Once you have begun learning the sky, you can start thinking about equipment. As far as things go, astronomy is generally done with binoculars and of course, telescopes. I’ll deal with binoculars first, because they are probably MORE useful to a beginner than a telescope to start off with, for reasons I’ll explain below.
However, first a vitally important point about any piece of astronomical viewing equipment:
BE CAREFUL WHEN BUYING ANYTHING FROM A DEPARTMENT STORE, ESPECIALLY TELESCOPES!!!
Department stores are notorious among astronomers – they usually sell low quality, poorly constructed and poor performing binoculars and telescopes. Especially beware of advertisements for 500+ x magnification on small scopes. While they can theoretically achieve high magnification, the images stink. When it comes time to buy, go to a telescope store, or at least a photographic shop. Especially beware of the brand names Tasco, Jason, and Simmons. They are cheap, they are not solidly constructed and their optics are not terribly good.
It is best to start with binoculars: They are inexpensive, offer a wide field of view, and allow you to use both eyes for viewing, rather than just one. They are also portable, require little maintenance, you can use them for both land and sky viewing, and best of all, they are easy to use.
There are several important things to think about regarding binoculars:
The most important thing is aperture: how large (wide) the lenses are. The more aperture there is, the more you will see – i.e. you want them to let in plenty of light. The front lens(es) by the way, are called the objective(s), which generally means the primary optical device(s).
The next consideration is coating of the lenses. Binoculars are typically multi-coated, or fully coated. The coating refers to special optical solutions applied to the lens at the factory that help reject stray light, and help cut down on false colour, which has traditionally been a major problem for lens based optical equipment. Multi-coated optics have optical solutions on all lenses that are exposed to air, but generally do not offer colour correction (removing false colour) that is as good as fully coated optics (several layers of coatings on all lenses).
The third point relates to what you will be able to see through binoculars, and refers to two concepts, plus the involvement of aperture: The magnification of the binoculars (one concept), the aperture of the lenses, and the field of view (the second concept).
To help put this into perspective:
Have you ever seen what looks like a mathematical formula on a pair of binoculars, such as 10 x 50 7º ?
What this really refers to is the magnification of the binoculars e.g. 10 x or ten times, the aperture of the lenses e.g. 50mm (a common size), and a field of view of 7 degrees, which is quite wide. Ideally, you want a field of view as wide as possible: this helps you to fit objects into the picture: for example, on my binoculars, I can fit the entire Southern Cross into my FOV.
I use fully coated Yashica 10 x 50 7ºs for my binoculars. They are over 15 years old now, but still deliver (in my humble opinion) outstanding views.
Binoculars have widely varying magnifications and objective sizes: Typically, they range from about 8 x 25 to 25 x 80, though there are some larger ones.
When you are out observing with binoculars, your arms will start to get tired after a while, as will your neck from looking up: so it helps if you can mount your binoculars on a stand. Some places that sell binoculars also sell stands for them, or you can make your own. Stands are mandatory for large binoculars, such as 25 x 80: I don’t know anyone who can hold them steady after 5 minutes, as they weigh so much.
The wide fields of view offered by binoculars allows you easily fit whole sections of constellations and entire star clusters into an image. This wide field of view is also very helpful in locating objects, especially if a particular one that you want to see is hard to find.
Binoculars can also offer very good contrast. On an average night, I can easily find many deep space objects just by noticing their image against the sky as opposed to a star.
When you look at the moon however, the flat, dark grey areas (“mares” or “seas”) stand out incredibly well from the brighter cratered and mountain regions. Binoculars also cool down in the night air quickly. With telescopes (discussed soon), some time must be spent waiting for heat waves (that distort the viewing) to dissipate.
There are two basic types of binoculars: Porro Prism and Roof Prism.
Porro Prism binoculars are historically the most common design. Light coming in through the objectives is focused by a pair of prisms so that the path is folded in a zig-zag pattern, and out through the eyepiece, which also ensures that the image comes out the right way up.
Porro Prism binoculars can be quite rugged. If the binoculars receive a good deal of knocking around however, or are dropped, the prisms can come out of collimation, or alignment. With the light path now skewith, the image will be somewhat blurred, or you may not be able to see anything. Fortunately, they can be realigned, though repairs should be handled by someone who knows what they are doing…
Roof Prism Binoculars
Roof Prism binoculars are a more modern design. Instead of “folding” the prisms and zigzagging the light path, Roof Prism binoculars allow for a “straight through” design, which helps prevent tenuous loss of light. They are also able to better withstand the knocks.
Because of their straight through design, Roof Prism binoculars can be made into compact sizes: generally speaking, many of the smaller size binoculars of today such as 8 x 25 are of Roof Prism Design.
Telescopes come in a very wide variety of types, shapes, and sizes. Before I get into specifics however, there are a few vital considerations:
As with Binoculars, the most important consideration is APERTURE. Aperture is even more important for telescopes than binoculars, and I’ll explain why later.
Before acquiring a scope, think about what you will want to see with it. Some scopes are best suited to the Moon and Planets, others for deep space objects.
Don’t rush to buy a telescope. Talk to owners, look through their scopes, and try out a variety first. You may find that the right scope for you is different from your expectations.
Just before we get the the good part about telescopes, there are some telescope mounting considerations:
There are three kinds of bases commonly available for mounting a telescope on: Tripod, Dobsonian, and a Permanent Pier.
This is without a doubt the historical type of mount for amateur astronomy. Little needs to be said, other than to look for two main things in a tripod: A solid, stable construction (try to get wood over lightweight aluminium, unless weight is really an issue), and a compact design for portability.
A tripod is illustrated in the refractor picture below. However, a tripod is potentially the least stable design, when compared to the other mounts.
This type of base is also a mount. It is named after an astronomer who popularized the design. This is little more than an open box placed on a free rotating platform. The telescope has large knobs fixed to the side of it that rest in curvatures in the top of the box when the telescope is placed into the mount, as is illustrated in the picture of a reflector telescope below. The telescope can move up and down (be moved in altitude), and be rotated left to right (be moved in azimuth). Movement in altitude and azimuth means the telescope has an altazimuth mount.
The Dobsonian mount is almost rock solid, and as simple as can be.
This is where a pylon (pier) is secured to the ground, and the telescope is mounted on top of it.
The pier is also very solid, especially if the base is on concrete.
Except in the case of Dobsonian mounts, which are both base and mount, other telescopes have a separate base and mount. Mounts also feature controls (called slow-motion controls), that allow a user to manoeuvre the telescope on its mount. However, Dobsonian mounts do not have these controls. The telescope is moved by hand.
The mount is what connects the telescope to the base. There are two types of mount: Altazimuth, and Equatorial. One thing I want to stress at this point: Virtually any type of telescope can be attached to either type of mount.
Altazimuth mounts, as already explained, allow the telescope to move in altitude (up / down) and azimuth (left / right). An altazimuth mount is shown in the picture of a refractor below. When using an altazimuth mount, you can just set up and go. Altazimuth mounts are not suited to astronomical photography except for quick snapshots, unless a field de-rotator (see the Photography Page) is used. Even then, photography is not the forté of the altazimuth mount because it does not easily allow tracking of objects across the sky.
Equatorial mounts allow a user to track an object as it moves across the sky (due to the Earth’s rotation). Whereas altazimuth mounts only move in altitude and azimuth, equatorial mounts can function in a similar way, but are used almost exclusively in right ascension and declination.
Right ascension means that the telescope can track an object as it moves across the sky, from east to west. Declination allows a telescope to follow the rising and setting of an object.
Essentially, right ascension is equivalent to longitude on the Earth, and declination is the same as latitude. Celestial co-ordinates, or the position of an object in the sky is given in right ascension and declination co-ordinates. To use an Equatorial mount, you must first polar align the telescope, to make sure that tracking will be done accurately, as objects seem to rotate around the poles. The further north or south one is, the more pronounced the effect.
Polar Alignment is done by lining your telescope exactly (or to within half a degree or so is usually good enough) with the north or south pole respectively, depending on which hemisphere you are in. You will need to adjust setting circles which are dials with hours, minutes, and degrees to match your latitude.
Using the slow motion controls, you can manually track an object. However it can be more fun (and less work) to attach a motor to your mount, and have it move the telescope for you.
Equatorial mounts are more complex than altazimuth mounts, but they are useful for doing photography, where an object must be tracked.
I will discuss these in more detail on the Going Further page, but for now, a simple explanation is due. Most small telescopes are manually driven – the user does the work of finding an object and moving slow motion controls etc. to keep the image in the eyepiece. Computerized telescopes however, make use of electronics and motors to do the finding of an object and the tracking of it for you. These two functions are huge advantages to computerized telescopes. The most popular example I can think for for an amateur astronomer is the Meade LX200. Having a computerized telescope means you don’t actually have to learn the sky – however, it is highly advisable! The only downside to a computerized telescope is the cost. They can be rather pricey.
Now we are ready to discuss telescopes proper.
So without any more ado:
The are fundamentally three different kinds of telescope, classified by their optical arrangements, with each kind having various subtypes. The three primary kinds are are Refractors, Reflectors, and Catadioptric.
This is probably what most people think of when they hear the word “telescope”. Refractors use their objective lens to focus incoming light to a point.
The light rays converge at a diagonal prism or mirror, that reflects the light rays up through the eyepiece. Refractors can be used for either astronomical or land (terrestrial) viewing. For terrestrial viewing, the image actually appears upside down, and perhaps inverted right to left. To correct this, an erect image focuser needs to be used, instead of the 90 degree astronomical focuser. The focuser is the 90 degree device you see at the back of the telescope. The eyepiece is attached to it, and the focuser can be moved in and out (forwards or backwards) until image focus is achieved.
Refractors are the simplest type of telescope, and require little maintenance. This is reflected by the fact that they do not really require collimation of the lenses; they are fixed, just like binoculars (unless knocked around a lot). Refractors tend to typically have smaller objectives than the other types of telescopes, mainly because it is expensive and difficult to produce high quality lenses in larger apertures.
Aperture sizes commonly vary between 60mm (2.4″) to 178mm (7″), though a few are more. The most common sizes are 60mm (2.4″ most common), 70mm (2.9″), 80mm (3.3″ common), 90mm (3.5″ also common), 102mm (4″), 127mm (5″),152mm (6″) and 178mm (7″). The latter four larger apertures are generally (but not exclusively) reserved for very high quality refractors that use lenses made of ED (Extra low Dispersion) glass and / or special coatings (such as magnesium fluorite) that provide images virtually free of false colour (apochromatic). Smaller refractors frequently suffer from some false colour, often a shade of blue or violet when looking at stars and planets.
Refractors can best be used to advantage on the moon and planets. Despite false colour problems (in smaller, non-apochromatic scopes), refractors offer high contrast, which is an advantage when looking at these objects. For example, when looking at Jupiter, it becomes relatively easy to discern different bands around the planet.
Refractors can also be used on deep sky objects, but are not as good as reflectors, because their generally smaller apertures do not let in as much light.
Whereas refractors use lenses to focus light, reflectors use mirrors. Reflectors have many designs, but I’ll look at the most common two: the Newtonian (illustrated), and the Cassegrain.
As a whole, reflectors share certain properties. Among the advantages, reflectors do not generally suffer from false colour like most small refractors do (i.e. they are apochromatic), because light is reflected, which means that light of different wavelengths (colours) do not get bent differently.
Reflectors, by nature of design, allow for large apertures at reasonable price, which is one important factor most astronomers look for (remember, the more aperture you have, the more light you will let in, and so the more you will see).
However, reflectors generally require more maintenance than refractors, because they come out of collimation more easily. The Newtonian design is best known for this, but it happens less frequently on Cassegrains too. Fortunately, re-collimation is an easy job that can be done in a matter of minutes (or seconds).
Aperture in the case of reflecting telescopes refers to the diameter of the main mirror (the primary mirror). Sizes vary from about 76mm (3.1″ xxx) to 915mm (36″), or more. The most common sizes for commercially made reflecting telescopes are 152mm (6″), 203mm (8″), 254mm (10″), 318mm (12.5″) and 406mm (16″). There are larger sizes, but they are less common. The biggest aperture telescopes are made by companies specializing in large sizes, or are home made.