts can offer some impressive functionality but
it’s wise to treat them with caution for lower-priced telescope packages. If
you’re paying a set amount for a computerized telescope package, it’s useful
to keep in mind that the computerization comes at a cost and may detract
from the size and possibly the quality of the telescope that is supplied with it.
Go-To capability is offered on many computerized mounts. Here, as long
as the mount knows where it is in the world, what time it is, and the direction
in which it’s currently pointing, you can select the target you’re after from
a huge database of objects. The computer then does the rest, slewing the
telescope round so that it’s then pointing in the right direction. The term Go-
To describes the action of selecting your target and pressing a button to "go
to" it.
Although this may sound the perfect way for a beginner to find objects
in the night sky, there are issues. For example, letting the computer do
the work will not help you learn your way around the sky. Here, there’s no10911