|Australia and New
Zealand share some of the finest dark skies in the world and if you're looking to buy a
telescope in Australia then you come to the right place. Astronomy is a very rewarding
hobby but it pays to know a little bit about the various types of telescope available to
view our southern celestial wonders at their best.
Finding the right telescope for your needs can be a daunting task if
you're new to the subject. In Australia, almost all telescopes you'll come across are
imported from several manufacturers around the globe. Telescope designs are essentially:
Telescopes - (a lens at the front which bends the incoming light to a point
of focus at the other end where an eyepiece is placed to magnify the image).
Reflecting Telescopes -
(incoming light passes down a long tube to a curved mirror which focuses the light back up
the tube to another mirror which then diverts this beam of light to an eyepiece where the
image is then magnified.) Three common reflecting telescopes are available in Australia
and they are the Newtonian, the Schmidt-Cassegrain and the Maksutov-Cassegrain. To see
diagrams of the light path in each one of these designs
The key to higher power is larger aperture.
In other words, the larger the telescopes lens or mirror, the more light it can gather and
focus so that you can see more when using higher power eyepieces. But there is a practical
limit to how much magnification you can use and this has to do with the ocean of air that
circulates in our atmosphere. If it is turbulent then high power views seem like peering
at an object behind running water. But this is yet another subject.
REFRACTING TELESCOPE AUSTRLAIA
Found in most camera shops and department
stores are classic entry-level 60 x 700mm telescopes. The '60' represents the objective
lens aperture in millimetres while the '700' represents the focal length in millimetres.
It is generally held that these telescopes should be avoided and unfortunately, even in
Australia, some retailers posing as specialist astronomy stores, do sell them. A telescope
of this specification can produce nice simple views of the Moon and very modest views of
the planets. Larger aperture, high performance achromatic or apochromatic refracting
telescopes are better suited for observing the planets and deep sky but are proportionally
more expensive per centimetre of aperture when compared with more cost effective
Refracting telescopes suffer from varying degrees of an optical defect known as chromatic
aberration. Chromatic aberration is the failure to bring light of all wavelengths to a
common point of focus. In other words, blues and reds appear slightly shifted.
To correct the problem, modern day refractors are
comprised of two objective lenses coupled together and
these are called achromatic refractors. But even an
achromatic (abbreviated achromat) optics still exhibit
some amount of chromatic aberration. When looking at
bright stars, the limb of the Moon or planets such as
Jupiter or Venus in particular, the effect is a slight
purplish halo around the subject. The most efficient
refractor design is called an apochromatic refractor,
which uses 3 coupled lenses of crown and flint glass
greatly minimising the effect to almost negligible
levels thus producing much sharper, contrast rich
images. Such designs are quite expensive however new ED
(extra low dispersion glass) refractors like those
produced by Sky-Watcher and Vixen Japan offer truly
excellent apochromatic performance in a more affordable
two element objective design.
But should you decide on a popular achromatic design, their are filters called Minus-Violet
or fringe killing filters that simply screw in to the barrel of an eyepiece to greatly
reduce visible chromatic aberrations without greatly altering the natural appearance of the
Newtonian reflectors are comprised of two
mirrors. The larger primary mirror is parabolic and reflects incoming light rays to a
smaller flat secondary mirror mounted diagonally. This secondary mirror diverts this light
to an eyepiece near the skyward end of the telescope where the image is subsequently
brought into focus with a sliding tube rack and pinion arrangement. Since they reflect
light, Newtonian telescopes do not suffer from chromatic aberration found in refracting
While equatorial mounted systems are the desirable option for protracted lunar and
planetary observations another popular design is the
or rather Newtonian on Dobson mount system. This is a Newtonian
optical tube assembly mounted on a basic squat-style alt-azimuth platform usually made of
wood. At high powers these systems require constant and re centring of the target on both
axis. They are best suited for casual observing and deep-sky.
|CATADIOPTRIC (Cassegrain) AUSTRALIA
There are two commonly available
catadioptric designs. They are the Maksutov-Cassegrain and the Schmidt-Cassegrain. Both are
extremely portable telescopes based around a mirror-lens design involving greater costs to
manufacture compared with a Newtonian yet their versatility makes them an appealing
option. Unlike the conventional rack and pinion focussing system used in most refractors
and Newtonian telescopes, catadioptric scopes are focussed by the inward-outward
adjustment of the primary mirror via a focus knob attached to an internal threaded rod.
While refractors have the advantage over reflecting telescopes of an unobstructed primary
objective, Matsukov and Schmidt Cassegrains have an inherently larger secondary central
obstruction compared with Newtonians. They do however share the common advantages of a
sealed tube and longer primary focal lengths with ratios of f/10 to f/15 most common. Like
refractors, both these instruments are also susceptible to dew settling on the primary
lens and dew covers are a favourable option.
These are excellent all-round instruments offering the best of both worlds. Many are
supplied today with smart GOTO electronics on an Alt-Azimuth driven mount which can be
mounted to an optional equatorial wedge for more accurate astrophotography.