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 Do I Need Extra Eyepieces?
Buying extra eyepieces besides those that come with the telescope can make a huge difference to your experience in observing the night sky.
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.
About Telescope Eyepieces
You may think about buying an eyepiece kit, but three or four quality eyepieces will serve you better than 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 all telescopes. 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. This is how:
Magnification of telescope = Focal Length of Telescope ÷ Focal Length of Eyepiece
That result? Large eyepiece focal lengths would result in low power when compared to small eyepiece focal lengths with the same telescope.
In other words, the smaller the focal length of an eyepiece the higher the magnification for a specific telescope. Here’s an example. A telescope with a focal length of 800 mm and an eyepiece with one of 10 mm will result in a telescope magnification of 80x. Whereas, an eyepiece with a focal length of 25 mm will yield a magnification of 32x.
Field of View
This is how much night sky you will see. Look at this in terms of 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.
Now the TFOV is the AFOV ÷ magnification and is the amount of sky you will really get to see.
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)