Altazimuth Telescope Mounts

A telescope mount serves two purposes: it supports the telescope and keeps it steady, and to allows it to swivel around so that you can point it to objects in the night sky. The cost of a good quality mount can often form a large proportion of the overall cost of a telescope. There are two types of mount: the altazimuth mount, and the equatorial mount. Here we discuss the former.

The altazimuth mount is the simpler design and the cheapest to construct. In operation it essentially resembles the mount of a cannon: it allows for up-down motion of the telescope tube (known as the altitude), as well as horizontal motion (known as the azimuth). Many entry level refractor telescopes employ an altazimuth mount, which usually sits on top of a tripod. Dobsonian telescopes are also of the altazimuth type, here the resemblance to a cannon is most striking.

The advantage of the altazimuth mount is its simplicity, ease of use, and cheaper construction (especially true of Dobsonians which tend to be the cheapest telescopes of any given size).

One disadvantage of altazimuth mounts is that they don't have the ability to automatically track objects as they move during the course of the night due to the earth's rotation. In order to keep objects in the field of view one needs to continually adjust the telescope's orientation in both the up-down and left-right directions. (This is less of a problem with equatorial mounts, which are designed to allow for easier tracking by adjusting along just one axis). However, in recent years computerized altazimuth mounts have appeared which use motors to automatically compensate for the sky's motion, though these are generally a great deal more expensive.

When deciding which telescope mount to buy, one important factor to consider is how good the mount is at damping vibrations. The slightest vibration in the telescope apparatus will result in dancing images which are unsuitable for viewing; a good mount should therefore provide a rock solid support for the telescope which minimises vibration.

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Aperture Size Versus Magnification

A common misconception among those new to astronomy is that magnification is the most important factor when it comes to choosing an astronomical telescope. This is simply NOT true. Telescopes which are advertised on the basis of high magnification are usually of poorer quality and should therefore be viewed with suspicion. While the ability to magnify an image is of course necessary, the most important feature of a telescope is its light gathering ability. The objects we see in the night sky are very distant from us and therefore appear very faint. The primary purpose of the telescope is to gather as much of the incoming light as possible, in order to show these faint objects better.

The measure of a telescope's light gathering ability is given by its aperture size. The aperture size is the diameter of the telescope's primary mirror, or lens. The larger the aperture the brighter the images the telescope will give. (This is why many of the telescopes used in astronomy research are very large, this allows more and more faint objects to be brought into view which would otherwise not be visible in a smaller aperture telescope). It is often stated that the mimimum useful aperture for astonomical purposes is 3" for refracting telescopes, and 6" for reflecting telescopes.

So what role does magnification play? Magnification enlarges the image so that distant objects appear to be closer, thereby allowing us to study them in greater detail. However magnification comes at a price: each time an image is magnified, the already scarce light which was gathered by the telescope is `diluted', so that the image appears dimmer. In addition, the resolving power of a telescope is limited by the quality of the optical components and by a physical effect known as diffraction; so that even if we push up the magnification, no extra detail will be revealed. At very high magnifications images therefore appear blurred and dim.

So what is the maximum usable magnification of a telescope? This depends on many factors, including: the aperture size, the quality of the optical components, the degree of atmospheric disturbance at the time (which causes the image to appear `wavy' at higher magnification - analogous to rising hot air from a radiator), the location above sea-level (if you are at the top of a mountain, the starlight travels through less atmosphere to reach you so that images appear brighter and more steady). As a rule of thumb the maximum useful magnification (under ideal seeing conditions, and excellent optics) is approximately 50x per inch of aperture. This means that the maximum magnification for, say, a 6" reflecting telescope will be 300x.

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Dobsonian Telescopes

The Dobsonian Telescope (or Dob) is credited to John Dobson who first started using them in the 1950s in an attempt to bring astronomy to the public. The Dob is essentially just a Newtonian Reflecting Telescope tube mounted on an inexpensive mount (called an alt-azimuth mount) which holds the tube like a cannon, allowing it to easily swivel up-down and left-right. The Dobsonian therefore shares many of the good/bad points of the Reflecting Telescope (click on the link for more information). The mount is usually of wooden construction, and, since the mount consitutes a significant proportion of the cost of a telescope, Dobsonians tend to be much cheaper than other similarly sized telescopes.

One big advantage of the Dobsonian is its ease of use: you can be outside and set up ready for viewing within minutes. The simplicity of the mount means you just point the telescope to where you want to look and that's it, no complicated polar alignment is required (as is the case with equatorial mounts). You will need to keep nudging the telescope to keep the object you're looking at in view as the night sky revolves, but in practice one gets used to that quite quickly.

The compact nature of the Dob's mount means that the Dobsonian is highly portable, and smaller models can easily be taken by car to dark-sky locations.

The low centre of gravity of the Dobsonian means the telescope is very stable and doesn't suffer as much from vibration, unlike some other tripod-mounted telescopes (vibrations cause the image in the eyepiece to dance around and one must wait for them to damp down before viewing - in a good mount vibrations will die away quickly leaving a steady image).

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Refracting Telescopes

The Refracting Telescope (also known as the refractor) precedes the Reflecting Telescope and was invented in 1608 by three Dutch spectacle makers. It was first used for astronomical purposes by Galileo a few years later, and it is still one of the most telescope designs due to its simple construction and ease of use.

The Refracting Telescope consists of a long hollow tube with a large lens, known as the objective lens, situated at the front end and an eyepiece at the opposite end. Starlight enters the telescope through the objective lens which gathers it and focuses it into the eyepiece. The eyepiece magnifies the image thereby making distant objects appear closer.

The Refracting Telescope has several advantages:

• its simple construction makes the refractor more sturdy and is therefore less liable to go out of alignment than a Reflecting Telescope; indeed a refractor can go many years without needing any realignment
• small refractors are extremely portable and very easy to set up, an important consideration if you need to travel long distances to reach a dark-sky site
• the telescope tube assembly is sealed at both ends (by the lenses), therefore providing protection against dust and moisture
  

Some disadvantages of Refracting Telescopes:

• all Refracting Telescopes suffer from chromatic aberration, which leads to false colour appearing around images, due to the fact that they use lenses to focus light
  
• small errors in the grinding of the lens can result in distorted images
• larger telescopes are less portable

Chromatic aberration (false colour) occurs because the objective lens focuses different frequencies of light at different distances. In the achromatic refractors the lenses are specially designed to reduce this effect; most refractors available to buy are achromatic refractors. Apochromatic refractors go even further and using exotic glasses they nearly eliminate chromatic aberration altogether; they are thought by many to provide the best image quality of all telescope designs, however they are generally very expensive (eg upwards of $500/£250 for a small 80mm refractor) and for that reason are not recommended for a first time buyer.

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Reflecting Telescopes

The Reflecting Telescope, also known as the Newtonian Reflector, was invented by Isaac Newton around 1680 and proved hugely successful. It remains one of the most popular telescope designs to this day.

The design is very simple: a large concave mirror (called the primary mirror), which sits at the back of the telescope tube, reflects incoming starlight back along the tube and brings it to a focus. A second small, flat mirror (the secondary mirror, or flat), which is angled at 45 degrees to the primary, then intercepts the focused light and directs it into the eyepiece located on the side of the telescope tube. The eyepiece magnifies the image thereby making distant objects appear closer.

The Reflecting Telescope has several advantages over the Refracting Telescope:

• it is simpler and cheaper to produce, which means for the same price Reflecting Telescopes will have a larger aperture
• larger apertures means larger light gathering capacity, which in turn means brighter images of faint, deep-sky objects
• Refracting Telescopes suffer from chromatic aberration (false colour around images) due to the fact that they use lenses to focus light; Reflecting Telescopes use a mirror to focus the incoming light and therefore do not have this problem.

Some disadvantages of Reflecting Telescopes:

• the mirrors tend to go out of alignment relatively quickly and will need to be periodically realigned, however this is a fairly simple process (compare to Refracting Telescopes which can go for many years without the need for realignment)
• all Reflecting Telescopes suffer from light loss due to the presense of the secondary mirror in the light path, and the fact that the mirrors only reflect around 90% of the incoming light
  
• the reflective coating on the mirrors will fade with time, which means the mirrors will need to be resurfaced every few years to maintain good reflectivity
  
• small errors in the grinding of the mirrors can result in distorted images
• larger telescopes are less portable, an important factor if you intend to transport your telescope to a dark-sky site by car
  

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Introduction - Choosing and Buying a Telescope

Through a series of posts I will attempt to explain some of the basic facts about telescopes which will hopefully prove interesting to the newcomer to astronomy, and enable them to make a more informed choice about what telescope to buy. I intend to post more information here in the coming weeks and I will add links to any new posts at the bottom of this page, so check back regularly!

To those starting out in astronomy buying a first astronomical telescope can seem a little daunting. So many different types are available: reflector telescopes, refractor telescopes, dobsonians, some with computerised `goto' capability etc etc. What's more the difference in prices can be staggering, for example a small 50mm refractor can be bought for as little as $60 (or £30), while a top of the range 16" Meade LX200R would set you back a whopping $12,999 (or £6500)! So given this huge choice what type should I get? Is an expensive telescope really necessary for me, or will a cheaper model suffice? Should I buy a refractor, a reflector, or a dobsonian? These are just some of the questions I asked myself when I first started out in astronomy, after reading my posts below you will hopefully know the answers!

Click on a topic to find out more:

Main Page - Introduction
Reflecting Telescopes
Refracting Telescopes
Dobsonian Telescopes
Aperture Size Versus Magnification
Altazimuth Telescope Mounts