Types of Telescopes
This tutorial assumes that you know the difference between an equatorial and
an altitude-azimuth (or alt-az or altazimuth) telescope mount. If you don't,
check out the many articles that can be found on the Internet. To summarize, an
equatorial mount, when polar aligned, allows the telescope to follow the path of
the stars across the sky by moving in only one axis. An altitude-azimuth mount
is like a gun turret which requires movement in both axes to follow the stars.
Types of Computers
There are two basic types of telescope computers—Guiding and GoTo. The first
telescope computers for the amateur were simply digital versions of the
mechanical setting circles that show the orientation on the two axis (left/right
and up/down). These two positional indicators are called right ascension and
declination (for equatorial mounts) or azimuth and altitude (for
altitude-azimuth mounts). The early computers were called Digital Setting
Circles or DSCs as they duplicated the two mechanical indicators.
Add computer control and a database of objects in space and you have a
guiding computer. Guiding computers allow one to pick an object in the database
and have the computer guide the movement of the telescope to the correct
position where you can see the object through the scope. The computer detects
the current orientation of the telescope and indicates the movement necessary to
arrive at the correct position. This is accomplished by reading encoder data
from two encoders, one on each axis.
You must move the scope to the position indicated by the computer display. This
can be a number showing how far you are from the desired location and arrows
showing the direction to move or it can include a computer display (e.g. a
laptop computer) showing a map of the sky with a crosshair showing the current
location of the scope and the same basic information indicating the direction to
move the scope. By the way, these computers as still sometimes referred to as DSCs and they usually have a simple setting circles mode which displays the
position of the two axis.
The "go to" computer "goes to" the location you choose all by itself. The
difference between a guiding computer and a GoTo computer is the motors that
drive the telescope to the desired location.
If your computer "knows" its position and either guides you or moves itself
to the position you choose, it has encoders. Encoders are necessary for the
computer to get "feedback" as to its current position. The movements of the
telescope are encoded in digital form via voltage fluctuations and fed to the
computer, thus the term "encoder."
Related Sensing Equipment
In order for a computer to know which direction to move and the distance
necessary to reach the destination (object), it must first know its current
location. Giving it this information is called aligning the scope. Telescope
alignment on a guiding computer is usually done manually by the operator.
However, many of today's GoTo telescopes include sensing equipment to
automatically align the scope.
This equipment includes a Global Positioning System (GPS) to determine the
latitude and longitude or location on the Earth's surface. The next thing it
must know is how level the scope is. Internal leveling sensors give that
information. A compass tells the computer which direction the base of the
telescope is facing. Finally, the date and time must be known by the computer. When all this data is available, the computer will know where to point the scope
to find any object in the database.
Stand-along, PC Interface or Integrated
The original DSCs were a separate handheld box that connected to encoders
and included a display to show the position of each axis. The guiding computer
started out as a separate box also. Then a connection was added to interface
with a Personal Computer (PC) with planetarium software to show a map of the sky
and the position of the scope. The guiding computer became the interface between
the telescope and the PC. The main interface with the user is the PC keyboard,
mouse and display. The integrated computer is built in to the telescope
electronics and is controlled by the telescope's hand control unit.
||Type of Equipment
Stand-alone Guiding Computer
Stand-alone Guiding Computer or PC Interface*
(same as NGC-MAX)
SGT-MAX / BBOX
* Go-To capability is determined by the computer software and telescope drive
The JMI Team