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Telescope Eyepieces: A Beginner's Guide

by: OZScopes - The Australian Telescope Experts

No telescope works without an eyepiece. So, just what is a telescope eyepiece? They come in a range of sizes, magnification factors, styles, types and manufactuers.

To understand how your telescope's eyepiece works with your telescope towards understanding the power (or magnification) achievable, read on.

Celestron Omni Series Telescope Eyepieces
Celestron Omni Series Telescope Eyepieces

First, we have to understand Focal Length.

Focal length is typically measured in mm, and is the clearest marker of how powerful your telescope is. It refers to to the distance between the lens or primary mirror, to the point where the telescope is in focus. This point is known as the Focal Point.

The longer the focal length of your telescope, the more powerful it is, the larger the image, and the smaller the field of view.
e.g. A telescope with a focal length of 2000mm has twice the power and half the field of view of a 1000mm telescope.

Now, we can calculate Magnification (Power).

To determine the magnification of your eyepiece, simply divide the focal length of your telescope with the focal length of your eyepiece.

Check out this awesome Eyepiece Magnification Calculator if you're getting stuck.

Magnification/Power = Telescope Focal Length / Eyepiece Focal Length

Thus, by using different eyepieces of different focal lengths, you'll be able to experience different magnifications.
e.g. a 25mm eyepiece used on a telescope with a 1000mm focal length would yield a power of 40x (1000 / 25 = 40) and a 10mm eyepiece used on the same telescope would yield a power of 100x (1000 / 10 = 100). 

Remember, the longer the focal length of your eyepiece, the less its magnification, and the wider your field of view.

A word of warning

There are practical upper and lower limits for telescopes. Determined by different optical laws and the nature of the human eye, there are maxium limits to which you'll be able to push your telescope. As a rule of thumb, the Maximum Usable Power (or Highest Usable Magnification) of your telescope will be 60x its aperture (in inches) under normal conditions.

Power above the Highest Usable Magnification will appear dim, and low-contrast.
e.g. The maximum power on a 4 inch telescope : 4 x 60 = will be 240x.

As power increases, sharpness and detail decreases - because you're pushing your telescope to the limits of its capability, the exit pupil will decrease in size (read more about exit pupils below), resulting in darker images. Higher powers are used mainly for lunar, planetary, and binary star observations, as they emit a larger amount of light.

Be wary of manufacturers that advertise ridiculously high-powered telescopes (e.g. 750x) with a mere 6mm aperture. Both false and misleading, these manufactuers leverage the fact that most consumers are in the dark about how telescopes operate. They pander to the common misconception that magnification is the most important thing in astronomical telescopes.

So what's this about Exit Pupils?

The exit pupil is the image of the object that's been formed by your eyepiece. It's also where you place your eye to see the full field of view. For an eyepiece with a particular focal length to work well with your telescope, you need to determine the exit pupil. The size of your exit pupil is merely:

Exit Pupil = Telescope Aperture / Magnification (Power)

As you increase the power of your scope, the smaller your exit pupil becomes.

Your eye's pupil cannot get wider than 7mm, nor smaller than about 0.5mm.

you should only purchase eyepieces that - when combined with your telescope - provides you with exit pupils no larger than 6mm, and no smaller than 2mm for optimum effect.

Telescope Eyepieces - General Applications

with credits to Universe Today


Plossl Eyepiece
Plossl Eyepiece

The majority of the eyepieces that we have here on OzScopes are Plossl Eyepieces. The Plossl seems to be the most popular eyepiece design today - using 4 or 5 elements, they are very well-corrected and have a wider (50-52 degree) FOV than Orthoscopic eyepieces.

2mm-4.9mm Telescope Eyepieces: These produce very high magnifications and work best on long focal length refractors and standard Schmidt-Cassegrains. Unless you have very steady seeing, this range more than likely will produce too much magnification for other telescope styles.

5mm – 6.9mm Telescope Eyepieces: These make good planetary detail and double star eyepieces for long focal length telescopes and will work satisfactorily in shorter focal length telescopes with steady seeing conditions.

7mm – 9.9mm Telescope Eyepieces: These are ideal high magnification eyepieces for shorter focal length telescopes, and serve as good planetary, double star and lunar details units.

10mm – 13.9mm Telescope Eyepieces: These are fine to use across all focal lengths and offer great background darkening capabilities for studying planetary nebula, small galaxies, planetary details and lunar details.

14mm – 17.9mm Telescope Eyepieces: These are a great mid-range magnification for all focal lengths and will help resolve globular clusters, galaxy details and spot planetary nebulae.

18mm – 24.9mm Telescope Eyepieces: These will work as nicely on long focal length telescopes to show wide field and more extended objects. Shorter focal length telescopes will enjoy great mid-range magnification for objects like galaxy clusters and large open clusters.

25mm – 30.9mm Telescope Eyepieces: These are extended field eyepieces for longer focal length – good for large nebula and open clusters. For shorter focal length, they are fantastic for large objects such as the Orion nebula, views of the full lunar disc, large open clusters and more. This multi-purpose magnification range also makes for good “locator” eyepieces in all focal lengths.

31mm – 39.9mm Telescope Eyepieces: These are well suited to shorter focal length telescopes for extended views and large, starry fields.

40mm Telescope Eyepieces and Above: These are exclusively the domain of shorter focal lengths. This magnification range is superb for showing large, starry vistas as well as extended nebula with star fields, etc.

Once you've seleceted an eyepiece, calculate the magnifications they will produce with your scope. For determining maximum power, remember that a good rule of thum is to use NOT MORE than 60x per inch of aperture with apertures up to 6". Higher magnifications may still look nice, but are unlikely to reveal any additional detail. Realistically, the atmosphere will limit your viewing to a maximum of about 300x, no matter how large your telescope's aperture.

Basically, you'll be choosing low and medium power eyepieces in increments to "frame" the subject, and high powered eyepieces to reach optimim contrast and resolution for viewing planets and double stars.

Using Telescope Eyepieces

Most of your observing should be done with lower powers, at roughly 6x to 25x the aperture of your telescope. At these powers, images will appear brighter and crisper, providing a more enjoyable viewing experience with wider fields of view.

Powers lower than 3x - 4x your telescope's aperture size at night are typically not useful at all - and might even show a dark spot in the center of the eyepiece in a Catadioptric (Schmidt-Cassegrain) or Newtonian Reflector Telescopes.

Remember this - your telescope is only as good as the eyepiece you'r using. Even if you have the most amazing telescope quality, but if you use a poorly manufactured eyepiece, you run the risk of wasting the full capability of your telescope. There has to be a balanace between the quality of your telescope and the quality of your eyepiece.

Here's an area where heaps of beginners tend to go overboard - you don't really need more than 3 or 4 carefully chosen eyepieces, but most of us do wind up with collections. Although not neccessary, they can be quite fun - and some can be quite impressive.

So what about these "Barlow Lenses"? What are they?

SAXON 2X 1.25
SAXON 2X 1.25" Achromatic Barlow Lens


Many, many observers still find that Barlow lenses prove to be a valuable accessory. They sit between yoru telescope's focuser and your eyepiece, and will typically double or triple the magnification of any eyepiece.

Thus, for about AUD$50 to AUD$60, you've effectually doubled your eyepiece collection! A Barlow lens also preserves the eye relief of your longer focal length eyepieces, reducing the amount of squinting you will have to face. Bear in mind though, that although Barlow lenses are an affordable accessory, they do compromise your viewing experience slightly, especially with your more powerful eyepieces. You might experience some slight blurring with higher powered eyepieces when you plug your Barlow lens in, and are also not recommended for use with Astrophotography.

You can read more about Barlow lenses in our guide: "What is a Barlow Lens? Choosing the right one."

Telescope Eyepiece Barrel Sizes - What size eyepiece should I buy?

There's a fundamental difference between an eyepiece's focal length and its size. The Focal Length commonly refers to measurements like 4mm, or 25mm - this refers to the kind of magnification you'll be able to get with the eyepiece. The Size of the eyepiece, on the other hand, refers to the diameter of the tube - and whether or not it will fit on the barrel of your telescope's focuser.

Althought there are several "exotic" eyepiece sizes, the most common one is the 1.25 inch eyepiece.

The 1.25 inch Telescope Eyepiece : Initially thought of a larger eyepeice diameter, it has now become industry standard and virtually everty telescope offered today is equipped with a 1.25" focuser assembly. It offers a more comfortable field of view, room for more elements and design improvements, brighter images and improved contrast.