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Important Telescope Terms 

Have you decided to purchase a telescope and wish to know more? Please read the following to understand the important variations and functions of the telescopes we have available.

Focal Distance 

The focal distance of a telescope refers to the distance measured between the main optical element of the telescope and the light at the point that it focuses. The focal distance value is important as it partially determines the magnifying power of the telescope.

Lens Diameter 

The amount of light that a telescope can collect is directly related to the diameter of the primary lens or mirror. The bigger the diameter of the lens, the more light is collect and thus, the sharper and clearer the object is seen through the telescope. As much as practicable and possible, it is always best to purchase a telescope with a large lens diameter. It is however important to keep in mind that the bigger the lens diameter is, the bigger and heavier the telescope will be.

Focal Ratio

The focal ratio is calculated by dividing the focal distance by the lens diameter. This value determines the clarity and field of view of the object(s) you are wishing to view. If for example, a telescope had a focal distance value of 800 and a lens diameter of 60 the focal ratio would equal 1:13 (800/60=13). A focal ratio above 10 indicates an extremely clear but narrow field of view and is perfect for observing the moon and planets. Below is a list of telescope focal ratios and their suitability.

• 1:10 or higher = suitable for the observation of the moon, planets and double stars.
• 1:8 = suitable for all round observation use
• 1:6 and lower = suitable for the observation of wide view objects such as galaxies.

Magnification

The magnification of a telescope is determined by a combination of the lenses used to make up the telescope and is affected by the focal distance of the eyepiece and the telescope itself. The general rule is the higher the magnification, the smaller the field of view and the lower the clarity of the image. The maximum magnification of a telescope is twice that of the lens diameter. For example, a telescope with a lens diameter of 60mm suggests that the maximum magnification of that particular telescope is 120x.

Additional Information 

Lens/Mirror

They are capable to collect far more light from an object that is far away and then bring this light (or image) back to one (focal) point. A lens is used in a refractor and a mirror is used in a reflector telescope. The Maksutov-Cassegrain telescope makes use of both the lens and mirror.

Eyepiece

Collects the light from the focal point of the telescope and magnifies this image.

Filters

These are pieces of glass placed in the tube of the eyepiece. This filters the light waves in order to display a clearer image.

Lens Diameter

The amount of light that a telescope can collect is directly related to the diameter of the primary lens or mirror.

Magnification 

The magnification is the factor to which the telescope can make an object look closer. This can be 10 times, but may also be 100x.

View Finders

Viewfinders work similarly to the sight on a rifle and are used to focus on an object (generally, stars or planets) you wish to observe with your telescope. There are three types of viewfinders available:

• Peep sights: Notches or circles that allow you to center your image.

• Reflex sights: A mirror box that shows the sky and illuminates your observation with a red LED dot, similar to that on a laser beam on a rifle.

• Telescoop Sight: A small, low magnification telescope (5x – 10x) mounted on the side of the telescope that has a cross and works like a riflescope.

Dew Shield

Observing at night during cold weather will often result in dew condensation on your telescope. To avoid this happening to the optical elements of your telescope (mirror or lens), a dew shield can be used.

Azimuth Tripod

An azimuth tripod has two rotatable axes; the horizontal axis and the vertical axis. If you wish to align your telescope on an object, you rotate the telescope over the horizontal axis (also called the azimuthal axis), then rotate the telescope up on the altitude (y) axis. This way, the object will also be visible on the vertical axis.

Equatorial Tripod

An equatorial tripod has two mutually perpendicular axes; a straight climbing axis (also known as RC-or RA-axis) and declination axis (also referred to as DEC or elevation axis). On this type of tripod, you do not have to move the telescope up and down (like with the azimuthal axis), but it compensates the telescope smoothly on the natural earth rotation.