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The Ultimate Telescope Eyepiece Guide: Everything You Need To Know

The Ultimate Telescope Eyepiece Guide: Everything You Need To Know

 

Eyepieces hold a ton of power over the quality of our skygazing experiences. Amateur astronomers often have a giant breakthrough moment when they look through a telescope paired with a high-quality eyepiece for the first time. The eyepieces included with your telescope starter pack are a good start, but if you’re ready to level up, high-quality eyepieces will be your next best purchase.

 

If you own a high-end telescope but your eyepieces don’t meet or exceed the optics of your scope, it’s going to bring the whole experience down. Alternatively, if you have an entry-level scope, the best thing you can do to elevate your experience is purchase quality eyepieces!

 

If you feel like there are too many components of an eyepiece to ever make sense of, this guide is for you. We've broken down everything you need to know about eyepieces so you can shop with confidence!

 

Key Factors To Consider:

  • What you wish to observe
    • Closer objects like planets and the moon, or distant objects like star clusters, nebulae, and galaxies.
  • How tolerant you are of optical imperfections
  • Whether you want a wide or narrow field of view (FOV)
  • Whether you wear eyeglasses while observing
  • Budget
    • Higher price typically indicates sharper image and wider field of view

 

Note: You can mix and match brands. If you have a Celestron telescope you can use Explore Scientific eyepieces, for example. You just have to make sure to choose an eyepiece with the right barrel size (typically 1.25” or 2”).

 

Primary Features of an Eyepiece

Focal Length

Source: https://stargazingpro.com
 

Telescopes and eyepieces each have their own focal length measurement. Focal length refers to the distance the light travels through the telescope or eyepiece to reach the eye, and is directly related to determining magnification. The amount of light your telescope gathers will always be the same, but you can swap out eyepieces with different focal lengths to view different magnifications.

 

Focal length is the number (measured in millimeters) you’ll see written on the eyepiece itself (for example, 10mm, 15mm, etc).

 

Magnification (Power)

Source: https://skyandtelescope.org/
 

As mentioned, magnification (which can also be thought of as how powerful your eyepiece is) is directly related to the focal length. You can calculate the magnification by dividing your telescope focal length by eyepiece focal length.

 

Magnification = Telescope Focal Length/Eyepiece Focal Length

 

For example, if you have a 2000mm focal length telescope with a 25mm focal length eyepiece, magnification will be equal to 2000/25 which is 80X. In other words, objects will appear 80 times closer than with the naked eye.

 

The focal length of an eyepiece has an inverse relationship with magnification. The shorter the focal length, the more the magnification. A 6mm eyepiece has more magnification power than a 25mm eyepiece, for example. You’ll sometimes hear eyepieces being referred to as “low-power” or “high-power”. A low-power eyepiece is simply an eyepiece with less magnification, and vice versa.

 

Different celestial bodies are best viewed at different magnifications. So the first thing you’ll want to do is decide what you plan to be observing the most. If you’d like to keep your options open, we recommend starting out with 1 low power, 1 medium power, and 1 high power eyepiece, and a barlow lens (more on barlow lenses below).

 

Telescopes are not unlimited in their viewing capabilities. Each telescope has a physical limit that restricts you from increasing magnification indefinitely. Telescopes always gather a fixed amount of light, so when you move to higher magnification eyepieces, the light is being spread over a larger area, resulting in the image being dimmer. Often, when using your telescope at different powers (magnification) you have the choice between small, bright images and large, dimmer images.

 

It’s important to know your telescopes highest usable magnification so that you don’t purchase an eyepiece that isn’t practical for your scope.

 

Highest usable magnification = 50X x Telescope Aperture

 

If you have a telescope with a 4” aperture, then 4x50=200X. 200X is the maximum amount of magnification you can expect your telescope to be able to handle.

 

The best practice when using your telescope is  to start with your low power (low magnification, long focal length – so, 25mm or 30mm, for example) eyepiece to get your desired object into the field of view of your telescope. Then you can start gradually moving to higher powered eyepieces (high magnification, shorter focal length) until the view improves and you reach your desired level of brightness, scale, and visibility.

 

Different focal lengths and their best uses are detailed below.

 

2mm-4.9mm Eyepieces

  • Very high magnification good for long focal length refractors and SCTs
  • For close up planetary viewing

5mm-6.9mm Eyepieces

  • Good planetary detail and double star eyepieces
  • For long focal length scopes & shorter focal length scopes with steady seeing conditions

7mm-9.9mm Eyepieces

  • Good high magnification eyepieces for shorter focal length scopes
  • Planetary, double star, and lunar viewing

10mm-13.9mm Eyepieces

  • Good across all focal lengths
  • Offer great background darkening for studying nebula, small galaxies, planetary details, lunar details

14mm-17.9mm Eyepieces

  • Mid range magnification for all focal lengths
  • Good for globular clusters, galaxy details, planetary nebulae

18mm-24.9mm Eyepieces

  • Longer focal length to view wide field and extended objects
  • Shorter focal length scopes great for mid-range magnification of galaxy clusters

25mm-30.9mm Eyepieces

  • These eyepieces paired with a longer focal length scope make for good viewing of large nebula and open clusters
  • With a shorter focal length scope, these eyepieces are good for viewing Orion nebula, views of the lunar disc, large open clusters
  • Good "locator eyepiece" in all focal length scopes

31mm-39.9mm eyepieces

  • Shorter focal length scopes for extended views of large, starry fields

40mm eyepieces

  • Exclusively for shorter focal length scopes
  • Large, starry vistas, extended nebulae w/ star fields

 

Apparent Field of View (AFOV):

Source: https://agenaastro.com/

 

Often just referred to as field of view (FOV), this measurement refers to how much width of the sky you can see through your eyepiece. FOV is measured in degrees and is usually advertised on the eyepiece. 25-30 degree eyepieces are considered super narrow, and are best used for planetary and lunar viewing. 80+ degrees indicate an ultrawide FOV, and are ideal for an immersive experience when viewing large, deep-sky objects.

 

True field of view is the actual angle of sky seen through your eyepiece when paired with your telescope. This is calculated by dividing apparent field of view by magnification.

 

True Field of View = Apparent Field/Magnification

 

Exit Pupil

Source: https://www.bestbinocularsreviews.com/

 

The exit pupil measurement is the size of the image that comes out of the eyepiece. For all light rays that enter your pupil, the exit pupil must be smaller than the pupil of your eye, otherwise light spills over and is lost. Our pupil diameter decreases as we age. On average, the size a dark-adapted pupil for a middle-aged adult is 5mm, and 7mm for younger adults. Don’t go smaller than 0.5 to 0.7mm.

 

Exit pupil size (mm) = eyepiece focal length/telescope focal ratio

Focal ratio (f/#) = Focal Length of Objective / Diameter of Objective

*A telescope with a smaller focal ratio is often called a "fast" telescope

 

Eye Relief

 

The eye relief measurement tells you how far your eye must be from the eyepiece to see the entire field of view. Eye relief is a measure of personal preference. Bigger distance (longer eye relief) is going to be more comfortable if you wear glasses when observing.

 

Eye relief is typically directly related to focal length where shorter focal length = shorter eye relief. However, new advancements have allowed some models to offer LER (long eye relief) with short focal length. In this case, you can expect to see it advertised on the eyepiece product page when you’re shopping.

 

Barrel Size

 

This is the key measurement you need to ensure your eyepiece fits in your telescope. The barrel of an eyepiece slides into the focuser or diagonal which connects to your telescope.  1.25” barrels are the industry standard though you’ll sometimes see 2” as well.  

 

Quality Decisions:

Eyepieces are made of multiple lenses stacked together. Non-reflective coatings are an imperative feature to cut down reflections between the lens surfaces that will reduce contrast and spoil the view. The interior of eyepiece and edges of eyepiece lenses will be painted matte black to reduce reflections. When looking for an eyepiece with the best non-reflective coatings, look for the terms "fully multi-coated" or "multi-coated" when shopping and avoid eyepieces advertised as simply just "coated" or "fully-coated".

 

Types of Eyepieces: 

Features and types are non-exhaustive, but this will be what you come across most often on the market.

  • Plossl
    • Wide FOV (around 52 degrees)
    • Often a shorter eye relief (how far your eye must be from the eyepiece in order to see he entire FOV)
    • Good all-round eyepiece choice for both planetary and deep sky observations
  • Radian
    • Similar FOV to Plossl
    • Longer eye relief (great especially if you wear glasses)
  • Orthoscopic
    • This was considered one of the most reliable eyepiece types before the Plossl was introduced, though it’s still a great option
    • Decent eye relief
    • FOV 40-45 degrees narrow but excellent imaging
    • Good for moon and planetary observations
  • Barlow Lens
    • Not an eyepiece but increases magnification of your existing eyepiece
    • Essentially lets you double the amount of eyepieces you have
    • Double or triple magnification
    • Attach your eyepiece to it and connect the whole thing to where eyepieces usually go
    • If you're purchasing a barlow lens with your eyepieces, stagger your focal lengths (don't purchase 10mm and 20mm eyepieces with a barlow lens, because your 20mm with a barlow is essentially a 10mm)

 

Telescope and eyepiece quality play a major role in your skygazing experience but they’re not the end-all-be-all. There are many other factors at play such as seeing conditions. Just because you have a high-end telescope and eyepiece, it doesn’t guarantee that you’ll have a perfect night of viewing – but it will certainly put the odds in your favor.

 

We hope this guide was helpful! We did our best to include all useful information but not overwhelm you with facts that you won’t need to know in day-to-day use. 

 

We'd love to hear your thoughts or questions regarding eyepiece selection. Leave a comment below, we'll read every one!

 

Shop Eyepieces

 

Explore Scientific 62 Degree Eyepiece 40mm

Shop: Explore Scientific 62 Degree 40mm Eyepiece

This is a low power eyepiece with a wider field of view, making it a great starter piece.

 

Explore Scientific 100 Degree Waterproof Eyepiece

Shop: Explore Scientific 100 Degree 9mm Eyepiece

This eyepiece offers an excellent balance between ultrawide viewing and magnification. 

 

Shop our entire collection of quality eyepieces at TelescopeTek

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