Understanding Telescope Eyepieces & Magnification

A Basic Guide To Telescope Eyepieces

Before we delve into Eyepieces, and how they directly affect the magnification of a Telescope, it's worth noting that all Telescopes will have what is known as a highest useful magnification.

The highest useful magnification can often be found in the specifications list or features list of a particular Telescope. Essentially, the highest useful magnification is the highest magnification that a particular Telescope can reach before the image becomes distorted, blurry etc. 

The highest useful magnification of a Telescope can be calculated using the following formula:

50 x Telescope Aperture (in inches) = Highest Useful Magnification

Now, onto Eyepieces! The purpose of an Eyepiece is to magnify the image projected by the objective lens or mirror of a Telescope. An Eyepiece’s magnifying power is directly related to its focal length. The Magnification of an Eyepiece is based on the following calculation:

Many Telescope packages will come with one or two different eyepieces that allow the user to get started; this may include a 10mm Eyepiece and a 25mm Eyepiece:

10mm Eyepiece	25mm Eyepiece

Once familiar with using Eyepieces on your Telescope, you may want to consider adding a few higher quality Eyepieces to your accessory kit, and a good quality Barlow Lens for further experimentation:

2x Barlow Lens	3x Barlow Lens

A Barlow Lens will increase the magnification of an eyepiece. Typically, you can purchase Barlows that will increase magnification by 2x, 2.5x, or even 3x. This is particularly helpful when working with lower power eyepieces.  For example, if you attach a 2x Barlow lens to a 24mm eyepiece, it will produce the magnification of a 12mm eyepiece. 

We suggest the purchase of 2 or 3 good quality Eyepieces that will deliver different levels of magnification. Be certain that the eyepieces are suited to your Telescope model.

A Low Power (20mm-30mm) wide field eyepiece will deliver a wide view of the night sky. Once objects have been located, then move on to a higher power eyepiece.

A Medium Power (10mm-20mm) eyepiece will darken the background sky and improve image contrast and detail. The details of the Orion Nebula and small star clusters will be magnified considerably.

A High Power (3mm-10mm) eyepiece will deliver detail from planets and enhance the observation of smaller distant objects.

Note: It can be possible to have too little or too much magnification. 

Field of View (FOV): 

This is the angle width of the night sky in front of your eye. Field of view is usually in the 40 degree to 100 degree range. A large field of view will show much more of the night sky in a particular magnification.

Barrel Size:

This is the diameter of the metal barrel that slides into the Telescope focuser. Most telescope eyepieces have a 1.25-inch barrel size, while others have 2-inch and 3-inch barrels.

Field Stop:

An eyepiece will have Field Stop metal ring on the inside that limits the Field of View. The metal ring produces an image with a sharp defined edge. 

Exit Pupil:

Is the diameter of the beam of light that is projected from the eyepiece to the entrance pupil of the eye.

Eye Relief:

Referred to as the maximum distance that the eye can be positioned on the eyepiece lens while observing the full field of view. A long eye relief measurement of 18mm to 20mm is recommended, especially for those that wear glasses.