The following guide will hopefully give some insight into the basic optical principles and design of telescopes which essentially fall into two main categories, refractors that use lenses and reflectors that use mirrors although there are some that use a combination of both lenses and mirrors called catadioptric! (Many telephoto lenses are catadioptric)

The main aim of any telescope is to collect light from distant objects which is then brought to a focus and viewed through an eyepiece, essentially the more light the better as this will allow you to see finer detail! It's also worth noting that low magnifications can be just as rewarding as high magnifications, indeed many deep sky objects and low light emmision nebulae are spread out over a relatively large area and require lower magnifications with greater field of views.

Before we move onto the telescope types it's worth looking at 'Magnification' and more importantly understanding the maximum useable magnification or power of a telescope, this is determined by the simple formula below:

So for a telescope with a focal length of 1000mm using a 25mm plössl eyepeice the magnification acheived would be as follows: 1000 / 25 = 40x   this being equivelent to 40 times the power of the unaided eye. Note that theoretically any magnification can be achieved regardless of the size of the primary optics however you can expect the image quality to depreciate with increased magnification especially with smaller telescopes, many high street stores sell telescopes such as 60mm refractors bosting powers of 400x or more! Beware of such claims! whilst theoretically these magnifications can be acheived the image quality will be so poor making the telescope all but unusable.

To work out the maximum usable magnification or power of a telescope, A good rule of thumb is 50x to 60x per inch of aperture under good conditions. For a 4" (102mm) refractor this means under good conditions the maximum magnification that you could acheive is 200x to 240x. With reflectors you should take into account that approx 20% of the mirror area is obstructed by both the secondary mirror and support veins which effectivley will reduce the overall aperture. Note: It is highly unlikely that you will be able to use magnifications much higher than 300x, this is mainly due the good old British weather and resulting turbulant atmosphere!

It's worth pointing out that all our telescope have equatorial mounts, this allows the telescope to be aligned with the Earth's axis so objects can be easily located by use of the setting circles. Optional motor drives can be fitted which is essential to track objects which would otherwise drift out of view (this is a result of not only magnifying the object being observed but also the Earth's rotation!) and also allowing long exsposure times for Astro photography. The movie clip above simulates views through a telescope at high power without a motor drive, the object has to be brought back into view manually by the use of slow motion controls.

Another consideration is that of 'Resolution', put simply this is the ability of a telescope to render fine detail. A higher resolution will allow you see more detail on the surface of a planet or seperate double stars that are very close together. Resolution is measured in terms of degrees of arc (called arc degrees), minutes of arc or arc minutes and seconds of arc (arcseconds). The full moon would roughly be 0.5 degrees or 1800 arcsec where as the Cassini division within Saturn's rings would be 0.7 arcsec. Resolution can be determined using this simplified formula:  R = 4.5 / D   where R = resolution and D is the aperture of the primary mirror or objective lens in inches. It follows that as the aperture of the telescope increases so does the resolution as well as the ability to see fainter stars.

The diagram above shows a binary star system that is separated by 1.8 arcseconds. When viewed through a 60mm (2.36") refractor which has a resolving power of 1.9 arcseconds, the resolving power will not be sufficient to split the two stars. With a 102mm (4") refractor the stars will appear seperated as the resolving power of this telescope is 1.1 arcseconds. Note at high powers and particularly with smaller refractors that the stars appear as small disks, this is actually as a result of the difffraction pattern created by focusing a star and reffered to as the Airy disk.

Refractors (also known as dioptrics) are what most people identify as being a "telescope", a long thin tube where light passes from the front objective lens, converging to a focus point and viewed through an eyepiece at the opposite end of the tube. Galileo has often been accredited for being the inventor of the refractor, however the Dutch eye glass maker Hans Lippershey discovered in 1608 that by placing one concave lens in front of another that the image was magnified.

Galileo can certainly be given credit for being the first person to use this type of telescope to undertake serious observations of the heavens. Galileo was able to see that the moon was not smooth, but covered with huge valleys and craters. He discovered four moons orbiting Jupiter and that Venus showed phases just like the moon. He realised that this meant Venus, and all the other planets, revolved around the sun not around the Earth, as many people believed at the time. In 1610 Galileo published his telescope observations.

Unfortunately this basic telescope design suffered from a defect that can still often be found in lower end specification and toy telescopes: ' Chromatic Aberration ' or CA. The diagram below shows the effects of light passing through a single objective lens (like the telescope Galileo used). The lens can be thought of as a collective series of prisms and like a prism the light is split up into the different colours of the rainbow, this happens because different wave lengths of light are refracted (bent!) accordingly by varying amounts resulting in a colour spectrum.

The diagram above shows a marked difference in the focal points of both blue light (A) and red light (B). Bright objects viewed through this type of telescope such as a full moon will appear to have a coloured fringe which will tend to be either a bluish or purplish haze (see diagram below) and in some cases all colours of the spectrum, this not only results in unwanted colour but also causes a loss in image sharpness because the light can't be brought to a single focus! In the past the only option was either to stop down the lens aperture and thus effectively increase the focal length (F) number but resulting in significant light loss and image quality, or to actually physically increase the focal length of the objective lens. Unfortunately the latter resulted in very long unwieldy telescopes that were very difficult to use!

Fortunately modern day refractors have overcome this issue by the use of an ' Achromatic objective ' lens. This usually consists of two elements of glass of varying density; one lens is usually concave with the other being convex. The diagram above shows how this set-up helps reduce ' Chromatic Aberration ' by effectively bringing all the different wavelengths of light closer to a common focus (A). Even Achromatic refractors are not completely free from false colour and may exhibit a slight blue / purple haze on all but the brighter objects such as a full moon or Venus depending on the quality of the instrument however this does tend to be minimal. Apochromatic refractors use a more complex objective lens made up of three elements and is virtually free from false colour however they tend to have a very high price tag!

The diagram above shows a typical modern day refractor setup. with advanced manufacturing techniques and the use of superior materials it is now possible to make much more compact refractors with virtually no false colour. Note that a 90° Star diagonal is often used especailly when viewing objects that are high up in the sky, the longer tube length of the larger refractors would otherwise be very awkard and difficult to use!

In 1668 Sir Isaac Newton avoided the problem of chromatic aberration inherent in refractors by inventing the reflecting telescope, which still bears his name. Newtonian telescopes use a curved mirror to focus incoming light to a second, flat mirror that directs the light to a convenient viewing position on the side of the telescope.

The Newtonian reflector designs like the basic refractor is not entirely trouble free! Concave primary mirrors that are spherical suffer from ' Spherical Aberration ' which is the in ability of the mirror to focus all the in coming light to a single focal point, whilst free from false colour the image can not be brought to a sharp focus. (See diagram below)

Spherical Aberration can be overcome by replacing the Spherical or concave mirror with a Parabolic mirror, the diagram above illustrates how the light is brought to a common focus resulting in a good sharp image. Many cheap reflector telescopes on the market utilise spherical mirrors; it's worth checking with the supplier whether or not their telescopes are made up with parabolic primary mirrors!

It is worth noting that there are some hybrid reflector designs which utilise spherical primary mirrors with either a glass front plate corrector lens, referred to as Catadioptric or an achromatic lens placed in the path of the secondary flat mirror such as the Bird - Jones reflector. The Catadioptric design has become very popular as this arrangement allows a fairly long focus telescope to be contained within a short tube length, unfortunately because the optical design is more complex this tends to increase the price tag accordingly! The diagram below shows a typical Newtonian reflector setup.

There are pros and cons for both style of telescopes, refractors tend to be more exspensive per inch of aperture than reflectors however refractors require little if any maintenance. Refractors tend to be very well suited for both planetary and lunar observations with their longer focal lengths where as reflectors being more compact in design with shorter focal lengths make an excellent choice for those interested in deep sky objects such as nebulae and galaxies.

Before taking the plunge consider where your main interest lies, size and portability are also worth taking into account especially if space is an issue! We have a comprehensive range of telescopes to cater for all levels whether you are just starting out in astronomy or perhaps a more advanced astronomer, what ever you decide please remember that we are only to happy to help and will gladly offer some friendly advice! Please don't hesitate to contact us at any time.

When using a telescope it's important to make your observations from a good viewing location. Many people think of the attic as an ideal setup with their telescope pointing towards the sky through a Velux window - how wrong they are! The problem is that when warm air and cold air mix this causes servere air turbulance which can completely blur an object when viewing through a telescope especially at higher magnifications. This is not unlike the heat shimmering effect on a tarmac road durring a hot summers day.

Ideally a location should be chosen well away from any buildings which could be a source of un wanted heat or any large areas of tarmac or paths which could have absorbed heat from the sun durring the day. Also a site well away from artificial lighting is preferable and ideally open and clear from any visual obstructions! You should allow some time for your eyes to become adapted to darkness, if you need to read star maps or make notes then if possible use red light as the human eye is not as sensitive to this unlike brighter white light.

The movie clip above shows the effects of atmospheric turbulance with Saturn appearing at times to be completely blurred with brief intermitant sharp views. A poor viewing location as mentioned above will only add to this. If the telescope is purely being used for terrestial use then the viewing location is not quite so critical however the magnifications used will probably be lower. Many Astronomers will refer to these atmospheric affects as 'Seeing' - an Italian observer Antoniadi devised a scale from 1 - 5 with 1 being perfect seeing, 2 - some quivering of the image, 3 - much quivering and unstable image, 4 - poor and 5 as either very bad or hardly worth observing! This system is still used today and is a very simple way of describing viewing conditions when recording details of observations

Note: when using your telescope ideally you should allow approximately ½ an hour for the optics to reach ambient temperature, especially if it has been stored at room temperature which is warmer than the outside temperature. Newtonian reflectors with their open tube design are particularily prone to air currents occuring within the tube so allowing the optics to cool down is essential when using higher magnifications, refractors with their closed tube design don't really suffer from such issues and allowing the optics to reach ambient temperature is not so critical. Below is a table that covers some of the do's and don'ts when using your telescope, it's worth taking note of some these points otherwise you may be rather dissappionted with the performance of your new telescope!

This section is only intended as a guide and nothing more! There are many books that cover these topics in far greater detail than is possible here however we hope that this guide will be useful, especially to those new to Astronomy. If you have any queries or require some general information then please feel free to contact us as we will be only to glad to help!