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Types of Telescopes
There are three basic types of
telescopes -- 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.
Refractors
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.
- Colour 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 deep sky objects more difficult.
- Some colour 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.
Newtonian 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 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 degrade 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."
Catadioptrics
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.
Schmidt-Cassegrain
Advantages
- Best all-around,
all-purpose telescope design. Combines the optical advantages of both lenses
and mirrors while cancelling their disadvantages.
- Excellent optics with
razor sharp images over a wide field.
- Excellent for deep sky
observing or astrophotography with fast films or CCDs.
- 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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