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 FACTS ON ASTROSCOPES
 
 
The Newtonian Reflector

This most common type of reflector telescope uses a concave objective mirror (primary mirror) at the back of the tube and a secondary mirror, which directs the light to the eyepiece.

Since the light rays entering the telescope reflect off the mirrors and don't pass through the glass, no false colour is produced. Orientation of the image is not important for astronomy so the image is left uncorrected.

The Newtonian reflector represents excellent value for money as typically they offer more aperture for a given price than other types of telescope.
 
 
 

The Refractor

Refracting telescopes have an objective lens at the front of the tube. The light exits out through the back of the tube to the eyepiece. Since many observations are made high in the sky, a right-angle diagonal is used to avoid neck strain. This also provides an upright image making them suitable for terrestrial observations. A refractor has several advantages over other designs. The tubes are enclosed so that dust and moisture do not enter the tube, they have fixed optics that do not normally require collimation, and they do not have a central obstruction which reduces the light entering the tube. A refractor typically will give higher quality images of planets than other telescopes of similar
 
 
 

The Catadioptric

Telescopes using a combination of both mirrors and lenses are called catadioptrics. There are many different designs. Examples of these are the Schmidt-Cassegrain and the Maksutov-Cassegrain.

Usually a full aperture lens is used to correct aberrations in a compound reflecting telescope. The corrector lens also increases the performance of the instrument as air currents are eliminated. The main advantage of the design is that, because the light path is folded back on itself, it provides a very portable, short physical length telescope with a long focal length.


Original Image

Inverted image

seen through a

reflector

Mirror reversed

image seen

through a

refractor


 
 
 
Astronomical Objects do not
require a telescope with erecting views as there is no 'Up' or 'Down' in space.


 
 
 
 
 
Telescope Mountings

A mounting with a tripod is necessary to point a telescope at its target, and to hold it there securely. A larger, sturdier mount is necessary for heavier telescopes. There are two basic types of mountings, and each is suitable for different applications.


The Alt-Azimuth Mount


The Alt-azimuth mounting is a simple system which moves in two directions; sideways, and up and down, and is sometimes provided with slow motion cable controls to accurately point the telescope. This type of mounting is ideal for terrestrial observations and can also be of use for basic astronomical observations.

 

The Equatorial Mount
There are several different models of equatorial mount offered in this website, and the principle of all of them is the same. The principle is to correct for the rotation of the Earth with one motion during astronomical observations. the Earth's rotation causes objects to rise in the East, follow a circular path across the sky and set in the West. When using an astronomical telescope at high power, it will be only take approximately 30 seconds for the object to move out of the field of view. The equatorial mounting allows the telescope to track the objects and to keep the image in the centre of the eyepiece. When the equatorial mount is correctly aligned, tracking can be achieved either by turning a gear manually or with a motor drive which tracks objects automatically.

 
 
 
 
 
 
 
Telescope Magnifications

Moon at appropriate

magnification under good sky

conditions.

Above the magnification limit,

the image becomes too blurry

for useful observing.


As a general rule of thumb, a telescope is capable of magnifying an object approximately 50x-75x per inch (or 2x-3x per mm) of aperture (objective diameter), so a 4" (100mm) telescope has the potential to magnify approximately 200x-300x. The optical quality, configuration of the telescope and seeing conditions however, will be deciding factors as to what useful magnification a particular telescope can achieve. Atmospheric turbulence usually restricts practical magnifications to a maximum limit of about 300x, or slightly more in rare cases. Above the magnification limit, the image becomes too blurry for useful observing.

It should be carefully noted that for many astronomical observations high magnifications are not necessary and much better results are often achieved by using lower power. The telescopes described in this catalogue are all supplied with various combinations of eyepieces and in some cases a Barlow lens.

The magnification ranges quoted for many of the telescopes on this website are simply those achievable with the standard accessories, and can be increased or decreased as necessary with the addition of the optional accessories, within the optimum boundaries of the telescopes capability.

Resolution

The most important factors in a telescope are the aperture, (or light gathering capability), and the quality and accuracy of their optics. The aperture determines the telescope's ability to resolve small or distant objects and to reveal fine detail. Resolution can be defined as how much detail a particular telescope can see. If the diameter of the aperture is twice as big on a similar quality telescope, then the resolving power should be twice as good. Resolution is stated in arc-seconds and there are sixty arc-seconds in an arc-minute and sixty arc-minutes in a degree.

In short: The bigger the aperture, the higher the resolution and therefore the better the image!!
* To calculate the magnification using a Barlow lens, use the formula (left) and multiply the result by the magnification of the Barlow lens - x2, x3 etc.

By using the magnification formula, one can calculate what accessories (eyepieces, Barlow lenses etc.) could be purchased for a telescope to achieve a greater range of useful magnifications than those achievable with the standard accessories supplied.