Types of Telescopes

Refractor

An optical system that utilizes a series of glass lenses to refract or bend the light from a distant object such that it can be focused to a point and magnified by an eyepiece.  Popularized by Galileo in the 17th Century, it is sometimes referred to as a Galilean refractor.

Reflector

The optical system in a reflector uses a primary mirror to reflect the light from a distant object such that it can be focused to a point and magnified by an eyepiece. Popularized by Sir Isaac Newton in the 17th Century, it is sometimes referred to as a Newtonian reflector.

Important Terms

Barlow Lens

An auxiliary lens that increases the power by a factor.  For example, a 2x barlow doubles the magnification of the telescope.

Erecting Lens

Lens used with a refractor telescope when viewing land-based objects to correct the normally upside down image provided by an astronomical telescope.

Finderscope

On most astronomical telescopes a lower powered finderscope with a wider field of view lets the user quickly locate the object to be viewed by the high-magnification main telescope.  There are two types of finderscopes, optical and red dot.

Focal Length

The measured light path of the optical system (tube length), typically measured in millimeters.

Focusing knob

Controls the rack and pinion focusing system.  Permits smooth movement of eyepiece for precise, sharp images.

Magnification (Power)

Telescopes are often referred to by two numbers separated by an "x".  For example: 100x4.5".  The first number is the power or magnification of the telescope.  With a 100x4.5" telescope, the object being viewed appears to be 100 times closer than you would see it with the unaided eye.   The magnification of a telescope is determined by dividing its focal length by the focal length of the eyepiece being used.  For example, a 500mm telescope with a 5mm eyepiece would magnify objects 100x.  Thus, a telescope can provide nearly any magnification required depending on the focal length of the eyepiece used.

Since most objects in the sky are rather large, high magnification isn't necessary.  Rather, a telescope that gathers lots of light is needed to make dim objects appear brighter and sharper.  The power or magnification contributes to the overall size and bulk of the telescope.  This is because as the power increases, so must the physical size of the objective lens.

Objective Lens Size (or Aperture)

The second number in the formula (100x4.5") is the diameter of the objective lens or mirror.  The aperture relates directly to how bright images will appear and how much detail is revealed.  The larger the objective lens, the better.  The size of the telescope's objective lens, or aperture, limits the amount of power that can be used effectively.  As the magnification of an object increases, the brightness of the image decreases.  This is because the light gathered by the telescope is being spread over a larger area.  Aperture also limits the clarity of an image at increased magnification.  When magnified beyond an aperture's recommended power, the image dims and becomes fuzzy.

Penta Mirror Technology

This patented Bushnell technology breakthrough allows for left-to-right correct view of the stars (not backwards, as in most telescope models) taking the guesswork out of where to move the telescope.  This feature provides unparalleled ease of use for finding and tracking objects with a 90 degree viewing angle and 1.25" eyepiece adapter.

Resolution

Resolution, or definition, is the ability of a telescope to distinguish fine detail and retain clarity.

Rotary Power Turret

This patented Bushnell feature allows for magnification and accessory changes to occur through a simple twist of the turret or accessory dial.  Accessories are incorporated into the telescope so there are no loose parts, and magnification and accessory changes can be made instantaneously to suit viewing conditions.

Types of NorthStar Telescopes

The NorthStar allows any beginner or amateur to navigate the skies like a professional.

Computerized Star Locator (CSL)

Utilizes an on-board computer to find over 20,000 astronomical objects.  Educational descriptions of the objects are provided at the touch of a button

"Go To" Computerized Star Locator

All the features of the Computerized Star Locator plus "go to" advanced motorized technology, where the computer automatically finds and follows any object from its 20,000 object database.

Types of Telescope Mounts

The mount secures the telescope to the tripod. It allows you to move your telescope horizontally and vertically. There are two primary types of telescope mounts, altitude-azimuth (or altaz) and equatorial.  Altaz mounts move up-down and left-right.  Equatorial mounts are tilted to align with the rotational (polar) axis of the Earth.

Easy Track® Mount

An altaz mount that allows quicker and easier set-up and storage, and reduces image shake caused by wind or ground tremors.

Equatorial  Mount

An advanced telescope mount that compensates for the earth's rotation.

Kinematic Mount

Undoubtedly the most precise, yet simple telescope  mount in years, this high-precision altaz mount uses the principle that it requires three points to form a plane.  Designed for optimum shape, the Kinematic mount features three points of contact in both horizontal and vertical planes.

Yoke Mount

A u-shaped altaz mount.

Tripod

A tripod serves as a steady platform for the telescope to allow hands-free use. Because of its powerful magnification, even the slightest jiggle of the telescope is enough to move it from its target.  Tripods feature three adjustable legs so the telescope can be used even when the ground is uneven.

How To Choose

When selecting a telescope, consider which target objects you plan to explore as well as your level of experience.  For viewing most night sky objects, you'll want a telescope with as much aperture as possible.  Keep in mind that as the size of the objective lens or mirror increases, so does the size of the telescope.  So, be sure to select a telescope that isn't too heavy to manage or too complicated for you to set up...especially if portability is a necessity.

Refractors

Refractors are less affected by atmospheric instabilities making them perfect for observing the Moon and planets.  Perfect for first-time and novice astronomers, small refractors are lightweight, portable and require very little maintenance.  And, if you're interested in viewing both land and celestial objects, refractors are equipped for both uses.

Reflectors

Reflectors feature larger apertures for a wide range of viewing at an affordable price.  Designed with the eyepiece located at the top of the tube, reflectors are more comfortable to use for viewing night-sky objects such as nebulae, the Moon, planets and galaxies.  Reflectors tend to be heavier and larger than refractors.

Eyepieces

Lower power eyepieces provide a wider field of view and a brighter image making them ideal for viewing the full Moon and planets, star clusters, nebulae and the constellations.  To focus in on the finer details of the Moon and planets such as mountains, ridges and craters, use an eyepiece with higher magnification.

Mounts

There are two basic types of telescope mounts, the altazimuth and the equatorial.  Altazimuth mounts are the easiest to maneuver and are ideal for first-time astronomers.  Equipped with a manual control or motor drive that allows you to follow the movement of objects across the night sky, equatorial mounts are ideal for more experienced astronomers.