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TELESCOPES - OVERVIEW AND TELESCOPE TYPES
THREE GOLDEN RULES
The most serious rule of telescope use is obviously "never look at the Sun through a telescope". But when it comes to choosing a telescope in the first place, there are 3 golden rules to follow -
1. Buy your telescope from a specialist astronomical shop or store.
2. Buy a brand name with a good reputation, ie Orion, Meade, Celestron, Skywatcher, etc
3. Do research (such as reading the advice on this web site) to ensure that you get the telescope that suits you best, that is, the telescope that you will use most often.
The last rule is very important. The telescope you choose must be one that meets your aspirations, so that you will want to use it often. But, at the same time, the telescope must not be so big, heavy, or so difficult to set up, that you quickly loose interest for that reason.
Only you can decide the telescope that's right for you. And you can do this only by having some knowledge of the factors involved. This page (below) gives an overview. For more detail, click on the links in the text below, or use the menu at left.
The most important aspect of any telescope is its aperture. That is, the diameter of its main primary lens or mirror. The bigger the aperture, the more light that will be collected, and the sharper the view will be. For example, an 8 inch aperture telescope will collect 4 times as much light as a 4 inch diameter telescope, so the image seen will appear four times brighter. Also, the 8 inch telescope will have twice the "resolution" of the 4 inch telescope. That is to say, its views will be twice as sharp, and finer detail will therefore be visible.
FOCAL LENGTH AND MAGNIFICATION
The diagram (right) shows how light from distant stars is brought to a focus, to form an image. This image is then viewed by the eye by means of a small eyepiece lens.
The focal length is the distance from the primary lens or mirror to the image it forms. In our example this focal length is 1000mm. The focal length of our eyepiece is 25mm.
The linear magnification of the telescope is calculated by dividing the focal length of the primary by the focal length of the eyepiece.
In our example, magnification = 1000/25 = 40 (written "40x").
Now, this 40x magnification is linear, that is objects will appear 40x taller, AND 40x wider. So the area magnification is 1600x. But astronomers always refer to the linear magnification.
So, don't get hung up about magnification. You can change the magnification of telescope simply by changing the eyepiece. Also, high magnifications should be avoided as far as possible because -
1. The higher the magnification, the dimmer the image will be.
2. The higher the magnification the smaller will be the field of view
3. High magnifications magnify any optical defects, and any instability in the telescope mounting
4. Resolution is determined by the aperture, so increasing the magnification will eventually result in blurred images
As a rule of thumb, the useful highest magnification (in perfect conditions) is 50 times the aperture in inches, or twice the aperture in millimeters.
So you should not use a power greater than 200x on a 100mm aperture telescope, or 400x with a 200mm aperture.
After aperture, the next most important aspect of a telescope is its focal length, or more particularly, its "focal ratio". Now the focal ratio of any primary lens or mirror is simply its focal length divided by its aperture.
So, in our example, where the focal length is 1000mm, and the aperture 100mm,
focal ratio = 1000mm/100mm = 10 (usually written "f/10" or simply "f10")
So, assuming the same aperture, a telescope with a longer focal ratio will produce larger images, but will have a narrower field of view, and dimmer images. So those telescopes would be better suited to viewing brighter objects requiring high magnification, like the planets. Also, generally speaking, the longer the focal length of a primary, the better the quality of the images. A specialist telescope for planetary use would be f10 or more.
On the other hand, the shorter the focal ratio, the smaller the images, but the wider the filed of view, and the brighter the image. So better suited to lower-power wide field viewing and fainter objects. An F4 Newtonian would make a good specialist telescope for deep sky use.
All this means that, for a general purpose telescope, you should be looking for a focal ratio of roundabout f6.
A refractor is the normal type of telescope that everyone is familiar with - a long tube with a large lens at the top, and a small eyepiece lens at the opther end.
The main advantage of the refractor is that it has no central obstruction and can produce unbeatable sharp images that can take very high magnification. Refractors are often used for lunar and planetary observation at high magnification.
The second advantage of the refractor is that they are very rugged, requiring little in the way of maintenance, or the need for periodical realignment of the optics. Nor is it normally necessary to wait for a refractor is cool down to ambient temperatures before the optics will give of their best.
The main disadvantage is that refractors can suffer from chromatic aberration (color fringing of the image). Manufacturers can easily avoid this problem by the use of reasonably priced achromatic lenses (doublet lenses), provided the focal ratio is long enough. That's fine up to about 4 inches diameter or so. But, due to the long focal ratios required, achromatic refractors of 6 inches and over are unweildy and difficult and expensive to mount adequately.
The small aperture and long focal ratio makes most refractors unsuitable for viewing deep sky objects. Short Tube refractors are available, with a focal ratio of about f5. At lower magnifications, these give excellent views of deep sky objects. But they do not take higher magnifications well, and color fringing is evident. They do not make good all-round performers.
Lenses made of special glasses, and triplet lenses, are used in refractors at shorter focal ratios of about F7. These are really premium quality scopes, and can be affordable up to about 5 inch aperture.
So you may be tempted by a cheaper telescope design which uses a spherical mirror but includes a sub-aperture glass corrector lens either in the focusser tube (as shown, right), or in front of the secondary mirror. The idea is that the additional lens will correct for the shortcomings of the primary mirror. Now high-quality catadioptric systems with sub-aperture lens elements have been built. But the cheaper commercial scopes often come with second-grade optics. Reviews of these scopes show that this optical arrangement is often not successful.
Other catadioptric Newtonians have a large front corrector plate (similar to SCTs) and some fine models have been produced from time to time. You won't come across many of these, because it is probably cheaper to parabolise a spherical mirror than to figure a large, two-sided corrector plate. So, the parabolic Newtonian remains the best option.
The main advantage of the parabolic Newtonian is that it offers the best value-for-money in terms of aperture for the price, and Newtonian telescopes of up to 14 inches and more are perfectly within the reach of the keen amateur's pocket. This is especially true of the Dobsonian telescope. This is simply a Newtonian mounted on a very simple, but effecticve, mount.
The Newtonian is also very light and compact for its size, and produces a correct image at a suitable height for observation. Other telescopes (refractors, Scmidt-Cassegrains, Maksutovs, etc) all require the use of a star diagonal mirror to avoid cricked necks. This diagonal also laterally inverts the image.
Downside of the Newtonian is that it requires some maintenance. The telescopes mirrors will need to be re-aligned from time to time. You will cetainly need to do this when you first get your scope, and then every time you tranport it. But the Newtonian kept at home will probably not need re-colimating for months at a time.
The open tube means that the mirror will need to be carefully cleaned from time to time. The mirrors' aluminum reflecting surfaces will deteriorate over the years, and the mirrors will eventually need to be re-aluminised.
To sum up, Newtonians and Dobsonian telescopes offer unbeatable value for money. A focal ratio of about F6 makes for a good general-purpose scope - but make sure the primary mirror is parabolic.
CATADIOPTRIC (COMPOUND) TELESCOPES
The third category of instruments is the true catadioptric or compound telescope. They employ both lenses and mirrors to form an image. They use a large glass corrector lens at the top of the telescope tube. The most common forms are the Schmidt-Cassegrain (SCT) and Maksutov-Cassegrain (Maksutov).
The first advantage of these telescopes is that, because of their "folded" optical design, they are very compact. So you get a smaller, lighter scope thats easier to mount and very transportable.
Their long focal ratio (normally F10 or more) means that they are suited to high magnifications, and less suitable for deep sky observations and wide angle views. However, optional "Focal reducers" are usually available, which reduce the focal ratio to a value more suited for those purposes.
Manufacturers like Meade and Celestron have developed superb SCTs, incorporating high technology. Accessories are available which make them suited for a wide variety of purposes.
Unlike the Newtonian's open tube, SCTs and maksutovs are sealed units so that dirt and dust are largely excluded. But otherwise they do suffer some other of the Newtonians disadvantages, though not to the same degree. They will require occasional optical collimation, and though not as frequently required as for the Newtonian, the process is a little more complicated. The central obstruction (up to 30% of aperture) is larger than in the Newtonian, and degrades performance for critical lunar and planetary observations. Even so, when well made, a Schmidt-Cassegrain or Maksutov will deliver very fine images of a wide variety of celestial objects.
The thin corrector plate of SCTs makes them prone to "dewing". This is where dew forms on either the outer surface of the Corrector plate, or (less frequesntly) on its inner surface. These problems can be avoided by the use of a dew shield, or dew heater, or both. The Maksutov design, where the front corrector is much thicker than in the SCT, is less likely to suffer with this problem.
All-in-all, these catadioptric telescopes are highly versatile, very portable, and very good value for money. The wide range of available accessories make them excellent general-purpose telescopes. However, they do cost considerably more than Newtonians/Dobsonian scopes.