How do telescopes work?

Maybe you've been out looking at the stars in the night sky, searching for constellations; or maybe you've already learned your way around the constellations, and now you'd like to take a closer look at objects like the moon, planets or stars with the aid of a telescope.


Picture 1: Various telescopes and mounts.

A telescope is a device used to magnify distant objects. There are many types to choose from, and many price ranges to consider. How do you know which one is best for you? How can you be sure that you won't be disappointed when you take your new telescope out to see the stars?

A telescope is an amazing device that has the ability to make faraway objects appear much closer. Telescopes come in all shapes and sizes, from a little plastic tube you buy at a toy store for $2, to the Hubble Space Telescope, which weighs several tons. Amateur telescopes fit somewhere in between, and even though they are not nearly as powerful as the Hubble, they can do some incredible things. For example, a small 6-inch (15 centimeter) scope lets you read the writing on a dime from 150 feet (55 meters) away!

Most of the telescopes you see today come in one of two flavors:

• The refractor telescope, which uses glass lenses.
• The reflector telescope, which uses mirrors instead of the lenses.

Both types accomplish exactly the same thing, but in completely different ways.

To understand how telescopes work, let's ask the following question. Why can't you see an object that is far away? For example, why can't you read the writing on a dime when it is 150 feet (55 meters) away with your naked eyes? The answer to this question is simple: the object does not take up much space on your eye's screen (retina). If you want to think about it in digital camera terms, at 150 feet the writing on the dime does not cover enough pixels on your retinal sensor for you to read the writing.

If you had a "bigger eye," you could collect more light from the object and create a brighter image, and then you could magnify part of that image so it stretches out over more pixels on your retina. Two pieces in a telescope make this possible:

• The objective lens (in refractors) or primary mirror (in reflectors) collects lots of light from a distant object and brings that light, or image, to a point or focus.
• An eyepiece lens takes the bright light from the focus of the objective lens or primary mirror and "spreads it out" (magnifies it) to take up a large portion of the retina. This is the same principle that a magnifying glass (lens) uses; it takes a small image on the paper and spreads it out over the retina of your eye so that it looks big.

When you combine the objective lens or primary mirror with the eyepiece, you have a telescope. Again, the basic idea is to collect lots of light to form a bright image inside the telescope, and then use something like a magnifying glass to magnify (enlarge) that bright image so that it takes up a lot of space on your retina.


Picture 2: This is the simplest telescope design you could have. A big lens gathers the light and directs it to a focal point and a small lens brings the image to your eye.

A telescope has two general properties:

1. how well it can collect the light
2. how much it can magnify the image

A telescope's ability to collect light is directly related to the diameter of the lens or mirror -- the aperture -- that is used to gather light. Generally, the larger the aperture, the more light the telescope collects and brings to focus, and the brighter the final image.

The telescope's magnification, its ability to enlarge an image, depends on the combination of lenses used. The eyepiece performs the magnification. Since any magnification can be achieved by almost any telescope by using different eyepieces, aperture is a more important feature than magnification.

What is a refractor telescope?

The refractor telescope uses lenses for astronomical observations and is also called a "lens telescope" or "refractor". The refractor is the first optical instrument that was used for astronomical observations by Galileo around 1600.


Picture 3: A refractor telescope (also called a lens telescope)

The refractor uses a lens that is placed in the front of the tube. The light passes through the lens and is deflected to a focal point where the image is formed.
The lens telescope is a simple, sound and optical (almost) perfect instrument. A well-built lens telescope has, in fact, only 1 true optical error, which is indicated by the term "Chromatic aberration".

Chromatic aberration, also known as "purple fringing", is caused because a single lens is not capable to focus all colours in the optical spectrum at the same time. In short, this means that for example one will see a sep arate blue, a red and a green colour of the moon. Practically this means that if one wants to focus on the blue colour then the green and red will look a bit blurred. If one looks at the moon then this will result in a rainbow halo around the borders of the moon.

To minimize the effects of chromatic aberration two measures were taken about 10 years ago. The first measure was to give the telescope a long focal length to force the light as little as possible "around the corner". The second measure was to utilize two lenses of different kinds of glass. These "Achromatic lens telescopes" make use of a so-called 'Fraunhofer lens'.

Advantages of refractors

• Provides good images of planets
• Good for details of planets and binary stars.
• Affordable when the lens is smaller than 10cm (100mm)

Disadvantages of refractors

• Suffers from rainbow halo effects (chromatic aberration)
• Very expensive for lenses larger than 10cm (100mm)
• Refractor is less suitable to view nebulae, as this requires larger apertures because of the need for greater light collecting.

What is a reflector telescope?

A reflector telescope uses mirrors for astronomical observations and is also called a "mirror telescope", a "Newtonian telescope" or "reflector". The first practical application for astronomical observations was developed by Sir Isaac Newton in 1668.


Picture 4: A reflector telescope (also called a mirror telescope or Newtonian telescope)

The reflector uses a concave mirror that is placed in the back of the tube. The concave mirror reflects the light back to a much smaller mirror that is placed in the middle or front of the telescope. The little mirror then reflects the light to an eyepiece which in turn allows you to observe the universe. As a reflector utilizes a small mirror that is placed in the middle of the telescope it is said that the reflector performs not as well as the refractor because a small part of the light is blocked.

The real reason why reflectors sometimes have a lesser reputation than the refractor is because the building quality. The reflector has only 1 optical active element present (the mirror), which means that this mirror has to be of excellent quality. A refractor (lens telescope) has multiple lenses which means that if the first lens does not perform perfectly this can be corrected by the other lenses. With a reflector (mirror telescope) this is not possible because all the light is reflected by just one mirror. More expensive telescopes will have better and bigger mirrors than cheaper telescopes.

Advantage of reflectors

• Reflectors are cheaper than refractors (if they have the same aperture)
• Reflectors do not have chromatic aberration (colour shifting) while refractors do.
• Good for deep sky observations

Disadvantages of reflectors

• The telescopes need to be collimated once a while (line up the focal distances of the mirrors)
• Because the reflector uses a very small mirror in front of the telescope (secondary mirror) this causes a little bit of light loss which makes it refractors (lens telescopes) better for planetary observation

What is a Maksutov-Cassegrain Telescope?

A Maksutov-Cassegrain telescope uses mirrors and lenses for astronomical observations and is thus a combination of a refractor telescope and reflector telescope. The Maksutov-Cassegrain telescope was designed by Maksutov in Russia around 1940. The idea behind this kind of telescopes stems from combining the advantages of the mirror telescope (reflector) and the lens telescope (refractor). Maksutov has tried to create the 'perfect' telescope; a telescope without the rainbow halo effect (chromatic aberration) that can be existent in refractors, a short and closed tube (easy to take with you), a long focal point (higher magnifications) and not too expensive to produce.


Picture 5: A Maksutov-Cassegrain telescope (also called a dioptric telescope)

The Maksutov-Cassegrain uses a hollow mirror in the back of the telescope that reflects the light beams back to a lens (Meniscus lens) that corrects any image imperfections. The lens is covered with a small piece of aluminium which reflects the light to the eyepiece. The advantage of Maksutov-Cassegrains is that the light first passes through the Meniscus lens which corrects any image imperfections.

Advantages of Maksutov-Cassegrains

• Cheaper than a refractor telescope with the same optical qualities as a more expensive refractor telescope.
• No long tubes because the light travels a number of times through the same tube which still allows for a long focal distance.
• No chromatic aberrations that are apparent in refractor telescopes.
• A good all-rounder
• Compact building

Disadvantages of Maksutov-Cassegrains

• Reflectors are relatively cheaper, and you can therefore buy a reflector with a larger aperture for the same money as a Maksutov Cassegrain telescope.
• The lens that is placed in the front side of the telescope tends to make the telescope have a long cool down period, unless measures are taken accordingly (such as a dew cap)

Which telescope do I need?

Once you have decided that you want to buy a telescope you have many to choose from, we will first help you to decide what kind of telescope you need and then explain you all the different features a telescope has.

The type of telescope that you need depends mostly on the observing you want to do. Many amateur astronomers own more than one telescope, each specialized for a different type of observing. But if you are a beginner, you might want to look for a telescope that you can use for several different activities.

Remember that there are three basic types of telescopes:

• Refractor telescopes – a lens is the primary device for gathering light.
• Reflector telescopes – a mirror is the primary device for gathering light.
• Catadioptric telescopes – a combination of lenses and mirrors is used to gather light.

Each type has advantages and disadvantages with respect to optical quality, mechanical performance, maintenance, ease of use and price.

To help with matching the telescope type to the type of observing you plan to do, we have prepared a table that relates the design and aperture to the observing use (moon, planets, deep-sky, etc.).


Table 1: Performance indicators of various types of telescopes

As you can see the quality of the observation generally increases with the increase in aperture (diameter of the lens or mirror). However the price of the telescope also increases when the aperture increases. You should therefore decide for yourself where you want to use it for and how much you are willing to pay for a telescope. At telescope-expert.co.uk we can offer you a telescope in any price range.

Generally, refractor telescopes are good for lunar and planetary observing, while reflector telescopes are good for deep-sky observing. Catadioptric telescopes are good general observing instruments.

You should also consider where you will do most of your observing:

• Light-polluted urban skies - catadioptric telescopes and refractors tend to do better than reflectors.
• Moderately light-polluted suburban skies - all types tend to be equal.
• Dark, rural skies - catadioptric telescopes and reflectors tend to be slightly better than refractors because they are better able to collect light.

What kind of features does a telescope have?

The features of a telescope determines its price and the quality of your observations.
We illuminate the following features of a telescope that are important for you to consider:

1. Optical features (how a telescope captures and focuses light)
2. Non-optical features (the hardware such as the mount that can really make a difference)
3. Mounts
4. Eyepieces
5. Finders
6. Filters & other accessoriesL
7. Practical considerations (portability, maintenance, storage and price ranges)

Optical features

The telescope's ability to collect light is directly related to the optics that are used. Telescopes with poor quality optics can be very frustrating to use. Here are some optical considerations to think about when buying a telescope:

• Aperture
• Magnification
• Focal length
• Focal ratio (f/number)

Aperture

The telescope's ability to collect light is directly related to the size (diameter) of the objective lens or primary mirror. Generally, the bigger the lens or mirror, the more light the telescope collects and brings to focus, and the brighter the final image. Aperture is probably the most important consideration when buying a telescope, but it is not the only consideration. You want to try to purchase as much aperture as you can reasonably afford; however, you should also consider other factors that will be discussed below, including size, weight, storage space and portability. The biggest telescope is not always the best one for you!

Magnification

This consideration is perhaps the most misleading to novice telescope buyers. Often, manufacturers of "cheap, department store" telescopes will display "200x power or more" on the boxes of their products. The magnification or power has little to do with the optical performance of the telescope, and is not a primary consideration.

The telescope's ability to enlarge an image (magnification) depends upon the combination of the lenses used, usually a long focal length objective lens or primary mirror in combination with a short focal length eyepiece. As the magnification of an image increases, the field of view and the brightness of the image decrease.

A general rule about magnification is that the telescope's maximum magnification is 40x to 60x (average = 50x) per inch of aperture (or 2x total millimeters aperture). Since any magnification can be achieved for almost any telescope by using different eyepieces, aperture becomes a more important feature than magnification. Furthermore, most astronomical objects are best viewed on a low magnification or power to gather the most light possible.

Focal length

Focal length is the distance required by the objective lens or primary mirror to bring all of the light collected to one point (the focus or focal point). The focal length of the lens or mirror is usually printed somewhere on the telescope's tube; if not, it should be found in the instructions or on the box.

The focal length is an important number to know. As discussed above, magnification depends upon the focal length of the objective lens or primary mirror and the focal length of the eyepiece. Generally, long focal length telescopes are capable of delivering higher magnifications than short focal lengths. However, do not mistake the length of the telescope tube for the focal length, because compound telescopes have a folded light path, which delivers a long focal length in a short tube.

Focal Ratio (f/number)

Focal ratio or f/number relates to the brightness of the image and the width of the field of view. The focal ratio is the focal length of the objective lens or primary mirror divided by the aperture. The focal ratio concept comes from the camera world, where a small focal ratio means a short exposure time for the film, and was said to be "fast." Although the same is true for a telescope, if a "fast" and a "slow" telescope are compared at the same magnification for visual rather than photographic viewing, then both telescopes will have the same quality image. Generally, the following information about focal ratios can be helpful:

• f/10 or higher - good for observing the moon, planets and double stars (high power)
• f/8 - good for all-around viewing
• f/6 or lower - good for viewing deep-sky objects (low power)

Non-optical features

There are other parts of a telescope, besides the optical components, that you'll want to consider:

Eyepiece holder

Eyepieces come in three diameters -- 0.965 inches (2.45 cm), 1.25 inches (3.18 cm), and 2 inches (5.08 cm). However, the eyepiece holder in your telescope is fixed. The "cheap, department store" telescopes tend to have 0.965-inch holders. Most telescopes have 1.25-inch holders. Some have 2-inch holders. Eyepieces of differing diameters are not usable in all telescopes. Make sure that the eyepiece holder in your telescope can support the eyepieces you wish to purchase. You typically do not have an option on the size (i.e. the manufacturer sets the size with the telescope model), but you should definitely know what it is because it will affect other choices you'll be making.

The diameter of the eyepiece holder can be found in the description (diameter of eyepiece) of the telescope on our website (telescope-expert.co.uk)

Mounts

Telescopes must be supported by some type of stand, or mount -- otherwise you would have to hold it all of the time. The telescope mount allows you to:

• keep the telescope steady
• point the telescope at the stars or other objects (birds)
• adjust the telescope for the movement of the stars caused by the Earth's rotation
• free your hands for other activities (focusing, changing eyepieces, note-taking, drawing)

There are two basic types of telescope mounts:

• Alt-azimuth
• Equatorial

Picture 6: Various telescopes and mounts.

Alt-azimuth

The alt-azimuth mount has two axes of rotation, a horizontal axis and a vertical axis. To point the telescope at an object, you rotate it along the horizon (azimuth axis) to the object's horizontal position, and then tilt the telescope, along the altitude axis, to the object\'s vertical position. This type of mount is simple to use, and is most common in inexpensive telescopes. The alt-azimuth mount has two variations:

• Ball and socket – used in two inexpensive rich-field telescopes. It has a ball shaped end that can rotate freely in the socket mount.
• Rocker box – a low center-of-gravity box mount, usually made of plywood, with a horizontal circular base (azimuth axis) and Teflon bearings for the altitude axis. This mount is usually used on Dobsonian telescopes. It provides good support for a heavy telescope, as well as smooth, frictionless motion.

Although the alt-azimuth mount is simple and easy to use, it does not properly track the motion of the stars. In trying to follow the motion of a star, the mount produces a "zig-zag" motion, instead of a smooth arc across the sky. This makes this type of mount useless for taking photographs of the stars.

Equatorial

The equatorial mount also has two perpendicular axes of rotation -- right ascension and declination. However, instead of being oriented up and down, it is tilted at the same angle as the Earth\'s axis of rotation. The equatorial mount comes in two varieties:

• German equatorial mount - shaped like a "T." The long axis of the "T" is aligned with the Earth's pole.
• Fork mount - a two-pronged fork that sits on a wedge that is aligned with the Earth\'s pole. The base of the fork is one axis of rotation and the prongs are the other.

When properly aligned with the Earth's poles, equatorial mounts can allow the telescope to follow the smooth, arc-like motion of a star across the sky. Also, they can be equipped with:

• setting circles - allow you to easily locate a star by its celestial coordinates (right ascension, declination)
• motorized drives - allow you or your computer (laptop, desktop or PDA) to continuously drive the telescope to track a star.

You need an equatorial mount for astrophotography.

Eyepieces

An eyepiece is the second lens in a refractor telescope, or the only lens in a reflector telescope. Eyepieces come in many optical designs, and consist of one or more lenses in combination -- they are almost like mini-telescopes themselves. The purposes of the eyepiece are to:

• produce and allow you to change the telescope's magnification
• produce a sharp image
• provide comfortable eye relief (the distance between your eye and the eyepiece when the image is in focus)
• determine the telescope's field of view:
  • apparent - how much of the sky, in degrees, is seen edge-to-edge through the eyepiece alone (specified on the eyepiece)
  • true or real - how much of the sky can be seen when that eyepiece is placed in the telescope (true field = apparent field/magnification)

There are many types of eyepiece designs:

• Huygens (indicated with the letter H on telescope-expert.co.uk)
• Ramsden (indicated with the letter R telescope-expert.co.uk)
• Orthoscopic (indicated with the letter O on telescope-expert.co.uk)
• Kellner and RKE (indicated with the letter K on telescope-expert.co.uk)
• Erfle (indicated with the letter E on telescope-expert.co.uk)
• Plössl (indicated with the letter P on telescope-expert.co.uk)
• Nagler (indicated with the letter N on telescope-expert.co.uk)
• Barlow (used in combination with another eyepiece to increase magnification 2 to 3 times)

Picture 7: Schematic diagrams of various eyepieces.

Huygens and Ramsden eyepieces are the oldest designs. They suffer from chromatic aberrations and are often included with the least expensive and least effective telescopes.

Orthoscopic eyepieces were invented by Ernst Abbe in 1880. They have four elements and a 45-degree apparent field of view, which is somewhat narrow. The optical design gives a crisp view, has a good eye relief, and is considered excellent for planetary viewing. Orthoscopic eyepieces can range from $50 to $100 each.

Kellner and RKE(Edmund Scientific's patented modification of Kellner) are a three-element design that produce images in a 40-degree field of view, with some chromatic aberration. They have good eye relief. Kellner eyepieces work best in long focal length telescopes. They are a good balance between performance and economy. They vary from €30 to €50 each.


Picture 8: Set of various eyepieces

Erfle eyepieces were invented during World War II. They have a five-element design and a wide, 60-degree field of view. They suffer from ghost images and astigmatism, which makes them unsuitable for planetary viewing. Improvements on the Erfle design are called wide-field eyepieces.

Plössl eyepieces have a four-element or five-element design, with a 50-degree field of view. They have good eye relief (except for 10 mm and shorter lenses). They work best in the 15- to 30-mm size. The quality is good, especially for planetary viewing. They have some astigmatism, especially at the edge of the field. They are popular eyepieces.
Nagler eyepieces were introduced in 1982, advertised as "like taking a spacewalk." They have a seven-element design with an incredible 82-degree field of view. They come in 2-inch barrel size only, and are heavy -- up to 2 pounds (1 kg) -- and expensive.

Barlow lenses can be an economical way to increase magnification and/or provide better eye relief with an existing eyepiece. The eyepiece fits into the Barlow lens, which then fits into the eyepiece holder.


Picture 9: An eyepiece fits into a Barlow lens to increase its magnification

One final category of eyepiece is the eyepiece with illuminated reticles. These eyepieces come in many designs, and are used exclusively for astrophotography. They aid in guiding the telescope to track an object during a film exposure, which can take anywhere from 10 minutes to an hour.

Finders

Finders are devices used to help aim the telescope at its target, similar to the sights on a rifle. Finders can come in three basic types:

• peep sights - notches or circles that allow you to line up the target
• reflex sights - a mirror box that shows the sky and illuminates the target with a red LED diode spot, similar to a laser sight on a gun
• telescope sight - a small, low magnification (5x to 10x) telescope mounted on the side with a cross hair reticle, like a telescopic sight on a rifle

Picture 10: View finder that can be mounted on the side of a telescope

Some finders come standard on telescopes, while others are sold separately.

Filters & other accessories

Filters are pieces of glass or plastic that you can place in the barrel of an eyepiece to restrict the wavelengths of light that come through in the image.


Picture 11: Set of filters

Filters can be used to:

• enhance the viewing of faint sky objects in light-polluted skies
• enhance the contrast of fine features and details on the moon and planets
• safely view the sun (see Observing the Sun for details)

Picture 12: Screwing filter into eyepiece


Picture 13: Completed filter/eyepiece combination

Dew caps

Because you will observe during the night, when it may be cool, moisture can condense in your telescope and on the optics. To prevent this, you can use a dew shield, which wraps around the front end of the telescope. The shield extends the length of the tube and allows moisture to condense on the inside of the shield rather than in the tube. Some shields can be heated to prevent moisture from condensing at all.

Other detectors

Your eye is the principal light detector for any telescope. For most amateur astronomers, this is the only detector that they will ever need. You might want to take photographs of what you see, however, and you can do that with conventional lens and film cameras or with CCD devices/digital cameras. Some astronomers use their telescopes to make scientific measurements with photometers (devices to measure the intensity of light) or spectroscopes (devices to measure the wavelengths and intensities of light from an object)

Practical considerations

There are practical matters involved when purchasing a telescope. To get the most out of your purchase, these factors should also be considered:

• Portability
• Maintenance
• Storage space
• Price

Portability

Areas of dark skies are decreasing across Europe. If you are an urban astronomer, odds are that you will have to move your telescope to a site several miles away that has moderate-to-dark skies. If so, you need to make sure that it is light enough to carry in and out of your home and car, and that it will fit inside your car or van. Finally, you may want a telescope that needs minimal assembly (optics, mount) when you reach your observing site -- trying to assemble a telescope mount in the dark can be extremely frustrating.

Maintenance

Some telescopes, like reflectors, require periodic maintenance. The most common maintenance with reflectors is keeping the mirrors aligned, or collimated. This can be a simple or complicated procedure, depending upon the individual telescope. Sometimes, especially with open-ended or completely open telescopes, dust may enter the tube and settle on the primary or secondary mirrors; these mirrors may have to be cleaned and re-aligned. Finally, mirror surfaces can degrade with time, and may require re-aluminizing or replacement.

Storage

When not in use, telescopes must be stored somewhere. This can be a definite problem with a large aperture telescope like a 10-inch Dobsonian reflector. You want to find a place with sufficient room, that is as dust-free and moisture-free as possible. Store the telescope covered to prevent dirt and dust from getting into it.

Price

Telescopes vary widely in price. They can range from a few hundred dollars to several thousand dollars, depending upon the type:

• small Newtonian reflectors (6-inch / 150mm aperture or less) - £130 to £500
• achromatic refractors (2- to 3-inch / 50 to 80mm aperture) - £130 to £500
• large Dobsonian reflectors (6- to 18-inch / 150mm to 460mm aperture) - £150 to £1,020
• catadioptric telescopes (6- to 11-inch / 150 to 280mm aperture) - £500 to £1,500
• apochromatic refractors (3- to 5-inch / 80 to 127mm aperture) - £1,000 to £5,100

You can also consider price per unit aperture, and they would rate from high to low as follows:

1. apochromatic refractors
2. Newtonian reflectors, catadioptric telescopes, achromatic refractors
3. Dobsonian reflectors

Two things to remember:

• No matter how good the telescope quality is, you probably won't enjoy it if you have to bankrupt your savings or remortgage your house to pay for it.
• You will have to purchase other things to complete your observing equipment (eyepieces, finders, filters).

Generally, you should buy as much aperture as you can reasonably afford. But for most observers, the following sizes will be more than sufficient:

• Refractors: 3 inches / 80 millimeters
• Reflectors: 4 to 8 inches / 10 to 20 centimeters
• Catadioptric telescopes: 6 to 8 inches / 16 to 20 centimeters