So you are looking to buy a telescope or already have one but are wondering about the best telescope eyepieces to get better detail in your views. Here are some points to consider and the specifications that matter when choosing between telescope eyepieces for sale.
Why get extra eyepieces for your telescope?
Why do I need extra eyepieces? Eyepieces determine the magnification and field of view of your telescope and so sourcing extra eyepieces contributes to getting the most out of your telescope.
It’s about enhancing your experience.
The eyepiece is an important component of your telescope. It contributes to half of the optics in a refractor telescope and about a third in a reflector. Thus, improving the eyepiece goes a long way towards improving your experience.
So, buying extra eyepieces, besides those that come with the telescope, is about upping your experience in observing celestial objects of the night sky.
About Telescope Eyepieces
A typical eyepiece set would cover low power, medium power, and high power.
You may think about buying a ready-compiled eyepiece kit, but three or four quality eyepieces will serve you better than buying an entire telescope eyepiece set with pieces you might never use. Something to think about.
This telescope eyepiece guide covers some main considerations in choosing those extra eyepieces.
Type of Optical Lens
Another thing to know is that not all eyepieces are well suited to every telescope. Plossl eyepieces, for example, are not recommended for fast telescopes (f/5 or lower) or fast Dobsonians.
Fully multi-coated (FMC) glass optics enhances the light rays transmitted. This provides for high achromatic photos of distant objects such as Venus and Mars.
Focal Length Eyepieces
Telescope magnifications are directly related to the focal lengths of the telescope and the eyepieces.
magnification of telescope formula
How do you work out the telescope’s magnification? The ‘magnification of telescope’ formula is the telescope focal length (TFL) divided by the eyepiece focal length (EFL).
Magnification formula for telescope = TFL ÷ EFL
With a smaller eyepiece focal length, you’ll get higher magnification for an individual telescope. Here’s an example. A telescope with a focal length of 800 mm with an eyepiece with FL of 10 mm means a telescope magnification of 80x. Whereas, an eyepiece with a focal length of 25 mm with the same telescope would yield a magnification of 32x (800÷25).
Tip: Always make sure you are using the same units.
The limits of your telescope’s magnification range need consideration when buying eyepieces, as the aperture size may restrict how far you can go.
Field of View
This is how much night sky you will see. There is both the apparent (AFOV) and the true field of view (TFOV).
The larger the AFOV, the more sky you’ll see at a certain magnification, being the apparent angular sky width, ranging from 40 to 100 degrees. The AFOV is governed by the size of the eyepiece field stop.
The TFOV is the amount of sky you will really get to see.
TFOV = AFOV ÷ magnification.
Eye relief is the max distance where you can position your eye away from the top eyepiece lens and still see full field of view. It’s about comfort while observing. Having your eye jammed up close say with a 5 — 8 mm eye relief can become uncomfortable. Eye relief especially matters if you need to wear eye glasses while observing. This may be the case if you have a strong astigmatism. For this, look for a long eye relief, say 18–20 mm, to help.
This is the diameter of the light beam exiting the eyepiece and entering your eye. Generally the larger the exit pupil, the brighter the image you will see. But the limit is no more than 7mm as an average (the users pupil diameter under dark condition) or else the light is wasted.
In this, you need to consider the user’s age. So, the useful exit pupil can vary between 0.5–0.7 mm minimum and 5–7 mm maximum depending on light conditions and the users age. For users over 40–50 years of age, the maximum the pupil of the eye opens drops to about 5–6 mm and over 60 to about 4–5 mm.
The exit pupil is the eyepiece focal length ÷ telescope focal ratio.
Eyepieces for Telescopes
Some good eyepieces for general observing of the night sky include Gosky plossl, Celestron X-Cel LX, Celestron 93220, Celestron 93432 Luminos, Baader Hyperion, Orion Lanthanum, Orion 8728 Sirius Plossl, and Televue Nagler.
Many consider Tele Vue as the best telescope eyepiece brand especially when it comes to Nagler eyepieces, which have a wide range of focal lengths.
- Featured image source: Nick Kinkaid, Attribution-NoDerivs 2.0 Generic (CC BY-ND 2.0)
- FOV, Randy Culp, http://www.rocketmime.com/astronomy/Telescope/Magnification.htm