Questions to ask yourself before buying
Important features to look for in a telescope
Understanding telescope specification

There are lots of telescopes available on the market, and for the beginner, selecting one can be a bewildering experience. Before buying a telescope it is important to ask yourself the following questions:

What kind of observing would you like to do? Astronomical, Terrestrial or both?
Selecting a telescope for both astronomy and land viewing sounds attractive, but these applications can work against each other, and your choice will usually be a compromise. However, once you have decided on the telescopes main purpose, choosing one can become much easier.
If you have decided that your telescope will be used primarily for observing the night sky, the instrument required does not necessarily need to give a right side up image and is not required to focus on nearby objects. With the exception of the moon, planets and close star clusters, interesting night sky objects are faint, in fact most are very faint. As a new observer you may be mainly interested in viewing the moon and planets, and if this is the case, a telescope with a small objective (primary mirror or lens) may be sufficient. However, most observers quickly graduate to galaxies, nebulae, globular clusters, open clusters etc. To view these objects you will require a telescope with the largest aperture that is possible for your circumstances, which will include things like cost, weight, portability, etc.

Newtonian reflector telescopes are a popular choice for astronomical use because they have the lowest cost per inch of aperture. Observation of faint deep sky objects, such as nebulae and galaxies, can be achieved at a relatively reasonable cost by reflectors having mirror diameters of 150 to 200mm (6 to 8 inches).
Refractor telescopes are good for achieving high power and contrast when viewing the planets and the moon. They have a reputation of providing crisp, sharp-quality images. Since they are virtually maintenance free, they are easy to operate, but due to high costs for the large aperture scopes, most beginners will choose a Newtonian reflector as a first scope for all round astronomy. Short-tube refractors are now another low cost option for beginners. Their smaller size makes them an excellent choice for a portable telescope and the beautiful wide-field star vistas which they provide, are great for learning your way around the night sky.

Terrestrial viewing can easily be achieved with both short and long optical tube refractors, but shorter refractors with apertures of 70mm to 100mm, and focal lengths of 400mm to 700mm are preferred. Shorter scopes have wider fields of view than longer scopes and for land as well as night viewing, gathering more light gives better resolution which is more important than magnification. When choosing a scope for this purpose keep in mind that you will also be trying to look through the air in front of you, and at higher magnifications the image will shimmer, especially on hot, windy days. Newtonian reflectors are not suitable for terrestrial viewing because observing is done through the side of the tube and you therefore have to turn sideways to the target. Since there is no reasonable way to compensate for this, refractors make better terrestrial scopes. For dual purpose viewing, the refractor is a better choice but with larger apertures you do not have to magnify as much to achieve the same resolution. When using your refractor for daytime viewing, a 45 degree erect image diagonal is required to turn the image right side up. For night sky use, a 90 degree star diagonal is almost a necessity to avoid a strained neck.

Where will the telescope be used

When astronomical viewing in urban areas, using a longer tube will increase the contrast, but it will give you a smaller field of view. If you do want to use larger aperture scopes in the city, just remember to use lower power. All telescopes will perform far better in darker sky areas.
Important considerations include convenience of use, portability, ease of storage and ease of transportation to dark sites. Short tube Catadioptric-Newtonians (Sky- Watcher 1141EQ1, 1301EQ2, and 1501EQ3-2) and short tube refractors (Sky-Watcher 705AZ3, 804AZ3, 1025AZ3) are good choices if there are any restrictions.

Important features to look for in a telescope



Focuser

The focus assembly is a device to bring the image into sharp focus. It usually moves the eyepiece back and forth with a rack-and-pinion or with a screw-in assembly. It should move smoothly and the image should not jump around as it is adjusted. Standard focusers accommodate eyepieces with 1.25 inch diameter barrels although 2 inch are also becoming more common.
Coatings

Coatings are thin layers applied to lenses and mirrors to enhance light transmission, protect mirror surfaces and suppress flare and color aberrations. See Importance of Coatings.
Finderscope



A finderscope is a small auxiliary telescope, attached to and aligned with the main telescope. It has low power magnification, a wide field of view and a crosshair to aid in centering. It assists you in finding and aiming at objects which you want to observe through the main telescope. Most finderscopes give an upside down or left-right reverse image.

Understanding telescope specification

Aperture Diameter

The clear aperture of a telescope is the diameter of the objective lens or primary mirror specified either in inches or millimeters. The larger the aperture the more light it collects and the brighter (and better) the image will be. Greater detail and image clarity are observed as aperture increases.
Focal Length
This is the distance (usually in millimeters) of an optical system from the lens (or primary mirror) to the point where the telescope is in focus (focal point). The longer the focal length of the telescope, generally the more magnifying power it has, the larger the image and the smaller the field of view.
Focal Ratio
This is the ratio of the focal length of the telescope to its aperture in the same units of measurements. For example, the f/ratio of a telescope with a 200mm aperture and a focal length of 1000mm is: 1000 / 200 = 5, or f/5. Telescopes of f/4 to f/6 are called "fast" systems. They offer lower magnifying power and wider fields of view than slow f/8 to f/15 systems.
Magnification
The most common misunderstanding of telescopes is that they are rated by their magnifying power. The fact is, telescopes are rated by their aperture or light gathering capability. The aperture of a telescope is far more important than the magnifying power, because it determines the telescopes ability to resolve small or distant objects. See Usable Magnifications.