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 (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.
- Easy to use and reliable due to the simplicity of
- 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
- 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
- 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
- Focal ratios are usually long (f/11 or slower)
making photography of deep sky objects more difficult.
- Some color aberration in achromatic designs
- Poor reputation due to low quality imported toy
telescopes; a reputation unjustified when dealing with a quality refractor
from a reputable manufacturer.
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.
- 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
- Good for deep sky astrophotography (but not as
convenient and more difficult to use than Catadioptrics).
- Low in optical aberrations and deliver very bright
- 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
- Slight light loss due to secondary (diagonal)
obstruction when compared with refractors.
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 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.
- Best all-around, all-purpose telescope design.
Combines the optical advantages of both lenses and mirrors while canceling
- Excellent optics with razor sharp images over a
- Excellent for deep sky observing or
astrophotography with fast films or CCD's.
- Very good for lunar, planetary and binary star
observing or photography.
- Excellent for terrestrial viewing or
- 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
- Closed tube design reduces image degrading air
- 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
- Best near focus capability of any type
- More expensive than Newtonians of equal
- It is not what people expect a telescope to look
- Slight light loss due to secondary mirror
obstruction compared to refractors.
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