Activities such as astronomy, nature studies and viewing sports must often be done from a distance.
For various reasons we cannot get close enough to the subjects to view them in the detail that is needed.
Our eyes are general purpose tools and their resolution is limited, their magnifying properties are minimal and they are limited in how much light that they can gather.
We must use optical devices such as telescopes to increase our visual range.
A telescope is an optical device which makes far objects appears closer.
It samples a small area of view, a field, and then magnifies it so that distant objects appear larger.
Parallel light rays entering the telescope are focused to a single point, called the focus or focal point.
These focused rays are then magnified with a very powerful lens, or more commonly a set of lenses, called an eyepiece, to give enlarged views of distant objects.
The eyepiece acts in the reverse direction to the telescope lens, taking the focused rays and sending them to the eye as parallel rays.
The diameter of the observed circle depends on the field of view of the eyepiece.
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.
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.
Secondary Mirror Diameter
The second, smaller mirror in the light path of a reflecting or catadioptric telescope.
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.
Highest Practical Power
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 telescope's ability to resolve small or distant objects.
Faintest Steller Magnitude
Stellar magnitude is measure of the brightness of a star or other celestial body. The brighter the object, the lower the number assigned as a magnitude.
Resolving Power (arc sec.)
The ability of a telescope to separate closely positioned points.
A low power telescope attached parallel to the main instrument which provides easy object locating and telescope aiming.
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.
A mirror or prism system which changes the angle and orientation of the light rays coming from the telescope to the eyepiece.
Also called an ocular. This is a small tube that contains the lenses needed to bring a telescope's focus to a final image in the eye. Telescopes usually come with at least two eyepieces: one for low power and a second for a higher power view.