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05
TELESCOPE BASICS
C14 • Horsehead Nebula • Image courtesy of Richard Hedrick
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TELESCOPE TYPES
Newtonian Reflector Telescope
A Newtonian reflector uses a single
concave mirror as its primary. Light
enters the tube traveling to the
mirror at the back end. Light is then
“bent” forward in the tube to a
single point, its focal plane. A flat
mirror called a “diagonal” intercepts
the light and points it out the side of
the tube at right angles to the tube
through the eyepiece. The eyepiece is placed there for easy viewing.
Newtonian Reflector telescopes replace heavy lenses with mirrors to collect and
focus the light, providing much more light gathering power for the money. You can
have focal lengths up to 1000mm and still enjoy a telescope that is relatively
compact and portable. Newtonian Reflector telescopes do require more care and
maintenance because the primary mirror is exposed to air and dust. However, this
small drawback does not hamper this type of telescope’s popularity with those
who want an economical telescope that can still resolve faint, distant objects.
Newtonian reflectors produce a “right-side-up image” but the image will appear
rotated based on the location of the eyepiece holder in relation to the ground.
Newtonian reflectors are best for astronomical use where right-side-up does
not matter.
Newtonian 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)
• Adequate for lunar and planetary work
• Good for deep sky astrophotography (but not as convenient and more difficult to
use than Catadioptrics)
• Free of color aberration due to the use of a primary mirror
Newtonian Disadvantages
• Generally not suited for terrestrial applications
• Slight light loss due to secondary (diagonal) obstruction when compared
with Refractors
Catadioptric Telescopes
Catadioptrics use a combination of mirrors and lenses to “fold” (reflect) the light
path and form an image. There are two popular designs: the Schmidt-Cassegrain
and the Maksutov-Cassegrain. In a Schmidt-Cassegrain, the light enters through
a thin aspheric Schmidt correcting lens. It then strikes the spherical primary mirror.
It 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 and most modern type
of telescope optical design and are marketed throughout the world in 3.5” and
larger apertures.
Catadioptric telescopes combine the practical advantages of lenses and mirrors
while eliminating their disadvantages. They offer the clarity and contrast of
refractors with the low aberration of reflectors. Catadioptrics have an average focal
ratio of f/10 which is wide enough for all types of photography. They are also
easier to maintain because all optical elements are solidly mounted and rigidly
collimated. Catadioptric telescopes provide the best possible combination of light
gathering power, long focal length, portability and affordability.
Schmidt-Cassegrain Advantages
• Best all-purpose
telescope design
• Combines the
optical advantages
of both lenses and
mirrors while
eliminating their
disadvantages
• Excellent optics and
razor sharp images over a wide field
• Excellent for deep sky observing and astrophotography
• Very good for lunar, planetary and binary star observing
• Excellent for terrestrial viewing and photography
• Focal ratio generally around f/10
• Closed tube design reduces image-degrading air currents
• Extremely compact and portable
• Easy to use
• Durable and virtually maintenance free
• Large apertures at reasonable cost 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 of telescope
Schmidt-Cassegrain Disadvantages
• More expensive than Newtonians of equal aperture
• Slight light loss due to secondary mirror obstruction compared
to refractors
Maksutov-Cassegrain Telescopes
The Maksutov-Cassegrain is similar to the Schmidt-Cassegrain with
essentially the same advantages and disadvantages. 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 which gives it
slightly better resolution for planetary observing.
Advantages of Maksutov-Cassegrain Compared
to Schmidt-Cassegrain
• Smaller secondary
obstruction results
in a slight increase
in planetary detail
and contrast
• Less expensive to
manufacture
• Longer focal
lengths resulting in higher magnifications for planetary viewing
Disadvantages of Maksutov-Cassegrain Compared
to Schmidt-Cassegrain
• Slightly heavier because of the thick meniscus correcting lens
• Increased time to reach thermal stability in larger apertures
over 90mm
• Longer focal lengths resulting in smaller field of views
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