Telescope Basics

This section is not intended as the definitive work on telescopes but rather to give the beginner an understanding of what will be important when they are considering their first telescope.

First up, if there is a local astronomy club near you then the best advice is to go along and talk to the people there, you’ll get far more out of that than you will from reading these or other pages on the web. If not, then read on –

It’s all about magnification right!
Actually, this is one of the greatest myths about astronomical telescopes.  The single most important attribute about a telescope is its ability to gather light cleanly rather than pure magnification.  This ability is mainly governed by aperture size (the size of the lens or primary mirror of the telescope), the bigger the aperture the more light the telescope can capture.  The ability to magnify is useless if not enough light is captured in the first place (all that you will get is a bigger blur). It is also often the case that high magnification is not desireable for viewing particular types of astronomical objects, for example some nebula are actually vast in size and magnification which is too high will not allow all of the object to be viewed. For many nebula, open star clusters and globular star clusters it is much better to have lower magnification but as much aperture as you can afford.

Having said that; good, usable magnification can be very useful when viewing the lunar surface and the planets.  So what is usable magnification exactly?  A general rule of thumb for how much usable magnification a telescope can handle is often calculated as 100x per 50mm of aperture (e.g. a 100mm telescope with the right eyepieces should be good up to around 200x).  Be aware that there are many other considerations such as quality of the lenses or mirrors within the telescope which can affect the image quality.  There are two main ways that a telescope can deliver magnification, focal length and eyepiece power. 

Both of these described below – 

  • At it’s simplest, focal length is usually the length of the telescope tube in mm as this is the distance that the light has to travel from the aperture to the eyepiece.  The longer the focal length, the higher the inherent magnification.  There are a couple of types of telescope which are exceptions to this such as Cassegrain and Catadioptric telescopes which have a relatively short physical tube but a very long light path through the use of internal lenses and mirrors.
  • Eyepieces also have a focal length, the shorter the eyepiece focal length the greater the magnification (e.g. an 8mm eyepiece will provide double the magnification of a 16mm eyepiece).

The calculation for the magnification provided is usually calculated as the telescope focal length divided by the eyepiece size.  So for example using a 20mm eyepiece on a telescope which has a focal length of 700mm would provide a magnification of 35x.  Using a 4mm eyepiece on the same telescope would provide a magnification of around 175x. 

You will also hear some telescopes described as fast or slow.  This description refers to a thing called the focal ratio.  Focal ratio is calculated as the focal length divided by the aperture size (both in mm). The lower the focal ratio, the faster the light gathering.  Aperture for aperture, fast telescopes usually have a shorter focal lengths than telescopes with a slow ratio.  For this reason, slow scopes are often better for planetary viewing and fast scopes are more suited to deep space objects such as nebula where the light is dimmer but the objects themselves are larger requiring a lower magnification.  Typically, a fast telescope might have a focal ratio of 5 whereas a slow telescope might have a ratio of 12. 

Telescope Types
There are several types of telescope on the market, a short description is provided for the three main types which are commonly available – 

  • Refractor Telescopes – Refractor telescopes work by using a lens to gather and then bend (or refract) light which causes the rays to converge at a focal point forming an image which can then be viewed via an eyepiece of some description.  Refractor telescopes have been around since the time of Gallileo and are the most commonly used type of low cost telescope.  Refractors are simple to use and require very little maintenance. However, they do start to become very expensive for larger aperture models and can produce some false colour, known as chromatic abberation (or CA) around the image due to the refracting method deployed.  Typically, refractors with longer focal lengths produce less CA than those with short focal lengths.  The exception to this are apochromatic refractors which have very exotic glass and are corrected so that CA is negligible.  A small 80mm APO will set you back around £500 however and are outwith the reach of most first time buyers.    The quality of lens glass and coatings used on regular achromat (non-apo) refractors can vary enormously and some produce images which are substantially better than others, both in minimising CA and in general image and colour sharpness.  Refractors normally use a mirror star diagonal between the main tube and the telescope for more comfortable viewing. This provides an image which is the correct way up but inverted left to right. You can also get an amici prism diagonal which will provide a correct vertical and horizontal image. 
  • Reflector (or Newtonian) Telescopes – Reflector telescopes work by gathering the light on a main mirror and then reflecting it to a secondary mirror where the image can then be viewed via an eyepiece.  The reflector was used by Issac Newton and they do not suffer from chromatic abberation as refractors do, although they can suffer from spherical aberration.  Reflectors with parabolic mirrors generally perform much better than those with spherical mirrors.  They can also be built with large scale apertures for less cost than the equivalent refractor.  The downside is that they are often bulkier than refractors and require more collimation maintenance.  Collimation is ensuring that the optics within the telescope are aligned properly.  This is done with special tool called a collimator and by adjusting the collimation knobs on the telescope.  It is not a difficult task but it does require to be carried out regularly on shorter length reflecting telescopes.  Images through reflector telescopes are normally inverted (i.e. upside down and with left and right reversed) which isn’t really a problem in astronomy (there’s no right way up in space). You can get an erect image corrector lens but this introduces more glass and often detracts from the quality of the view. 
  • Cassegrain Telescopes – Cassegrain telescopes work by using a mixture of lenses and mirrors and generally provide a very well corrected view.  They often feature longish focal lengths but due to the internal lens/mirror arrangement, the focal length is acheived by the light being bounced around internally and so the physical tube lengths are often relatively short.  There are two main model types available, the Schmitt Cassegrain (SCT) and the Maksutov Cassegrain (MCT).  There are subtle differences between the two but the afformentioned features hold true for both. Maks are often available in smaller aperture sizes than SCTs and often have slightly higher focal ratios.  Maks use diagonals and fittings in the same way as refractors but SCTs normally need a special fitting type. 

Eyepiece Fitting Sizes
There are three main eyepiece fitting sizes in common use.   For some reason these are usually defined in inches rather than mm.  The oldest type is 0.965″ and is often found on low cost telescopes.  By far and away, the most common size of eyepiece in use is the 1.25″ size which is found on most serious astronomical telescopes.  The 2″ standard of eyepiece is used on some telescopes too, these can produce a better apparent field of view than 1.25″ eyepieces on longer focal length eyepieces.  On the whole 1.25″ and 2″ eyepieces are preferable and nicer to use than 0.965″ eyepieces, there is also a much wider choice of eyepiece type and size available these days.  Your telescope can be fitted with an eyepieces from any manufacturer provided that the eyepiece size is the same as the holder on your telescope focusser (e.g. 1.25″ holder and 1.25″ eyepiece). 

Eyepiece Types
There are now more eyepiece types than you can shake a stick at but I’ve listed some of the more common ones in ascending order of general quality. 

  • Huygens – These are one of the earliest twin lens eyepieces and are often shipped free with low cost telescopes.  They work reasonably well with high focal ratios but can struggle with shorter tube length models and can suffer from some aberration, distortion and poor eye relief. 
  • Ramsden – These are similar to the Huygens in terms of performance and stick with the 2 lens formula.  They are slightly better in quality but a long way behind the Kellner below. 
  • Kellner/Achromat – Kellner/Achromat are Achromatic Doublet eyepieces which are far superior to either the Huygens or Ramsden and are the entry level for serious observing.   They are a three lens design which offer very good value for money.    There is also a Rank Kellner version which usually offers a slightly better field of view. 
  • Plossl – Plossl eyepieces are 4 element (sometimes more) and are superb all round eyepieces.  They tend to be considerably more expensive than Kellner/Achromat lenses however and can be a bit short on eye relief. 
  • Others – There are a host of other more expensive eyepiece design including Ortho, Erfle and Nagler. These tend to climb in price quite dramatically and in my opinion should only be considered once you get really serious. 

Barlow Lenses and Focal Reducers
The Barlow lens is indeed an ingenious piece of optical equipment.  By inserting a Barlow lens between the focusser and the eyepiece the magnification normally acheived is doubled, trebled or in some cases by a multiplier of up to 5.  The Barlow acheives this by effectively increasing the focal length of the telescope which in turn multiplies the magnification by the same factor.  The downside of this can be that, because additional glass is being inserted in the optical path, the image can sometimes become less sharp.  In fact a poor quality Barlow can cause otherwise good optics to produce a magnified image which is downright blurred.  It is also easy to forget that the amount of magnification produced maybe too much for the telescope’s aperture as the telescope’s light gathering ability does not increase with the introduction of the Barlow.  Barlow’s can be useful for acheiving very high magnifications for planetary and lunar work. 

A Focal Reducer works almost like a Barlow in reverse by shortening the telescope’s focal length and thus reducing the magnification.  These are very useful for viewing nebula and other large astronomical objects which require wide field capabilities in telescopes which have a high focal ratio and thus a narrower field of view.


Mounts and Tripods
The importance of the Tripod and Mount is usually underestimated by beginners to astronomy.  Above almost all other considerations, it is very important that the tripod and mount arrangement is strong enough to securely support the telescope tube being used.  There is nothing worse or more frustrating than trying to sight your telescope on an astronomical object with a wobbly tripod or a shaky mount.  Many mounts and tripods have a maximum load weight guide, however this is often an overestimate and it is safer to be comfortably under this limit.

There are two main types of mount in use with astronomical telescopes.   These are described below – 

  • Alt-Az (Altitude-Azimuth) Mount – This type of mount is the simplest to use and consists of an up/down and left/right type axis.  Nearly all telescopes in this website use this type of mount and they come as standard on most lower cost telescopes.  They do not require any particular type of alignment and can be used as soon as they are set up.  Nearly all photographic tripods can be considered Alt-Az Mounts. 
  • Equatorial Mount – This type of mount is more complex to use and in addition to the axis found on the Alt-Az, also have an additional axis (almost like a second Azimuth axis) which allows the telescope to follow the rotation of the celestial sphere.  This means that once an object is found, it can be tracked simply by moving this one single axis.  Equatorial mounts are more complex to use than Alt-Az mounts as they have to be polar aligned.  They are often fitted with motors to allow and object to be tracked automatically and are essential for deep space imaging where extremely long exposure photography is required. 

Finderscope Types
There are two main types of finderscope available on the market.  These are – 

  • Regular Optical Finders – These are the most common finders and work like a mini refractor telescope and come in various different sizes, usually denoted by the magnification and the objective size in mm (e.g 8×24).  Note that most standard straight through finderscopes views are inverted. 
  • Red Dot Finders – These finders work by a providing an illuminated red dot within the finder which has the illusion of projecting the dot onto the sky.  These finders are often preferred to the regular finders as they are usually the correct way up and have zero magnification making it somewhat easier to navigate the night sky. 

Finderscopes come in a variety of fitting but the two standards which are widely in use are – 

  • Standard Finder Shoe – This is a shoe fitting where the finder base slips into a shoe holder which is usually then secured with a small screw.  This standard type of fitting is used widely by Celestron, Skywatcher, Orion and others.  Makers of these types of shoe fitting are a standard size across the range and can be used interchangeably. 
  • Two Hole Finder – Not so standard are two hole finders.  These do not use a shoe type fitting but rather have two thumbscrews which attach the finder to the telescope via the two holes within the finder base (hence the name). Since these often clamp directly onto the curved surface of the telescope, care needs to be taken when purchasing these since they can vary in terms of the base curvature and (less often) the spacing between the holes. 

Computerised Telescopes
Comparatively affordable computerised (or GOTO) telescopes have undergone something of a revolution in the last 10 or so years.  These motorised mounts require a short alignment process, usually by levelling the telescope, entering the current geographic location co-ordinates and manually sighting and confirming two or more stars.  After this is done the telescope will automatically move (or GOTO) and sight the required object which is selected by keying it into an attached keypad. 

Even the entry level GOTO telescopes usually cost at least £200, however some secondhand examples can be purchased for around the £100 mark. These are generally small aperture (60mm) examples and the best advice when you are on a tight budget is to put the money towards better optics and a sturdy manual mount instead of spending money on a GOTO mount.  An inexpensive Planisphere will allow you to manually locate objects in the night sky and there are also free astronomical packages available for computers such as Stellarium ( which will show you details of the night sky for your location and time. 

Further Reading
An excellent book for beginners to astronomy who wish to learn more about the objects in the night sky and how to find them is a title called Turn Left at Orion which is available from Amazon and other suppliers.