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Introduction
traditionally, a system of lenses, mirrors, or both, used to gather
light from a distant object and form an image of it. Traditional
optical telescopes, which are the subject of this article, also
are used to magnify objects on earth and in astronomy; other types
of astronomical telescopes gather radio waves (see radio astronomy
), X rays (see X-ray astronomy ), or infrared or ultraviolet radiation.
Astronomy is a fascinating lifetime hobby
enjoyed by young children to centenarians, by people from all walks
of life and with varied interests.
You can observe or photograph the heavens on a casual or serious
basis, undertake scientific study or marvel at the wonderment of
our existence. Astronomy can be a fun and relaxing way to soothe
our minds and bodies from our hectic everyday life. It is a way
to enjoy nature, being outside and marveling at the night sky.
Astronomy is fun and easy to learn! You don't have to be a scholar
in physics or math to enjoy our universe. Besides binoculars or
a telescope you will need star maps or books listing the location
of various objects in the sky. Now even computerized telescopes
are available making it very easy to observe numerous objects in
an evening. Much useful information for all levels of interest is
available from amateur astronomy clubs, college astronomy professors,
libraries, planetariums, telescope dealers and other hobbyists.
A basic understanding of telescope and astronomical terminology
is useful and this knowledge briefly covers some of the items that
will be helpful to get you started.
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*** Types of Optical Telescopes
Astronomical telescopes come in a wide variety, ranging from the
plastic tube sold in toy stores to the famous Hubble telescope flying
in the sky. Here, we only focus on the most basic information necessary
for a astronomer of backyard or flat roof .
An astronomical telescope is used to collect light from a celestial
object and produce a magnified image.
In addition to these two common types, there are a number of other
designs which use a combination of lenses and mirrors to take advantages
of both refractors and reflectors, such as Schmidt-Cassegrain telescopes
and Maksutov telescopes.
There are three basic telescope designs that
pertain to the pursuits of amateur astronomers,classified according
to the element that gathers and focuses the incoming light.. They
are Refractors, Newtonian
Reflectors, and Catadioptrics.
All these designs have the
same purpose, to collect light and bring it to a point of focus
so it can be magnified and examined with an eyepiece, but each design
does it differently. All designs can perform satisfactorily if properly
and responsibly manufactured and all have their own special virtues.
Choosing a particular telescope depends on your individual needs
including cost, portability, versatility, usability, appearance,
etc. You should also contemplate what you plan to do with the instrument
both now and in the future. Many amateurs own two or more telescopes
to satisfy their varied interests.
Some amateur astronomers build their own telescopes but this market
has rapidly declined due to the abundance of affordable commercial
telescopes available and the time, materials and equipment needed
to hand-construct an instrument.
We will briefly discuss the most popular types of telescopes and
describe advantages and disadvantages of each.
1) Refractors
The refracting telescope (shown above) was invented in 1609 by Galileo
Galilei who used it to discover Jovian satellites, lunar mountains,
et al. Galileo's refracting telescope suffered from a number of
defects in image formation, particularly aberrations, due to the
use of simple lenses. Modern optical telescopes use compound objective
lenses to minimize aberrations. The compound lens consists of one
convex lens of crown glass and one concave lens of flint glass.
In addition, all lenses are coated with anti-reflective coatings
to maximize transmission. In common designs for amateur astronomy,
light from the objective lens is often turned 90 degrees by a flat
mirror before entering the eye piece.
Refractors provide the best image quality among amateur telescopes,
thanks in particular to the compound objective lens. Images of planets
and the Moon are extremely crispy and sharp. They are also used
for viewing objects on land, when an erecting eye piece is used.
In addition to their high performance, refractors are also maintenance-free
and very portable due to their light weight and compactness. Thus,
refractor has always been one of the most favorable among amateur
astronomers. The disadvantage of a refractor telescope is that it
is the most expensive in terms of dollar/aperture ratio. However,
thanks to the web, you can now directly buy from the factory at
very affordable prices.
Refractors (also known as dioptrics) are
what the average person identifies with the word "telescope", a
long, thin tube where light passes in a straight line from the front
objective lens directly to the eyepiece at the opposite end of the
tube.
Advantages
- Easy to use and reliable due to the
simplicity of design.
- Little or no maintenance.
- Excellent for lunar, planetary and binary
star observing especially in larger apertures.
- Good for distant terrestrial viewing.
- High contrast images with no secondary
mirror or diagonal obstruction.
- Color correction is good in achromatic
designs and excellent in apochromatic, fluorite, and ED designs.
- Sealed optical tube reduces image degrading
air currents and protects optics.
- Objective lens is permanently mounted
and aligned.
Disadvantages
- More expensive per inch of aperture
than Newtonians or Catadioptrics.
- Heavier, longer and bulkier than equivalent
aperture Newtonians and catadioptrics.
- The cost and bulk factors limit the
practical useful maximum size objective to small apertures
- Less suited for viewing small and faint
deep sky objects such as distant galaxies and nebulae because
of practical aperture limitations.
- Focal ratios are usually long (f/11
or slower) making photography of deepsky objects more difficult.
- Some color aberration in achromatic
designs (doublet).
- Poor reputation due to low quality imported
toy telescopes; a reputation unjustified when dealing with a quality
refractor from a reputable manufacturer.
2) Reflectors
The Newtonian Reflecting telescope (shown
above) was first designed by Isaac Newton in 1668. Here, a primary
parabolic mirror reflects light rays to an inclined flat mirror,
which, in turn, reflects the rays to an observer located at the
side of the tube. This design has essentially remained the same
today in most amateur reflectors.
Newtonians (also known as catoptrics) usually
use a concave parabolic primary mirror to collect and focus incoming
light onto a flat secondary (diagonal) mirror that in turn reflects
the image out of an opening at the side of the main tube and into
the
Newtonian Reflectors use a highly polished,
curved mirror at the rear of the telescope tube to collect light.
This mirror reflects and converges the light, where it is intercepted
by a small mirror at the top end of the telescope tube. In turn,
the light is reflected into a focuser and eyepiece mounted on the
side of the tube. As such, the observer is positioned at the top
of the telescope. They are generally regarded as the best all-around
telescopes, as their large apertures gather plenty of light, and
allow for good planetary views, all at a moderate price.
The most attractive feature of a reflector telescopes is that it
provides the lowest dollar/aperture ratio of any telescope type.
Therefore, for the same money, a reflector gathers most light, which
is important in deep space viewing. Small amateur reflectors, such
as a 3" (76 mm) or a 4.5" (114 mm) are also light enough for easy
transportation. Because of the open tube design, a reflector requires
alignment and cleaning once a while. The image from a reflector
is usually upside-down and is therefore not suitable for terrestrial
viewing.
The refractor is the design most people envision
when they think of a telescope. They are, generally, long, thin
telescopes that use a lens at the front of the telescope tube to
collect light. The observer looks through an eyepiece at the back
of the telescope, where the light is focused, forming an image.
Refractors are highly regarded for their sharp, high contrast images.
They are best suited for viewing the moon and planets.eyepiece.
Advantages
- Lowest cost per inch of aperture compared
to refractors and Catadioptrics since mirrors can be produced
at less cost than lenses in medium to large apertures.
- Reasonably compact and portable up to
focal lengths of 1000mm.
- Excellent for faint deep sky objects
such as remote galaxies, nebulae and star clusters due to the
generally fast focal ratios (f/4 to f/8).
- Reasonably good for lunar and planetary
work.
- Good for deep sky astrophotography (but
not as convenient and more difficult to use than Catadioptrics).
- Low in optical aberrations and deliver
very bright images.
Disadvantages
- Open optical tube design allows image-degrading
air currents and air contaminants, which over a period of time
will dergrade the mirror coatings and cause telescope performance
to suffer.
- More fragile than Refractors or Catadioptrics
and thus require more maintenance (such as collimation).
- Suffer from off-axis coma.
- Large apertures (over 8") are bulky,
heavy and tend to be expensive.
- Generally not suited for terrestrial
applications.
- Slight light loss due to secondary (diagonal)
obstruction when compared with refractors.
DOBSONIAN TELESCOPES
Most Newtonian Telescopes
have been supplied on equatorial mounts. The last few years have
seen a new commercial telescope available on the market - the Dobsonian.
A Dobsonian is a simple altazimuth mounted Newtonian telescope which
is excellent for beginners and in large sizes is an economical "Light
Bucket."
3) Catadioptric
Catadioptric telescopes use compound optical
systems. That is, both mirrors and lenses are employed to collect
and focus incoming light. The observer peers through an eyepiece
at the rear of the telescope tube, where the light is focused. The
two most commercially available catadioptric designs are Schmidt-Cassegrains
and Maksutov-Cassegrains. They offer excellent portability, as the
optical tubes are compact in design, as well as very good optical
quality. They are the most popular type of telescopes for astrophotography.
They tend to be more expensive than reflectors, and less expensive
than refractors of the same size.
Catadioptrics use a combination of mirrors
and lenses to fold the optics and form an image. There are two popular
designs: the Schmidt-Cassegrain and the Maksutov-Cassegrain. In
the Schmidt-Cassegrain the light enters through a thin aspheric
Schmidt correcting lens, then strikes the spherical primary mirror
and is reflected back up the tube and intercepted by a small secondary
mirror which reflects the light out an opening in the rear of the
instrument where the image is formed at the eyepiece. Catadioptrics
are the most popular type of instrument, with the most modern design,
marketed throughout the world in 3 1/2" and larger apertures.
The Schmidt camera telescope, invented in 1930 by Bernard Schmidt,
is a catadioptric system used for wide-angle photography of star
fields. The primary mirror is spherical instead of paraboloidal,
which requires that a special correcting lens be used on the front
of the tube. The Maksutov telescope, invented by D. D. Maksutov
in 1941, is similar in design and purpose to the Schmidt telescope
but has a spherical meniscus in place of the correcting plate of
the Schmidt.
Schmidt-Cassegrain
Advantages
- Best all-around, all-purpose telescope
design. Combines the optical advantages of both lenses and mirrors
while canceling their disadvantages.
- Excellent optics with razor sharp images
over a wide field.
- Excellent for deep sky observing or
astrophotography with fast films or CCD.
- Very good for lunar, planetary and binary
star observing or photography.
- Excellent for terrestrial viewing or
photography.
- Focal ratio generally around f/10. Useful
for all types of photography. Avoid faster f/ratio telescopes
(they yield lower contrast and increase aberrations). For faster
astrophotography, use a Reducer/Corrector lens.
- Closed tube design reduces image degrading
air currents.
- Most are extremely compact and portable.
- Easy to use.
- Durable and virtually maintenance free.
- Large apertures at reasonable prices
and less expensive than equivalent aperture refractors.
- Most versatile type of telescope.
- More accessories available than with
other types of telescopes.
- Best near focus capability of any type
telescope.
Schmidt-Cassegrain
Disadvantages
- More expensive than Newtonians of equal
aperture.
- It is not what people expect a telescope
to look like.
- Slight light loss due to secondary mirror
obstruction compared to refractors.
Maksutov-Cassegrain
The Maksutov design
is a catadioptric (using both mirrors and lens) design with basically
the same advantages and disadvantages as the Schmidt. It uses a
thick meniscus correcting lens with a strong curvature and a secondary
mirror that is usually an aluminized spot on the corrector. The
Maksutov secondary mirror is typically smaller than the Schmidt's
giving it slightly better resolution for planetary observing.
The Maksutov is heavier than the Schmidt and because of the thick
correcting lens takes a long time to reach thermal stability at
night in larger apertures (over 90mm).
The Maksutov optical design typically is easier to make but requires
more material for the corrector lens than the Schmidt-Cassegrain.
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*** Adhere to the American Eyepiece Size Standard
There are two size standards associated with
telescope eyepieces: Japanese and American. Eyepieces built to the
Japanese standard have a barrel diameter of 0.965". Those built
to the American standard have a barrel diameter of 1.25". Generally
speaking, inexpensive beginners' telescopes are usually outfitted
with Japanese standard eyepieces. As such, telescopes in this category
have 0.965" focusers, or eyepiece holders. Much more desirable are
telescopes that are designed to accept American standard eyepieces.
These scopes are generally built to better standards, and are able
to utilize much better quality eyepieces.
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*** The Optical Tripod
Like the tripod that it stands on, a telescopes performance
balances on three legs:
1) Magnification (or Power, how close it makes
things appear)
Ignoring a single leg of the tripod at the expense of the others
erodes a telescopes optical quality and your enjoyment of it. A
high magnification is useless if the image is dim or fuzzy.
The eyepiece is the part of the telescope that you look into. It
is also the part of the telescope that controls the magnification
or power.
The magnification, or power, of the telescope is relevant only when
an eyepiece, or ocular, is used to magnify the image for visual
inspection. The angular size of the virtual image seen by the observer
will be larger than the actual angular size of the object. The ratio
of these two sizes is the magnifying power and is equal to the ratio
of the focal lengths of the objective and ocular. Any desired magnification
can be obtained with a given telescope by the use of an appropriate
ocular, but beyond a point determined by the resolving power, higher
magnification will reveal no further details.
In addition to diffraction, other defects limit the performance
of real optical systems. The most serious of these for lenses is
chromatic aberration . Other defects include coma, astigmatism,
distortion, and curvature of field. In general, it is easier to
eliminate these faults in the reflector than in the refractor.
What you can see is dependent on a lot of
factors. The most important of these for astronomy is aperture.
Other important factors are optical quality, steadiness of your
tripod and mount, seeing conditions, your location (city or rural),
brightness of the object and your experience. You won't be able
to see the American flag on the surface of the moon or black holes.
You won't see as much color as you see in astrophotographs (photos
of celestial objects) because these utilize long exposure times
which allow the light and color to build up on the film.
Most telescopes can be used to see things on the Earth. You can
use them for long distance terrestrial viewing, nature study, sports
action, surveillance or general land usage. You can also easily
photograph terrestrial objects since a telescope can be used as
a long telephoto lens by attaching the body only of a 35mm SLR camera.
Our T-Ring and T-Adapter accessories are also required.
On our Terrestrial telescopes the eyepiece is usually not removable.
It has been specially selected to provide an optimum range of powers
such as 30-90 and will also ensure that the image is shown the right
way round. Astronomical telescopes show the image upside down and
back to front, by inserting an erecting eyepiece?(supplied with
the telescope) the image is slipped the right way round for when
you wish to look at a terrestrial object such as a distant building.
Our Astronomical telescopes are provided with several eyepieces
that will give a wide range of powers. They are also supplied with
a special lens called a “Barlow Tube?which is inserted between the
telescope and the eyepiece to double or even triple the power of
the eyepiece.
Each eyepiece is referred to by the focal length of the eyepiece
in millimetres eg 20mm, 12.5mm, 4mm.
To determine the effective magnification that each lens produces
divide the focal length of the telescope itself by the size of the
eyepiece. Thus smaller eyepieces will produce higher magnifications.
e.g.a reflector telescope has a focal length of 900mm, so using
a 4mm eyepiece will produce a magnification of 225x (900 ¡Â 4) or
225 times closer than it appears to your naked eye.
2) Resolution (sharpness and clarity)
The resolution of the telescope is a measure of how sharply defined
the details of the image can be. The laws of diffraction make a
certain amount of blurring unavoidable, because of the wave nature
of light. If two stars are very close, a given telescope may not
be able to separate them into two distinct points. The smallest
angular separation that can be unambiguously distinguished is called
the resolving power of the telescope and is proportional to the
ratio of the wavelength of light being observed to the diameter
of the telescope. Thus, the larger the diameter, the smaller the
minimum angular separation and the higher the resolving power.
High power has its drawbacks however and two areas that are affected
are Resolution and Field of View. When high powers are used the
view becomes less clear. The clarity of the view is known as resolution.
So the higher the power, the greater the loss of resolution. This
is an inherent factor in optics. Also the field of view or the area
that you can see from edge to edge in your eyepiece decreases as
the magnification increases. In essence you see smaller and smaller
areas.
Extremely high powers are best used for detailed viewing on brighter
objects such as the moon, major planets and double stars. With practice
and experimentation you will learn which objects are best viewed
through which eyepiece and when is the best time to view.
This area is also covered in our Frequently Asked Questions section
(see table of contents).
3) Brightness (called light transmission)
Brightness is important for astronomical use because you are looking
at such faint objects. The main factor influencing the brightness
of the view that you see is the diameter of the telescope. A larger
diameter has more light passing down the barrel of the telescope
which is why reflector telescopes are generally preferred for night
use.
Today's telescopes have something that early astronomers never had
- the coated lens. Coatings in use today actually increase the amount
of light that passes through them by reducing the amount of light
that bounces off he surface of the lens. The result is brighter
clearer views than ever before.
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*** What you can see with a telescope
Some of the types of celestial objects
you can view are:
THE MOON--Prepare for an awesome spectacle. The
moon's disk has a pastel-cream and gray background, streamers of
material from impact craters stretch halfway across the lunar surface,
river-like rilles wind for hundreds of miles, numerous mountain
ranges and craters are available for inspection. At low or high
power the moon is continually changing as it goes through its phases.
Occasionally you will be treated to a lunar eclipse.
THE SUN
-- It is quite safe to view the Sun
if you utilize a proper solar filter. The Sun is fascinating to
inspect as you detect and watch the ever-changing sunspot activity.
If you are fortunate enough, and are willing to travel to remote
locations, you may at some point experience a solar eclipse.
THE PLANETS -- Observation of planets will keep you very busy. You can see
Jupiter with its great red spot change hourly, study the cloud bands
and watch its moons shuttle back and forth. Study Saturn and its
splendid ring structure, watch Venus and Mercury as they go through
their moon-like phases. Observe Mars and see its polar cap changes
or watch the dust storms and deserts bloom with life. Uranus, Neptune
and Pluto can be seen easily with 8" or larger telescopes.
STAR CLUSTERS
-- There are two types of star clusters-
(1) open star clusters (also called galactic clusters) which are
loosely arranged groups of stars, occasionally not too distinctive
from the background stars, and (2) globular star clusters which
are tightly packed groups of many millions of stars.
NEBULAE
-- These are glowing clouds of gas
falling into two types- (1) planetary nebulae which are relatively
small ball-shaped clouds of expanding gases and are believed to
be the remnants of stellar explosions, and (2) diffuse nebulae which
are vast, irregularly-shaped clouds of gas and dust.
GALAXIES-- These are vast, remote "island universes," each composed of many billions
of stars. Galaxies exist in a variety of sizes with regular and
irregular shapes.
COMETS
-- Magnificent comets are routinely
visible through telescopes.
DOUBLE (BINARY) STARS -- These are pairs of stars orbiting around
a common center of gravity, often of different and contrasting colors.
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*** Images Produced by Optical Telescopes
The properties of the image produced by a telescope are similar,
whether formed by lenses or mirrors. The real image produced is
inverted; i.e., top and bottom are reversed, as are left and right.
In a terrestrial refracting telescope used to view objects on the
earth, an additional lens is used to invert the image a second time,
so that objects appear as they do when viewed with the unaided eye;
in an astronomical telescope, image inversion is unimportant and
no lens is added to invert the image a second time. The angular
size of an object as seen from the position of the telescope may
be expressed in degrees or in radians (1 radian equals about 57¡ã).
The angle in radians determined by the object is given by the ratio
of the object's diameter to its distance from the telescope. The
size of the object's image is the product of this and the focal
length of the image-forming lens or mirror. For example, the angular
size of the moon's diameter is about 12 ¡ã, or roughly 1100 radian;
a telescope with a focal length of 60 in. (152 cm) would produce
an image of the moon 0.6 in. (1.52 cm) in diameter. The brightness
of the image depends on the total light gathered and hence is proportional
to the area of the objective or the square of the diameter of the
telescope.
Astrophotography is also a rich and rewarding
experience. With many telescopes it is relatively easy, but takes
patience and experience to produce excellent results. Taking your
own astrophotographs is a thrill as you can share the results with
others.
CCD IMAGING -- The last few years have brought to the amateur astronomer a large assortment
of CCD (Charge Coupled Device) cameras. Electronic imaging opens
up a whole new vista for amateur astronomers who can obtain images
quickly and from urban locations.
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*** General Information
Seeking Conditions
"Seeing" is the term astronomers use to
describe the sky's atmospheric conditions. The atmosphere is in
continual motion with changing temperatures, air currents, weather
fronts and dust particles. These factors cause the star images to
twinkle. If the stars are twinkling considerably we have "poor"
seeing conditions and when the star images are steady we have "good"
seeing conditions. Poor seeing is most noticeable when observing
planets and the moon, whereas deep sky objects such as nebulae and
galaxies are less affected by poor seeing conditions. On deep sky
objects, the most important factor is the transparency of the atmosphere
(a measure of how dark the sky is on a given night-determined by
clouds, dust, haze and light pollution). Seeing conditions and transparency
will vary widely from site to site, from season to season and from
night to night.
Some manufacturers of small aperture telescopes would like you to
believe that they can routinely outperform larger aperture telescopes
because of atmospheric turbulence (poor seeing conditions). Occasionally
this may be true on planets and the moon (you can stop down the
larger aperture simply with cut-out masks to alleviate this problem),
but it is never true on deep sky objects (nebulae, galaxies and
star clusters) where maximum aperture is needed.
Portability
This is a very important
factor in choosing a telescope. If you live in a city polluted with
lights you may want to transport your telescope to a dark sky location.
If you livein a dark sky location you may have to take the equipment
out and set it up. Consider the overall weight and bulk that you
will be working with. If you are fortunate enough to have a telescope
permanently mounted (or set up), then you should consider the largest
aperture telescope you can afford (albeit still considering which
type of telescope design fits your needs).
Versatility
Look for a telescope
that can grow along with you as your experience and interest expand.
Make sure the manufacturer has a complete line of accessories so
that your telescope and your fun are not limited by lack of equipment.
Most manufacturers offer accessories that may be added on at a later
time.
If you want maximum versatility, consider that some telescopes are
multipurpose for the following- (1) terrestrial viewing, (2) terrestrial
photography with the attachment of a 35mm SLR camera, (3) astronomical
observing and (4) astronomical photography (astrophotography).
Quality
Most manufacturers are
reputable and make good quality products. However, even with the
same optical design and same type of mount there are distinct differences
between similar units. You need to inspect the units and rely on
the advice of telescope dealers, educators, members of astronomy
clubs or professional astronomers.
Another very important point is the after-purchase service. Does
the manufacturer have a technically competent staff to answer your
questions? Can you later purchase an assortment of accessories to
fulfill your expanding interest? If you have equipment problems,
can you get them repaired promptly?
Also consider the type and length of the product warranty.
The Celestial-Coordinate system
The celestial coordinate
system is an imaginary projection of the Earth's geographical coordinate
system onto the celestial sphere which seems to turn overhead at
night. This celestial grid is complete with equator, latitudes,
longitudes and poles.
The Earth is in constant motion as it rotates on its axis. Actually
the celestial-coordinate system is being displaced very slowly with
respect to the stars. This is called precession and is caused by
gravitational influences from the Sun, moon and other celestial
bodies.
The celestial equator is a full 360-degree circle bisecting the
celestial sphere into the northern celestial hemisphere and the
southern celestial hemisphere. Like the Earth's equator, it is the
prime parallel of latitude and is designated 0 degree.
The celestial parallels of latitude are called "coordinates of declination
(Dec.)," and like the Earth's latitudes they are named for their
angular distances from the equator. These distances are measured
in degrees, minutes and seconds of arc. There are 60 minutes of
arc in each degree, and 60 seconds of arc in each arc minute. Declinations
north of the celestial equator are "+" and declinations south are
"-". The North Pole is +90 degrees and the South Pole is -- 90 degrees.
The celestial meridians of longitude are called "coordinates of
right ascension (R.A.)", and like the Earth's longitude meridians
they extend from pole to pole. There are 24 major R.A. coordinates,
evenly spaced around the 360?equator, one every 15 degrees. Like
the Earth's longitudes, R.A. coordinates are a measure of time as
well as angular distance. We speak of the Earth's major longitude
meridians as being separated by one hour of time because the Earth
rotates once every 24 hours (one hour = 15 degrees). The same principle
applies to celestial longitudes since the celestial sphere appears
to rotate once every 24 hours. Right ascension hours are also divided
into minutes of arc and seconds of arc, with each hour having 60
minutes of arc and each arc minute being divided into 60 arc seconds.
Astronomers prefer the time designation for R.A. coordinates even
though the coordinates denote locations on the celestial sphere,
because this makes it easier to tell how long it will be before
a particular star will cross a particular north-south line in the
sky. So, R.A. coordinates are marked off in units of time eastward,
from an arbitrary point on the celestial equator in the constellation
Pisces. The prime R.A. coordinate which passes through this point
is designated "0 hours 0 minutes 0 seconds." We call this reference
point the vernal equinox where it crosses the celestial equator.
All other coordinates are names for the number of hours, minutes
and seconds that they lag behind this coordinate after it passes
overhead moving westward.
Given the celestial coordinate system, it now becomes possible to
find celestial objects by translating their celestial coordinates
using telescope-pointing positions. For this you use setting circles
for R.A. and Dec. to find celestial coordinates for stellar objects
which are given in star charts and reference books.
Conclusion
We hope this discussion
helps in understanding telescopes and astronomy in general and that
you will begin the lifetime enjoyment of our fascinating universe.
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*** Mounting the Telescopes
Equal in importance to the mirrors and lenses constituting
the optics of a telescope is the mounting of the telescope. The
mounting must be massive, in order to minimize mechanical vibration
that would blur the image, especially at high magnification or during
long-exposure photography. At the same time, motion of the telescope
must be precise and smooth. To allow the telescope to be pointed
in any direction in the sky, the mounting must provide rotation
about two perpendicular axes. In the altazimuth mounting, one axis
points to the zenith and allows rotation along the horizon and the
other allows changes in altitude, or distance above the horizon.
This mounting is used for small terrestrial telescopes and, since
the 1970s, most new astronomical telescopes use altazimuth mountings
that are computer-driven in both axes. Before the 1970s, most astronomical
telescopes used the equatorial mounting, in which one axis points
at the celestial pole and hence is parallel to the earth's axis.
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