How to Choose an Eyepiece

Always remember that your eyepiece is half of your telescope – there’s no point buying a Ferrari and then not putting the wheels on! Choosing an eyepiece can be a daunting task, especially if you’re not familiar with how telescope optics work. We sell a range of high-quality eyepieces, with universal sizes to fit in any telescope, regardless of brand. It’s always worth having a variety of focal lengths to provide you with a range of magnification options for you night sky observing, but working out what to get can be a frustrating exercise, and involves mathematics! Please call our full-time staff in Sydney any time for a discussion if you’re having trouble, or simply have a read of our helpful hints below!

1: Focal length, and some simple maths!
All eyepieces have a number associated with them – the focal length. The smaller this number, the higher magnification the eyepiece will provide, and vice-versa. You can work out the exact magnification if you know your telescope focal length (look at the specifications of the model you own). For example, a Celestron Nexstar 4SE has a focal length of 1325mm. With a 10mm eyepiece, this telescope will magnify the target 1325/10 = 132.5x! With a 25mm eyepiece, it will magnify 1325/25 = 53x.

2: Don’t go too high!
While you can put any eyepiece in any telescope provided the barrel sizing is correct, you should make sure not to zoom too much. Putting an eyepiece providing 400x magnification into a small refractor will give a blurry and fuzzy image, simply because you are stretching the light too far. As a general rule, you should never magnify more than double the aperture of your telescope in millimeters. For example, the lens diameter of a Celestron Nexstar 4SE is 4 inches (~100mm). You should thus never use an eyepiece in a Nexstar 4SE which powers it over 200x. Similarly, if you own a Cosmos 70 AZ (70mm lens diameter), you should not exceed 140x.

3: Make sure to get the right barrel size!
Most eyepieces are associated with either 1.25″ or 2″. This refers to the diameter of the eyepiece slot at the end of your telescope. Almost all beginner telescopes use 1.25″ eyepieces, though some SCT and reflector telescopes (particularly higher end models which are convertible for photography) will fit to both sizes. If in doubt, give us a call!

4: Don’t forget barlows!
Rather than buying six different eyepieces to give you six different powers, one option is to invest in a barlow lens. Put simply, a barlow used in conjunction with an eyepiece will increase the power by a set amount. A 2x barlow will double the power of any eyepiece (eg: turning a 20mm into a 10mm with higher magnification), while a 3x barlow will triple the power, etc, etc. This means that you can get six different magnifications with only three different eyepieces – very convenient!

5: Quality is important!
Eyepieces vary greatly in price, from simple $49 eyepieces, to $1000 Tele Vue branded eyepieces. Naturally, a standard 10mm eyepiece will not have as high light transmission or clarity as a 10mm Tele Vue eyepiece for triple the price, but to get the most out of expensive eyepieces, you need to be putting them in a good telescope! With high light transmission eyepieces, fainter deep space objects like nebulas and galaxies will suddenly be within reach for many observers who previously were not able to resolve detail on these fascinating targets. You can usually spend up to the cost of your telescope on a matching eyepiece (eg: if your telescope is worth $400, you could buy a $400 eyepiece to put in, which would give excellent results). Exceeding this limit would probably be overkill in most cases – if you’re particularly keen, maybe think about investing in a newer, bigger telescope!

Check out our range of eyepieces & barlows, and give us a call if you have any questions!

Tele Vue Optics has been thrilling customers since 1979. The company founder, Al Nagler, put his lifelong love of amateur astronomy to work as a professional optical designer in 1958. He says that it was his involvement in designing NASA lunar landing simulators that most directly influenced the Tele Vue products of today – Al wanted his own observing equipment to deliver that seemingly limitless starry vista usually reserved for astronauts, floating thousands of kilometres above the Earth’s surface. Starting by improving the standard Plossl design, Al’s top-grade optical work has continued, with every new design bringing his dream closer to reality. First, the Nagler series was released, with 10 times the field sharpness of any previous eyepiece. Tiny tweaks and years of hard work finally paid off in the Ethos range, 100 degree field of view eyepieces with stunning clarity. Since then, Tele Vue has continued to push to realm of what is possible, producing more compact, lightweight, and economical versions of the revolutionary Ethos (namely the Panoptic, Delos and DeLite series), to the point that now their range of eyepieces is huge, and there is something for every die-hard amateur astronomer. The eyepiece is, after all, half of your telescope! Bintel is Tele Vue’s Official Australian Dealer.


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