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TELESCOPE DESIGNS - AstroShop

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:

Refracting 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 click here.

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 reflecting telescopes.

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.
 

Sky-Watcher Black Diamond ED100 refractor with equatorial mount
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 subject.

NEWTONIAN TELESCOPE AUSTRLAIA

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 instruments.

While equatorial mounted systems are the desirable option for protracted lunar and planetary observations another popular design is the Dobsonian 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.

 

Sky-Watcher 6 inch (150mm) Newtonian on classic EQ3 equatorial mount

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.

 


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