Download Advantages and Disadvantages

By Kimberley Evans, Huw Wells and Katy Langley
•Catadioptrics use a combination of mirrors and
lenses to fold the optics and form an image.
•There are two popular designs: the SchmidtCassegrain and the Maksutov-Cassegrain.
•Catadioptrics are the most popular type of
instrument.
•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 not only for deep sky observing and astrophotography, but also very good for lunar, planetary and
binary star observing
•Image degrading air currents are reduced due to the closed
tube design
•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 of telescope
•More expensive than Newtonians of equal aperture
•Like refractors, the lenses cannot be made much bigger
than a metre
•It is not what people expect a
telescope to look like
•Slight light loss due to secondary
mirror obstruction compared to
refractors
•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), yet also reasonably good for
lunar and planetary work
•Good for deep sky astro-photography (but not as
convenient and more difficult to use than Catadioptrics).
•Low in optical aberrations
and deliver very bright
images
•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
•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
•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 and also good for distant
terrestrial viewing
•Inexpensive in smaller diameters
•High contrast images with no secondary mirror or
diagonal obstruction
•Sealed optical tube reduces image degrading air currents
and protects optics
•Since the tube is closed off from the outside, air currents and
other effects are eliminated. This means that the images are
steadier and sharper than those from a reflector telescope of
the same size
•Objective lens is permanently mounted
and aligned
•Colour correction is good in achromatic designs and excellent
in apochromatic, fluorite, and ED designs.
•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
•It is technically difficult to make a glass lens with no
imperfections inside it, with a perfect curvature on both sides
of the lens
•How well the light passes through the objective
decreases as the thickness of the lens increases. But at
the same time, since the objective can only be supported
at the ends, the glass lens might sag under its own
weight
All refractors suffer from an effect called chromatic aberration (or colour
deviation or distortion) that can produce a rainbow of colours around the
image. As light passes through the lens, the longer wavelength corresponding
to redder colours is bent less than the shorter wavelength light (bluer colours).
There are a couple of ways to reduce
chromatic aberration. One way uses multiple
lenses to compensate the aberration. The
other way is to use the longest possible focal
length (distance between the focus and the
objective) to minimise the effect. This is why
the early refracting telescopes were made
very long.
Both reflector and refractor telescopes can suffer from a defect called spherical
aberration. In this case, if the mirror is not well curved or the lens is badly shaped,
not all light is focussed to the same point.
To compensate this problem corrective optics can be used to intercept and correct
the light beams from the secondary mirror before they reach the cameras and
spectrographs. The images obtained with a telescope that suffers from spherical
aberration can also be computer-enhanced to produce sharper images.