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Choosing the Right Eyepiece for Your Telescope

Choosing the Right Eyepiece for Your Telescope

AstroTelescopium Team |

The eyepiece you decide to use with your telescope can either enhance or hinder your viewing experience when observing celestial objects. An eyepiece has a significant effect on the optical system of your telescope and its ability to provide you with the best views possible. In fact, without the right eyepiece your high quality telescope is pretty much useless.

Oftentimes telescope manufacturers provide modest quality eyepieces with their telescope. Unfortunately, these do not necessarily provide the full range of observing that you're looking for. That's why most stargazers quickly replace the modest quality eyepieces with those of higher quality.

Although there are various types of eyepieces to choose from, this guide is intended to make the selection process easier for you. The goal of this article is to provide you with the basics for choosing the right eyepiece without wasting too much time or too much money.

 

What are telescope eyepieces?

To make sure we're on the same page, let's start off with a brief definition. A telescope eyepiece, or ocular lens, is an essential accessory that determines the magnification and field of view of your telescope.

Eyepieces consist of multiple lens elements constructed in a housing. They play a crucial role in determining the clarity, sharpness, and brightness of the images you see through your telescope. Ideally, you should use an eyepiece that matches or exceeds the quality of your telescope.

 

What factors should you consider when choosing eyepieces?

Focal Length

Focal length is the distance that light travels through the eyepiece. The focal length of the eyepiece is the determining factor in the resulting magnification. Higher focal length eyepieces provide lower magnification, while lower focal length eyepieces provide higher magnification.

More specifically, magnification is determined by using the following formula:

Magnification = Telescope Focal Length (mm) / Eyepiece Focal Length (mm)

For example, if your telescope has a focal length of 1800mm and your eyepiece has a focal length of 60mm, then 1800 / 60 = 30. Therefore, your image is magnified 30x.

In order to increase the magnification, you would need to use an eyepiece with a shorter focal length. Using the same example, if we select an eyepiece with a focal length of 10mm, then 1800 / 10 = 180. As you can see, by reducing the focal length of your eyepiece, you have increased the magnification to 180x.

Alternatively, if you already know the magnification you want, you can divide the telescope's focal length by the desired magnification in order to solve for the required eyepiece focal length.

For example, let's say your telescope's focal length is 2000mm and you want to select an eyepiece that provides 200x magnification, simply divide 2000 by 200 which equates to 10. Therefore, a 10mm eyepiece will give you the 200x magnification you desire.

Apparent Field of View

The apparent field of view (AFOV) refers to the angular diameter or width of the image you see through the eyepiece before it's attached to the telescope. A wider field of view allows you to observe larger portions of the night sky. AFOV's range from narrow (less than 40°) to an extra wide angle (80° or more).

For an immersive visual experience, there are generally three classes for field of view:

  • Standard-Field (AFOV = 40° to 60°)
  • Wide-Angle (AFOV = 65° to 75°)
  • Extreme Wide-Angle (AFOV = 80° to 100°)

True Field of View

The true field of view refers to the angular diameter or width of the image you see through the eyepiece when it's attached to the telescope. The following formula calculates the true field of view:

True Field of View = Apparent Field of View / Magnification

For example, let's say your telescope has a focal length of 1800mm and your eyepiece has a focal length of 20mm with a 30° apparent field of view. The magnification would be 1800mm / 20mm = 90x. In this case, the true field of view would be 30 / 90 = 0.33°.

Eye Relief

Eye relief is the distance between your eye and the eyepiece lens when you are viewing a focused image. While comfortable eye relief is a matter of personal choice, 10-20mm is generally easy to use for most.

Longer eye relief (15mm and more) is beneficial for eyeglass wearers or those who prefer to be positioned further away from the eyepiece during their observations.

Generally, the shorter the focal length, the shorter the eye relief. However, many modern eyepieces are designed to provide long eye relief irrespective of the focal length.

 

How do you choose the right eyepiece for your telescope?

Know your telescope

Understand the specifications and limitations of your telescope, including its focal length and maximum useful magnification. Typically, the maximum usable power is about 2 times the telescope's aperture (in mm) or 50 times the telescope's aperture (in inches).

However, it is important to note that as power increases the sharpness of the image decreases. In other words, if the telescope is not collecting enough light, when the image is enlarged it becomes too faint. Also, at very high power, the image is more easily blurred by atmospheric turbulence.

Make sure to understand which eyepiece size your telescope can accept. The barrel size of an eyepiece is the part that slides into the focuser or the diagonal. A 1.25" focuser can accept a 1.25" eyepiece and a 2" focuser can accept a 2" eyepiece. If your telescope comes with a 2" focuser but includes a 2" to 1.25" adapter, it can accept a 1.25" eyepiece.

Determine your observing goals

Since different eyepieces maximize different types of observations, you should first determine the type you are interested in. That is, planetary observations, deep-sky objects, or both.

  • Ultra short focal length (2-4mm) eyepieces provide very high power magnification and are best for observing the planets and the moon on shorter focal length telescopes.
  • Short focal length (5-10mm) eyepieces provide high power magnification and are best for observing the planets and the moon on medium focal length telescopes.
  • Medium focal length (10-20mm) eyepieces provide medium power magnification and are best for observing larger deep sky objects on average sized telescopes.
  • Long focal length (20-30mm) eyepieces provide low power magnification and are best for observing larger deep sky objects on any sized telescope.
  • Very long focal length (30mm+) eyepieces provide very low power magnification and are best for observing the largest deep sky objects.

Optical Quality

Because eyepieces come in a wide price range, it is imperative that you determine your budget and prioritize the factors that are most important to you. Do not skimp on quality however. Look for eyepieces with high-quality optics in order to ensure clear and sharp images. Consider eyepieces with multi-coated lenses to minimize reflections and improve contrast. 

If possible, test different eyepieces to see how they perform with your telescope. Visit a local astronomy club or attend star parties to get hands-on experience.

 

Which telescope eyepiece is best?

One single eyepiece that is best for all seeing conditions does not exist. The best option is to start with a range of at least three separate magnification eyepieces:

  • Low power magnification (longer focal length) for seeing large celestial objects or for easily finding targets before switching to higher magnifications
  • Medium power magnification (medium focal ength) for seeing more detail in deep sky objects such as galaxies and small star clusters
  • High power magnification (shorter focal length) for seeing planetary and lunar details, observing small celestial objects, and resolving double stars

Ideally selecting eyepieces with focal lengths that offer close to a doubling of magnification with each step. For example a 40mm, 18mm and 8.5mm eyepiece to provide your low, medium, and high power magnification range.

It is important to note that purchasing 2 to 3 premium eyepieces will provide a much more enjoyable experience than having 4 to 8 average quality ones.

So, which eyepiece do we recommend?

Explore Scientific Eyepieces

We recommend Explore Scientific's Waterproof Eyepieces. These premium eyepieces are argon-purged at all focal lengths. Basically, by making the eyepiece body argon-purged, the waterproof eyepiece internal elements will remain as pristine as the day they were assembled.

Why argon instead of nitrogen? Well, to further explain, the atomic radius of Argon is larger than Nitrogen by 0.13 angstroms and has a larger atomic volume by 11.2 cubic centimeters/molecule. Plus, Argon has superior inertness forming few or no chemical compounds with other elements versus Nitrogen. That inertness prevents adverse reactions with eyepiece components and therefore extends the longevity of the internal components.

Argon is also less conductive of heat than Nitrogen, and it cools slower, buffering the effects of rapid temperature changes of the optics.

To ensure a perfect seal Explore Scientific tests every eyepiece in one meter of water for 30 minutes.

Explore Scientific's premium eyepieces are optimized to produce high contrast, high resolution, and superior flat field characteristics. Your experience will be very sharp and clear across the entire field of view.

Explore Scientific 82° Series eyepieces

Price range: $229 - $499

  • Ultra-Wide Angle FOV
  • Most Popular Series
  • Argon-Purged
  • Waterproof & Fogproof
  • Fully Multi-Coated
  • Slip-Resistant Textured Surface
  • Foldable Soft Silicone Rubber Eyecup
  • Parfocal

Explore Scientific 68° Series eyepieces

Price range: $219 - $419

  • Wide Angle FOV
  • Argon-Purged
  • Waterproof & Fogproof
  • Fully Multi-Coated
  • Slip-Resistant Textured Surface
  • Foldable Soft Silicone Rubber Eyecup
  • Parfocal

Explore Scientific 52° Series eyepieces

Price range: $89 - $179

  • Standard FOV
  • Budget-Friendly
  • Argon-Purged
  • Waterproof & Fogproof
  • Fully Multi-Coated
  • Slip-Resistant Textured Surface
  • Foldable Soft Silicone Rubber Eyecup
  • Parfocal

 

Final Thoughts

Choosing the right telescope eyepieces is crucial for maximizing your observing experience. This article has covered key factors such as focal length, apparent field of view, eye relief, and optical quality.

By understanding your telescope and observing goals, and considering your budget, you can make an informed decision and enjoy breathtaking views of the cosmos.

Looking to extend your range of magnifications and essentially turn your 2-3 eyepieces into 4-6? Consider adding a 2x or 3x focal extender to your accessory list:

If you do select any of the Explore Scientific Waterproof Eyepieces, make sure to also take a look at this short video demonstrating the eyepiece cleaning process for proper maintenance of your premium optics.

 

 

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