Terms of Telescopes
MECHANICAL TERMS AND CHARACTERISTICS
OF TELESCOPES
TELESCOPE MOUNTS (STABILITY)
To a large extent, a telescope is only as good as its stand and
mounting. A telescope magnifies everything, including vibration.
That's why many telescopes with decent optics are rendered useless
when supplied on a cheaply made mount. The mount's adjustments
should be smooth, yet precise, as you'll be using them to track
the slow and steady apparent movement of the stars.
A telescope mount has two functions- (1) provide a system for
smooth controlled movement to point and guide the instrument,
and (2) support the telescope firmly so that you can view and
photograph objects without having the image disturbed by movement.
There are two major types of mounts for astronomical telescopes:
Altazimuth
The simplest type of mount with two motions, altitude (up and
down/vertical) and azimuth (side-to-side/horizontal). These type
mounts are good for terrestrial observing and for scanning the
sky at lower power but are not for deep sky photography.
Equatorial
A telescope on an equatorial mount can be aimed at a celestial
object and easily guided either by manual slow-motion controls.
The equatorial mount is rotated on one axis (polar/right ascension)
adjusted to your latitude and that axis is aligned to make it
parallel to the Earth's axis, so that if that axis is turned at
the same rate of speed as the Earth, but in the opposite direction,
objects will appear to sit still when viewed through the telescope.
EYEPIECE (OCULAR)
An eyepiece is simply a precision magnifier. The eyepiece
used with a telescope magnifies the image that is formed by the
main optical system. There are many
different kinds of eyepiece designs for various
applications. The design and quality of the eyepiece can drastically
affect the overall performance of the telescope system.
ACCESSORIES
Barlow Lenses, SunDial, Planisphere, Eye pieces, solar filters
are the names of a few accessories in an enormous offering of
accessories for telescopes. As your interests expand, accessories
are available to help you achieve maximum satisfaction.
OPTICAL TERMS & CHARACTERISTICS
OF TELESCOPES
APERTURE (DIAMETER OF THE LENS OR MIRROR)
This is the single most important factor in choosing a telescope.
The prime function of all telescopes is to collect light. At
any given magnification, the larger the aperture, the better
the image will be.
The clear aperture of a telescope is the diameter of the objective
lens or primary mirror specified in either inches or millimeters
(mm). The larger the aperture, the more light it collects and
the brighter (and better) the image will be. Greater detail
and image clarity will be apparent as aperture increases.
Considering your budget and portability requirements, select
a telescope with as large an aperture as possible.
FOCAL LENGTH
This is the distance (in mm.), in an optical system, from the
lens (or primary mirror) to the point where the telescope is
in focus (focal point). The longer the focal length of the telescope,
generally the more power it has, the larger the image and the
smaller the field of view.
RESOLUTION
This is the ability of a telescope to render detail. The higher
the resolution, the finer the detail. The larger the aperture
of a telescope, the more resolution the instrument is capable
of, assuming the telescope optics are of high quality.
RESOLVING POWER
For telescopes this is referred to as "Dawes limit."
It is the ability to separate two closely-spaced binary (double)
stars into two distinct images measured in seconds of arc. Theoretically,
to determine the resolving power of a telescope divide the aperture
of the telescope (in inches) into 4.56.
LIGHT GATHERING POWER (LIGHT GRASP)
This is the telescope's theoretical ability to collect light
compared to your fully dilated eye. It is directly proportional
to the square of the aperture. You can calculate this by first
dividing the aperture of the telescope (in mm) by 7mm (dilated
eye for a young person) and then squaring this result.
EXIT PUPIL
The exit pupil of a telescope is the circular beam of light
that leaves the eyepiece being used and is measured in mm.
POWER (MAGNIFICATION)
One of the least important factors in purchasing a telescope
is the power. Power, or magnification, of a telescope is actually
a relationship between two independent optical systems (1) the
telescope itself, and (2) the eyepiece (ocular) you are using.
To determine power, divide the focal length of the telescope
(in mm) by the focal length of the eyepiece (in mm). By exchanging
an eyepiece of one focal length for another, you can increase
or decrease the power of the telescope. For example, a 25mm
eyepiece used on the ZOOOM 200 (1220mm) telescope would yield
a power of 49x and a 17mm eyepiece used on the same instrument
would yield a power of 72x. Since eyepieces are interchangeable,
a telescope can be used at a variety of powers for different
applications.
There are practical upper and lower limits of power for telescopes.
These are determined by the laws of optics and the nature of
the human eye.
Do not believe manufacturers who advertise a 375 or 750 power
telescope which is only 60mm in aperture (maximum power is 83x),
as this is false and misleading.
Most of your observing will be done with lower powers (6 to
25 times the aperture of the telescope [in inches]). With these
lower powers, the images will be much brighter and crisper,
providing more enjoyment and satisfaction with the wider fields
of view.
There is also a lower limit of power which is between 3 to 4
times the aperture of the telescope at night. During the day
the lower limit is about 8 to 10 times the aperture. Powers
lower than this are not useful with most telescopes and a dark
spot may appear in the center of the eyepiece in a Catadioptric
or Newtonian telescope due to the secondary or diagonal mirror's
shadow.
NEAR FOCUS
This is the nearest distance you can focus the telescope
visually or photographically for close terrestrial work.
FIELD OF VIEW
The amount of sky that you can view through a telescope
is called the real (true) field of view and is measured in degrees
of arc (angular field). The larger the field of view, the larger
the area of the sky you can see.
COLLIMATION
The proper alignment of the optical elements in a telescope.
Collimation is critical for achieving optimum results. Poor
collimation will result in optical aberrations and distorted
images. Not only is the alignment of the optical elements important
but even more important is the alignment of the optics with
the mechanical tube-this is called opto/mechanical alignment.
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