Telescopes offer two advantages to the amateur astronomer. The most important is the ability to intercept more light from an object.
The second is resolving power, the ability to separate the fine detail of objects that are close together. This is very important when observing planetary detail or double stars.
The diameter (aperture) of the telescope’s principal light collector, which can be a lens or a mirror, determines both of these advantages.
Although most people think of telescopes it may be best to consider binoculars first. They are portable, convenient and relatively inexpensive. Cheap binoculars are better than a cheap telescope.
Consider something like 7 x 35 binoculars. The 7 is the magnification produced by the eyepiece, the other number is the aperture in millimeters. This size is light and easy to hold.
If you are considering a telescope there are essentially three types available to the amateur astronomer.
Refractors are the most readily recognized type and consist of lenses at both ends of a tube. Whatever you do avoid the cheap models sold by department, nature/science and toy stores.
They quote large magnifications, but their small apertures make them useless for astronomy.
Remember a telescope’s most important job is to collect light, not magnify a image. A normal terrestrial telescope has an extra lens to ensure the image is the right way up.
However extra lenses cut down the amount of light reaching the eye. The one thing an astronomer wants to avoid is reducing light and therefore information.
So, the correcting lens is omitted and a true astronomical telescope gives an upside down (inverted) image.
Lenses can create false rainbow tints around very bright objects like planets. This is called chromatic aberration and can be ignored or corrected by a filter.
Refractors generally cost more per inch of aperture than other types of telescope, and those of more than 4-inch (100mm) aperture are rather long and cumbersome. However smaller good quality ones can make excellent beginner instruments.
The second type of telescope is the reflector. Light travels down a tube before reflecting off a couple of mirrors and through an eyepiece on the side of the tube.
Reflectors need to be larger than a refractor to be equally useful. They do not suffer chromatic aberration, but the main mirror may occasionally need repolishing or realigning (collimating).
There are kits available for this. Reflectors are often the most comfortable telescopes to use because of the eyepiece position.
You don’t have to kneel and possess an elastic neck to look straight upwards as you would with a reflector.
A popular type of reflector is the Dobsonian, which has a mount near the ground rather than a tripod.
The third type of telescope is the Schmidt-Cassegrain. This uses lenses and mirrors to fold the light path back on itself within a compact tube.
They are generally cheaper than refractors, but dearer than reflectors. They are more portable and easier to handle than the other two.
Telescope specifications may quote f/numbers.These have no affect on the image you see, but can affect the exposure needed if you get into astrophotography.
It’s best to avoid astrophotography to start with. Become familiar with the sky and your telescope first.
You may be able to evaluate the different instruments by joining a local astronomy club or attending one of their star parties. Check at your local library.
You can also look in a reputable astronomy magazine for reviews and manufacturers advertising good quality telescopes.
Although you should buy the biggest aperture you can afford, don’t get a large telescope if you will have to carry it a long way.
A smaller telescope would be easier to set up and therefore more likely to be used. You will also be wasting your money on a large telescope if light pollution and atmospheric turbulence in your area are high, as you won’t be getting the best from the instrument.
The general rule for calculating the maximum practical magnification a telescope can achieve is to double the aperture in millimeters, e.g. a 100mm aperture telescope should have a maximum magnification of 200x.
So, spend the majority of your budget on getting a larger aperture, not more accessories. Do not be tempted to buy an eyepiece that will allegedly stretch magnification beyond the maximum value calculated from the aperture.
If given a choice of eyepieces start off simple. A Kellner is a good general-purpose eyepiece and a Barlow lens placed between a focuser and eyepiece can triple magnification.
A telescope also magnifies the movement of objects across the sky and the instrument will have to be moved repeatedly to keep the object in the field of view.
To cope with this different mounts and drives have been devised that enable the telescope to follow the moving object.
You can get electronic drives to point the telescope, but the most important thing is that the mount is sturdy.
It is important to realize that many textbook and telescope advertisement photos are long exposures with false colors added.
Stars seen through a telescope will always look like points of light, and you will not see color in dim objects. The eye is not sensitive enough.
So what can you expect to see with a beginners telescope? A 3-inch (75mm) refractor or 6-inch (150mm) reflector will allow you to see many galaxies and nebulae, Saturn’s rings, Jupiter’s largest moons and hundreds of craters on the moon.
Plenty of wonderful objects to serve as an introduction to the night sky.