Explore Scientific 100 Degree 9mm Eyepiece Review


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The Explore Scientific (ES) eyepiece is everything it says it is.


Waterproof and fogproof.


But is spending this much on a single eyepiece worth it?

Only if you want to squeeze out the last 10% of telescope performance to obtain the absolute best in viewing quality.

Zooming in…

✔️ Best Feature: 100-degree AFOV

Worst Feature: Short eye relief

👌 Ideal For: Low Focal Ratio Telescopes, High Magnification, Super Wide FOV, Planets, DSO, Intermediates, Experts

  • Barrel Size: 2”
  • Apparent Field of View: 100 degrees
  • Focal Length: 9mm
  • Eye Relief: 12.5 mm
  • Field Stop Diameter: 15.7 mm

Our Verdict: The Explore Scientific 100-degree 9mm eyepiece is a luxury eyepiece and has a price tag that matches its status, but what you get in return is excellence at its best. To see more than ever with contrast and sharpness that inferior eyepieces can only dream of providing makes the 100-degree eyepiece more than worth it.

Who is the Explore Scientific 100-degree 9mm Best Suited to?

Eyepieces of this quality are usually aimed towards intermediates and expert telescope users. While beginners will experiment with complete kits that are cheaper than this single eyepiece, it takes experience and time to come to the understanding that expensive eyepieces are worth it.

As such, those who have had some hands-on experience and are looking to squeeze out more performance across the entire field of view will likely be the ones to flip the bill on such an eyepiece.

How Does the Explore Scientific 100-degree 9mm Perform?

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The ES eyepiece provides a stunning view of the universe as it almost seems to put you right there into space. With its hyper-wide 100-degree AFOV and short focal length, you can achieve exceptionally clear and pristine views at high power.

Not only can you see more through the eyepiece, you can also keep that object within the FOV for a long time before any adjustments need to be made.

As expected, the 100-degree eyepiece is not a small thing. It’s almost 6” long and is 2.4” wide and weighs 21.2 oz. If you must, it also has a 1.25” barrel that allows you to use it with a 1.25” focuser.

Features & Benefits

100-degree Apparent Field of View

It’s no surprise to see this 100-degree AFOV eyepiece with a steep price tag. You don’t get super-wide FOVs without investing in it.

The entire FOV is sharp and false-color-free. High contrast is present and allows you to find many faint objects, easily locate objects, and fit the entire disk of the moon within the FOV at high magnification.

The ES 100-degree series are often compared to Tele Vue’s Ethos series, and while they may offer the same AFOV, there are other aspects to consider. The ES eyepiece is cheaper and is completely water and fogproof.

Corrective Eyepiece Technologies

As the AFOV goes up, so does possible aberration from the center to the edges of the FOV. So, not only are you looking for an eyepiece that has a high or wide AFOV, you also want an eyepiece with corrective technologies.

ES uses hyper-wide glass with what has to include corrective technology because you can use averted vision to see fainter objects around the entire FOV all the way to the edge.

Like many eyepieces of this caliber, it can correct for those aberrations that would normally be visible with inferior eyepieces, but this is no normal or inferior eyepiece. It has 12 lens elements in 8 groups and each lens element has blackened edges that provides superior contrast.

Water & Fogproof

The ES eyepiece is completely waterproof to handle the just-in-case moments when water may threaten the eyepiece. It’s also completely fogproof with its gas-purged sealed body.

 Nitrogen is usually the gas of choice for most weatherproof optics, but argon is the high-end gas of choice for high-end eyepieces.

You won’t experience any foggy lenses when you’re waiting for the right hour to view your desired targets for the night.


Short Eye Relief

In all reality, to have 12.5 mm eye relief from a 9mm eyepiece is actually great. It’s about as low as you want to go without having to compromise comfort. The only unfortunate thing about it is that people with glasses will struggle to use the ES eyepiece.

With no available technologies or diopter to correct for vision issues, it’s an eyepiece that is suitable for non-glasses-wearers but not for those who must wear glasses.

Popular Questions

How does the 100-degree 9mm Eyepiece Work for Planets?

With such high power, the ES 100-degree eyepiece works great for planets as it still provides a huge field of view with contrast and sharpness across the entire view. Not only can you see greater detail in planetary features, but you can also keep that planet within the FOV longer. You may need to use filters to help because they will be bright.

What Telescopes can be Used with the Explore Scientific 100-degree Eyepiece?

Virtually any telescope can be used with the 100-degree series eyepieces. As this model has a 2” barrel, it can be adapted to be used with 1.25” optical assemblies.

While it will obviously work very well with long focal length telescopes, it would be a fantastic eyepiece to incorporate with fast telescopes as it provides not only a wide FOV but the corrective technology needed to provide an excellent image.

What is the Warranty on the Explore Scientific Eyepiece?

The 100-degree 9mm eyepiece comes with an Explore Scientific 1-year warranty. It must be registered with the brand within 60 days of purchase for it to be covered for any warranty repairs or replacements for the warranty period.  


Why use a super-wide eyepiece? If you have a manual mount and you’re having a hard time keeping the object within the field of view to get decent views, you need a wide-angle eyepiece.

Enter here, the Explore Scientific 100-degree 9mm eyepiece.

It’s super wide, provides high magnification, and it’s completely water and fogproof. With no aberrations, sharp and round stars, and excellent contrast to see faint objects, your observation sessions may get longer and longer each time you head out. Be prepared to be blown away.

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The allure of the cosmos captivates Fern, with its endless wonders and celestial majesty. There’s a unique tranquility, yet an undeniable thrill, in uncovering the intricacies of our vast galaxy. Away from her telescope, Fern finds solace in the pages of a gripping novel, often accompanied by a cup of her favorite tea.