How Are Telescope Eyepieces Measured?

How Are Telescope Eyepieces Measured

The telescope is the optical system that creates images, just like the camera lens that creates the image on a film. The telescope comes with the telescopic eyepieces that help to focus on the objects far away. Typically, it has a complex system of positive lenses located between the objective image and the eye where the image form at different positions.

If the image created by the lens is in front of the focal plane of the eyepiece, the image position will be at infinity. The eyepiece receives the collected light and converts it into a clear image that will reach the retina. The image formed can be measured using a unique way using different parameters. In this case, you need to follow these parameters to get the right measurement of the image formed. These include focal length, magnification,upper limit, the field of view, eye relief, and eyepiece size.


Here Is How Telescope Eyepieces Are Measured

How Telescope Eyepieces Are Measured

Focal length

The focal length of the eyepiece can be obtained by measuring the object size (ho), the image size (hi), and the distance from the object (So). The image of the object can be measured by projecting it on a paper or look at it directly through the eye lens. The best type of object is where it is on a smooth, shiny surface with clear boundaries, for example, a well-lit window. Image size should be kept to a minimum to make an accurate measurement to minimize distortion that tends to enlarge the outer field portion.Also, when the distance from the object is longer, the error will be reduced, which is probably the distance from the field object to the lens.


The magnification will help to determine the measurement of the telescope eyepiece. The eyepieces magnification varies from one eyepieces and it depends on the field of view and focal length. Magnification is determined by simply dividing telescope focal length by eyepiece focal length.This means, when the number on the eyepiece is smaller, it will provide higher magnification.

For instance, the 10mm eyepiece will provide twice the magnification,like the 20mm eyepiece. Also, it means that similar eyepiece offers different magnifications in different areas. The 10mm eyepiece can have a low magnification with shorter focal length and high magnification with longer focal length. For instance, the telescope focal length of 80mm and eyepiece focal length of 10mm will give 8x magnification.

The typical eyepiece collection includes three eyepieces, mainly a low power, a high power eyepiece, and a medium power. The normal magnification range will depend on your telescope, but in many scopes, the usual range can be between 50x and 250x.

In general, the magnification produced by the eyepiece is not fixed, for it varies from one telescope to the other. When the focal length of the telescope is more significant, the magnification will higher. It is always best to have three eyepieces on your model: one for lower magnification, one for medium magnifications, and another for higher magnifications.

Upper limit

The human eye has many limitations, and one of them is that, at some point, it can no longer apprehend higher or lower magnification. This is called the limit at which you can push the telescope. Beyond that point, you will not have satisfactory views.

To get the measurement of the upper limit, you need to know the size of the telescope’s aperture. The formula for measuring the maximum limit is as follows: 60 x aperture size (inches). This means that, with the aperture telescope of 4 inches, the maximum magnification is 60 x 4 = 240x. When you go beyond that, you cannot apprehend what you see.

Field of view


When you want to measure the telescope eyepiece, the field of view is very crucial. Typically, to see the amount of sky through the eyepiece, it is determined by the apparent field of view.Visible field of view design the feature of the eyepiece. Some eyepieces have a wide apparent field of view and others havean apparent narrow field of view.If the magnification is kept constant, the eyepiece that has a widevisible area will have a broader field of view.

Exit pupil size

This is another critical parameter concerning the eyepiece that helps to determine the size of the telescope eyepiece. It directly determines the degree of brightness of the image about the object and the degree of eye aberrations. The exit pupil size of the eyepiece is inversely proportional to the magnification of the telescope. For a relative magnification with the aperture diameter m, the exit pupil size of the eyepiece is given with 1/m. For example, a relative magnification of 0.5 per mm of the aperture (50x in D = 100mm in m = 0.5) leads to an exit pupil diameter of 2mm, which is 12.7x per inch of an aperture magnification of m = 12.7.

With the focal ratio of the telescope,F = ƒobj/D, the diameter of the exit pupil is given as P = ƒe/F, given that ƒe is the focal length of the eyepiece. Another significant number with the diameter of the exit pupil is the angular diameter of the Airy disc, expressed in 4.6/P (mm) in arcs in minutes.

When the exit pupil size is more than 2mm in diameter, the eye aberrations start to control the diffraction effect and gradually increase with pupil size. Therefore, the telescopic resolution is limited by aberrations higher than 2mm and by the diffraction of smaller pupils.

Eye relief

The eye relief through the eyepiece, the distance between the exit pupil,and the eye are paramount for practical observation. Like almost everything, too little and too much eye relief is undesirable. Short eye relief of the telescope eyepieces prevents the viewer from putting the eye at exit pupil, hence reducing the visible field. Also, it causes eye fatigue, which affects the quality of perception. Too much eye relief often give a long focal length of the eyepieces, especially when using it with the convectional Barlow lens. This makes it difficult to locate and retain the exit pupil. The size of the eyepiece relief varies mainly, depending on the type and design of the eyepiece.

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