What functions does a telescope have?
The properties of a telescope determine the price and the quality of your observations.
We will discuss some more properties of a telescope below, that could be of importance for your choice. These properties are:
- Optical properties (how a telescope collects the light)
- Non-optical properties (the hardware, like a stand)
- Eye pieces
- Filters & other accessories
- Practical considerations(portability, maintenance, storage and price)
The amount of light a telescope can collect is directly related to the optics in the telescope. Telescopes that have a bad quality lens/mirror can be very frustrating to use. Below you will find a number of optical functions of a telescope that are important to consider when you are thinking about buying a telescope:
- Objective diameter
- Focal length
- Focal ratio
The amount of light a telescope can collect is directly related to the diameter of the primary lens or mirror. In general you can say that the larger the lens or mirror, the more light can be collected by the telescope, and the clearer the final observation/image is.
The diameter of the telescope is probably the most important consideration when buying a telescope, but it certainly is not the only thing you should think about. While you want to try and get the largest diameter possible for your money, there are other factors that also play an important role, and these will be discussed below.
Choosing a telescope purely based on the magnification is one of the most common mistakes made by beginner telescope buyers. Manufacturers of cheap department store telescopes often put 200x magnification or more on the box, but the magnification has nothing to do with the optical performance of a telescope, and should absolutely not be your first consideration when buying a telescope!
The capacity of a telescope to magnify objects properly depends on the combination of lenses being used. The magnification is usually achieved by a combination of a telescope with a long focal length and an eyepiece with a short focal length. However, the problem is that the higher the magnification, the narrower the field of view and the less clear the image.
A rule of thumb is that the maximum magnification should be 2x the diameter (in mm) of the telescope. So if a telescope's diameter (diameter of the lens or mirror) is 114 mm, then the maximum magnification is 228x. Because the magnification can be adjusted (in almost any telescope) by using different eye pieces, the opening (diameter) of the telescope is a much more important than the magnification of the telescope. Aside from that, most astronomical objects are best viewed with a low magnification, so that the maximum amount of light can be captured.
The focal length is the distance the light travels before it comes together in one point (focal point).At telescopeplanet.co.uk we have stated the focal length of each telescope in its description. You can also find the focal length written on the telescope itself, or in the manual.
The focal length is an important number to know. As mentioned above, the magnification depends on the distance of the focal point. In general a larger focal length means a higher magnification. However, don't mix this up with the length of the tube, because a longer tube does not always mean a larger focal length. Because catadioptric telescopes(telescopes that use lenses AND mirrors) 'fold' the light rays a couple of times until the light reaches the eye piece, a larger focal length can still be created, but with a relatively short tube.
The focal ratio (you can find this in the description of our telescopes), also called the f-number, applies to the clarity of the image and the range of view. The focal view is calculated by dividing the focal point of the telescope by its diameter. The term 'focal ratio' originates from the world of cameras, where a small focal ratio means a short duration of light exposure on the film. When the focal ratio is small, the camera is often referred to as 'fast'. The same can be said for telescopes. However: a 'slow' telescope (with a large focal ratio) and a 'fast' telescope that both have the same magnification will perform the same. In general however, the following is interesting to note:
- 1:10 or higher- good for observations of the moon, planets and stars (high power).
- 1:8- good for all-round observations.
- 1:6 or lower- good for observing deep-sky objects (low power).
Non-optical properties of a telescope
There are also other parts, aside from the optical parts of a telescope, that you can take into consideration:
Eye piece holder
Eye pieces come in three diameters: 0.964 inches (2.45 cm), 1.25 inches (3.18 cm) and 2 inches (5.08 cm). The fitting of a telescope however, comes in only one of the sizes mentioned above, and cannot be changed. This means that if you ever want to buy extra eye pieces, you have to take the size of the fitting on your telescope into account. The most common international standard is 1.25 inches. Most of the telescopes on telescopeplanet.co.uk come, for that reason, with a 1.25 inch eye piece fitting.
You can find the diameter of the eye piece fixture under the description 'Diameter of the eye piece' at the description of each telescope on
The telescope must be supported by a holder, or a stand, otherwise you would have to hold the telescope the entire time. The telescope stand ensures that you:
- Can keep the telescope stable
- Can aim the telescope at stars and other objects (for instance birds)
- Can adjust the telescope during the observations of stars, to compensate for the earth's rotation.
- Can free your hands so that you can do other things, such as focussing, changing eye pieces, noting down important observations or draw stars.
There are two types of stands:
Photo 4: Different types of telescopes and stands
The azimuthal stand has two axles for rotation: the horizontal axle and the vertical axle. To direct your telescope toward an object, you rotate it using the horizontal axle (also called the azimuthal axle), after that you rotate the telescope upwards using the altitude (y) axle.
The azimuthal stand is easy to use and very common for beginner telescopes. This stand has two varieties:
- Ball and socket- primarily used on cheaper telescopes. The end of the stand has the shape of a ball, so that it can be rotated in all directions.
- Rocker box- is used for Dobson telescopes and is made of a wooden stand with a low centre of gravity.
Despite the fact that an azimuthal stand is an easy and user-friendly, it is hard to follow the stars perfectly with this stand. If you are following a moving start, then you continuously have to move the telescope 'up-and-down' in a kind of zig-zag pattern, instead of one smooth movement. For this reason this type of stand is not suitable for taking pictures of stars.
A parallactic stand has two axles that are perpendicular to each other: a straight ascension axle (also called RK- or hour axle) and a declination axle (also called DEC or elevation axle). With this type of stand you don't have to move the telescope up and down (as with an azimuthal axle), but the telescope smoothly compensates for the earth's rotation, in the opposite direction. The parallactic stand is offered in two types:
- German parallactic stand- is shaped like a "T". The long axle of the "T' is lined up with the earth's pole
- Fork stand- a fork in two parts that is placed on a wedge and is also lined up with the earth's rotation.
When a telescope is aligned with the poles of the earth, parallactic mounts can follow the stars with a smooth motion. In addition, some parallactic mounts are equipped with:
- Circles- this allows you to easily locate a star with the coordinates (right ascension and declination)
- Motorized roller- This allows you to continuously adjust the telescope to track stars with your computer (laptop, desktop or PDA)
Please note: you aneed a parallactic mount if you want to get started with astrophotography.
An ocular, also know as an eye piece, is the second lens in a refractor telescope, and the only lens in a reflector telescope. Oculars can have a wide variety of optical designs and may consist of one or several lenses. An ocular can almost be regarded as a telescope itself. The goal of an eyepiece is:
- Producing and changing the magnification of the telescope, by changing eyepieces.
- Producing a sharp image.
- Giving you a comfortable observation.
- Determining the width of the image (image field) through: Apparent image field – how much of the sky, in degrees, can be observed from edge to edge by using only the eyepiece (you can find this on the eyepiece itself) and Real image field – how much of the sky can be observed when the eyepiece is placed in the telescope.
There are many different types of eyepieces, including:
- Huygens (indicated with the letter H )
- Ramsden (indicated with the letter R)
- Orthoscopic (indicated with the letter O)
- Kellner and RKE (indicated with the letter K)
- Erfle (indicated with the letter E)
- Plössl (indicated with the letter P)
- Nagler (indicated with the letter N)
- Barlow (used in combination with a second eyepiece in order to magnify the image 2 or 3x)
Photo 5: Schematic overview of different eyepieces
Huygens and Ramsden have the oldest design. These eyepieces suffer from chromatic abberation (colour shifting) and are usually supplied with the cheapest and least effective telescopes (they are also indicated as H-eyepiece or R-eyepiece)
Orthoscopic eyepieces were invented by Ernst Abbe in 1880. These eyepieces have 4 elements and a 45-degree image field, which is sometimes too small. The optical design is good, so they give a sharp image. They're considered good eyepieces for observing planets. Orthoscopic eyepieces can cost about £15 to £60 a piece.
Kellner and RKE eyepieces have three elements in their design and can display a 45-degree image with a little chromatic aberration (colour shifting). They are however agreeable to use and work best in telescopes with a large focal length. You will get value for money with these eyepieces and they cost between £12 and £35 a piece.
Photo 6: Set of different eyepieces
Erfle eyepieces were invented during the Second World War. These eyepieces have 5 elements in their design and a very wide (60 degrees) image field. The problem is that they are prone to illusions and astigmatis, which makes them unusable for planet observations. Erfle eyepieces are also known as wide-field eyepieces.
Plössl eyepieces have four or five elements in their design. They have 50-degree image field and are user-friendly (except for the 10 mm and shorter lenses). They're optimal if they're between 15 and 30 mm. The quality is good, especially for observing planets. They suffer from astigmatism a little bit, particularly on the edges of the image field. These are however one of the most popular eyepieces.
Nagler eyepieces were introduced in 1982 and were promoted at the time with the slogan "like taking a spacewalk". They have a design with seven elements with an unbelievably wide image field (82 degrees). They are only delivered in 2 inches and are heavy, sometimes as much as 1 kg. In addition they are expensive.
Barlow lenses are a cheap way of increasing the maximum magnification of a telescope even more. You use a Barlow lens by placing another eyepiece inside it.
Photo 7: An eyepiece fits in the Barlow lens so that the magnification increases.
Another category of eyepieces are eyepieces with an illuminated iris. These eyepieces can have various designs and are exclusively used for astrophotography. They eyepieces are used to support the telescope while a photo is being taken. Taking a picture takes on average between 10 minutes and an hour.
Viewfinders are used to aim the telescope on a specific object (for example a star or planet) that you want to observe. A viewfinder works in the same way as a scope on a gun. Viewfinders are supplied in three types:
- Peep sights- circles that help you get your observation into view.
- Reflex sights- a mirror box that shows the sky and illuminates the observation with a red LED point.
- Telescope sight- a small, low magnifying (5x to 10x) telescope that is mounted on the side of the telescope uses crosshairs to aim.
Photo 8: Viewfinder that can be mounted on the side of a telescope.
Viewfinders are usually supplied with the telescope. Some are also sold separately.
Filters & other accessories
Filters are pieces of glass that you place in the tube of your eyepiece. This filters the light waves and you will not see certain colours or see other colours better.
Filters are used to:
- To augment the image of nebula in a sky in areas with a lot of light pollution.
- Sharpen the contrast of small objects and details of the Moon and other planets.
- Be able to observe the sun.
(DO NOT LOOK DIRECTLY AT THE SUN WITHOUT A FILTER, THIS COULD PERMANENTLY DAMAGE YOUR EYES)
Photo 9: Mounting a filter in an ocular.
Photo 10: filter/ocular
If you are going to observe overnight when it is cold, dew can condense on your telescope, especially around or on the optical elements (mirror or lens). To prevent this you can use a dew shield. This is folded around the front of the telescope and extends the tube. As a result, the condensation is collected in the dew shield instead of in the tube. Some dew shields can even be heated so that the dew no longer condenses.
Your eye is probably the most necessary element for making observations. For most amateur astronomers, this is probably the only detector that they will ever need. However, you can also take pictures of your observations. You can do this by using a conventional lens of a camera or with CCD devices and/or digital cameras. Some astronomers use their telescope to make entire exact observations with photometers (devices to measure the light intensity), or with spectroscopes (devices to measure the wavelengths and light intensities of objects).
There are a number of practical considerations that you should take into account before purchasing a telescope. To use your telescope optimally in the future you should take the following factors into consideration:
There are less and less places in the UK and Europe that are completely dark. If you live in a city you will probably want to take your telescope a few miles out of the bright lights of the city, so that you can go to observe the stars in a totally dark environment. Should you want to do this (or you want to take the telescope on holidays) it is necessary that the telescope is not too heavy to carry. In addition, it is also important that the telescope fits into your car. It is also useful if you have a telescope that can be put together quickly and easily. Putting together a telescope in a dark environment can be very frustrating.
Some telescopes, such as reflectors, require periodic maintenance. The most common maintenance with reflectors concerns aligning the mirrors (also called collimation). The alignment of mirrors can be both a simple and a complex procedure, depending on each individual telescope. Sometimes in half open, and even more so with completely open telescopes, dirt particles can end up on the mirrors. Because of this you will need to align and clean the mirrors every now and then. With very expensive telescopes it can be worth it after a long time to re-aluminate the mirrors or even replace them.
When anot in use the telescope will need to be stored somewhere. This can be a problem in telescopes with large dimensions (such as a 250 mm Dobson telescope). Before you decide to buy a telescope, it is important to see what a telescope weighs and where you want to store the telescope when you are not using it. The space where you store the telescope must be free of dust and moisture. We advise you to cover the telescope with a cloth to prevent dust and dirt from entering.
Telescopes can vary greatly in price. They start at less than £40 and can go up to thousands of Pounds, depending on the type:
- Small reflectors (150mm or smaller in cross-section) £175 to £600.
- Achromatic refractors (50 to 80mm in cross-section) £140 to 450.
- Large Dobson reflectors (150mm to 460mm in cross-section) £180 to £1000.
- Catadioptric telescopes (150 to 280 mm in cross-section) £500 to £1400
- Apochromatic refractors (80 to 127 mm in cross-section) £900 to £5000
If you were to look at the price per diameter, so for every mm that the mirror or lens would be larger then the following telescopes would be roughly in this order (high priced to low priced):
- Apochromatic refractors (lens telescopes)
- Newton reflectors, catadioptric telescopes, achromatic refractors
- Dobson reflectors
Two things you must remember:
- However good the telescope is, you probably can not enjoy the telescope if you spend all your savings on it or take an additional mortgage on your house.
- Besides the telescope you will probably want to buy other accessories (such as additional oculars, another finder scope, filters or an motor so you can control your telescope automatically)
Generally, it is wise to buy as much 'diameter' as possible within your price range. However, the following dimensions are sufficient for most (starting) amateur astronomers:
- Refractor telescopes: 80mm
- Reflector telescopes: 10 to 20cm
- Catadioptric telescopes: 16 to 20cm
Different kinds of telescopes
You have probably looked up at the sky at night before, looking for constellations, stars or the moon. Maybe you already know how to locate a constellation, and you would now like to see the stars, the moon or other planets from up close. This is possible with a telescope.
A telescope is a device that allows you to magnify your view of objects. There are many different telescopes and many different price ranges. So how do you determine which telescope suits you best? And how can you make sure that you aren't disappointed when you use your telescope to look up at the stars?
A telescope is a fantastic device that has the power to make far away objects seem very close. They come in a variety of sizes and prices; ranging from a small child's telescopes that you can buy relatively cheaply, to the famous Hubble Space telescope that cost over 1.7 billion Pounds to construct. Telescopes for adult amateurs are somewhere between those two in terms of size and price!. Even though they cannot magnify as well as the Hubble telescope does, a decent telescope is still capable of unbelievable things. With a small telescope measuing only 15cm long you can read the writing on a Pound coin from 55 metres away!
Most of the telescopes available on the internet or in stores nowadays can be split in three different types:
A refractor telescope is based on lenses, like a pair of binoculars, and for that reason refractor telescopes are also known as lense telescopes. A reflector telescope meanwhile works by using mirrors, and as a result is otherwise known as a mirror telescope, or sometimes as a Newton telescope. This is in honour of Sir Issac Newton, the British-born physicist and astronomer who invented the reflector telescope in 1668.
The three different types of telescopes mentioned above work in completely different ways.
How does a telescope work?
In order to understand how a telescope works, ask yourself the following question- why are you not able to read the text on a Pound coin when the coin is 55 metres away? The answer to this question is simple: the Pound coin does not take up enough place on the screen inside your eye (the iris). To speak in digital camera terms, the coin does not cover enough pixels on your iris for you to read the text.
If you had a bigger (physical) eye, then you would be able to capture more of the light reflecting off the surface of the Pound coin, giving you a sharper and clearer image of it. You could then be able to focus in on the Pound coin, so it would cover more pixels on your iris.
This is exactly the effect youn get with a telescope, and two parts in particular make it possible:
- The lens (in refractor telescopes) or the mirror (in reflector telescopes) is capable of capturing much more light from an object that is far away, and concentrates that reflected light (and image) into one point: the focal point.
- The eye piece gathers the light from the focal point of the telescope and diffuses it again (magnifies it), so that it covers a larger part of your iris. The eye piece works roughly in the same way as a magnifying glass that you would hold in your hand to read small written text./li>
When the main lens (in a refractor telescope) or the main mirror (in a reflector telescope) is combined with the eye piece, then we have a telescope. The idea is simple: capture as much light as possible and focus it onto a specific point within the telescope (the focal point), and in turn diffuse it again (with the eye piece) so that it covers enough space on your eye's iris.
The performance of a telescope
The performance of a telescope in general depend on two things:
- How well the telescope can collect the light
- How well the telescope can magnify.
The power of the telescope to collect light depends entirely on the diameter of the lens or mirror that is being used. The larger the diameter of the lens or mirror, the more light can be collected, and the sharper the final image will be.
The power of a telescope to magnify images depends on the lenses used. The eye piece handles the magnification, and as you can decide which eye piece to use and thus determine the magnification, the lens diameter of a telescope is much more important than the magnification.
What is a refractor telescope?
A refractor telescope uses lenses to carry out astronomical observations, and as a result is also known as a 'lense telescope'. The refractor was the very first optical instrument used for astonomical observations, by famous Italian astronomer Galileo Galilei during the first part of the 17th century.
Image 1: A refractor telescope (also known as a lense telescope)
The refractor uses a lens that is placed in the front of the tube. Light passes through the lens and is deflected toward the focal point, where the image is formed. The lense telescope is a simple, durable and optically speaking (almost) perfect instrument. A well-built refractor telescope in fact has only one real optical issue, and that is what is referred to as 'chromatic aberration'.
Chromatic aberration, or colour separation, occurs because a single lens is not able to focus all the different colours that make up white light into a single focal point. In short, this causes separate images of the object being observed to be created, for instance in red, blue and green. Then if you try to focus in on the blue, the green and red images of the object remain out of focus. This can be seen as a red and/or green blur of light around the edge of the object being observed.
To prevent this colour issue, two remedial measures were taken. The first was to make the telescope as long as possible (creating long focal lengths) in order to force the light 'around the corner' as little as possible to reach the focal point. The second corrective action was to use two lenses made from different types of glass. These 'Achromatic lenses telescopes' contain a so-called Fraunhoferlens.
Advantages of refractors
- Produces good images of planets.
- Planet details are displayed fantastically.Very affordable for lenses smaller than 10 cm (100mm).
Disadvantages of refractors
- Suffers from colour issues (chromatic aberration).
- Very expensive for lenses larger than 10 cm (100mm).
- For viewing nebulas the refractor is not suitable, because there is not enough light.
What is a reflector telescope?
A reflector telescope uses mirrors to make astronomical observations and is also called 'mirror telescope' or 'Newton telescope'. The first practical, usable reflector was designed in England by Sir Isaac Newton in 1668.
Photo 2: Design of a reflection telescope (also known as mirror- or Newton- telescope)
The reflector uses a hollow mirror in the back of the telescope, that reflects the incoming light and focuses it in a focal point on a supplementary mirror. The supplementary mirror reflects the light into an eye piece, through which you can observe the universe. Because a reflector uses a 'supplementary mirror' it is often said that a reflector doesn't perform as well as a refractor, because a small disturbance in the image can occur. A lot of manufacturers solve this however, by putting a larger mirror in the back of the telescope in order to minimize the disturbance.
The actual reason why reflectors sometimes have a worse reputation than refractors is that the quality of the construction is sometimes not as good as it should be. The reflector has only one active optical element (the mirror), so a lot depends on that single element. The refractor meanwhile has multiple mirrors, so that if the first mirror does not give a perfect image, it can be corrected by the other (remaining) mirrors. This is not the case with the reflector, because everything depends on just one mirror.
Advantages of reflectors
- Reflectors are more affordable than refractors of the same size.
- Reflectors do not suffer from colour issues.
- Reflectors are very suitable for deep-sky observations.
Disadvantages of reflectors
- The telescope has to be collimated every now and then.
- Collimation means the lining up correctly of the mirrors.
- Because a 'supplementary mirror' is used, there is less detail on planets than you would see on a refractor (lense telescope)
- A reflector is not as suitable for observing planets.
What is a Catadioptric Telescope?
A Maksutov-Cassegrain/Catadioptric telescope uses mirrors and lenses to perform astronomical observations, and is thus a crossover between a reflector and a refractor. The Maksutov-Cassegraintelescope was designed by Russian/Soviet optical engineer Dmitri Dmitrievich Maksutov in around 1941. The idea behind this type of telescope is to combine the advantages of the reflector telescope with the advantages of the refractor telescope. Maksutov tried to design an 'ideal' telescope; a telescope without chromatic aberration, and a short tube, so that the telescope could be transported easily. Besides that, the idea behind the Maksutov-Cassegrain telescope was to create a telecope with a long focal length without high costs.
Photo 3: A Maksutov-Cassegraintelescope (also known as a catadioptric telescope)
The Maksutov-Cassegrain uses a hollow mirror in the back of the telescope to reflect the incoming light rays onto a double hollow lens (the Meniscus-lens). In the back of the Meniscus-lens is a thin layer of aluminum, that serves as a capture mirror. This capture mirror reflects the light to the eyepiece, through the hole in the main mirror.
The main benefit of the Maksutov-Cassegrain is that the light first passes through the Meniscus-lens, so that image issues are corrected.
Advantages of Maksutov-Cassegrain
- Affordable reflector telescope with the same optical quality as a refractor.
- No more heavy, long telescope tubes.
- No colour issues.
- A good all-rounder.
- Compact construction.
Disadvantages of Maksutov-Cassegrain
- Reflectors are relatively cheaper, so for the same price you could buy a reflector with a larger diameter.
- The corrective lens on the front of the Maksutov-Cassegrain has a tendency to fog up if nothing is done to prevent this.