Explore Scientific 82° 18mm Waterproof Eyepiece - 2"

The 18mm Focal Length in an 82° Field: For Clusters and Nebulae

With its 18mm focal length, this eyepiece sits in the upper-medium magnification class and suits globular star clusters, planetary nebulae, the bright cloud structure near the nucleus of comets and initial observations of planets.

The 82° apparent field shows these objects with plenty of surrounding sky. The effect of the long eye relief and 82° field is a full-immersion experience.

A 7-Element Design with Enhanced Multilayer Deposition (EMD) Coatings

The optical system is a computer-designed layout featuring 7 lens elements arranged in 4 groups. This configuration uses combinations of low-dispersion and high-refractive-index optical glasses which are optimized for high contrast, high resolution and flat field characteristics.

The lenses carry durable Enhanced Multilayer Deposition (EMD) coatings. To further boost contrast, all internal surfaces of the eyepiece barrel are blackened, and the edges of the lenses are darkened to prevent stray light from scattering and reducing image definition.

The 82° Immersion Experience and Averted Vision

The ultra-wide 82° apparent field of view creates a comfortable, engaging window on the universe that naturally promotes relaxation at the eyepiece. With a relaxed eye, you can observe for longer periods without fatigue, allowing faint details to materialize that might otherwise be missed. The 18mm has generous eye relief.

The wide field also makes it easy to use averted vision: looking slightly to the side of a faint object puts its light on a part of your eye that is more sensitive to light, allowing you to see fainter images.

Waterproof, Argon-Purged Housing for Field Durability at 390g

Each eyepiece is assembled into precision machined metal lens barrels that keep the optical train aligned for years of rugged use. It is O-ring sealed and purged with an inert gas to create a waterproof, fog-proof housing.

A tapered 2" barrel adds extra security. At 390 g, it may need you to rebalance the telescope when swapping with much lighter eyepieces.

Using the 25.3mm Field Stop to Calculate True Field and Magnification

The eyepiece's 25.3mm field stop is the physical aperture inside the barrel that defines the edge of the field of view. You can use this value to calculate the true field of view (TFOV) this eyepiece will deliver on your specific telescope using the formula: TFOV ≈ (Field Stop Diameter / Telescope Focal Length) × 57.3.

• Magnification Example: On a telescope with a 1200mm focal length, this 18mm eyepiece provides 1200mm / 18mm = 67x magnification.
• True Field Example: On that same 1200mm telescope, the true field would be approximately (25.3mm / 1200mm) * 57.3 ≈ 1.2 degrees.

Pairing with Explore Scientific 2" Focal Extenders

Explore Scientific Focal Extenders raise magnification while keeping the eyepiece's native apparent field and eye relief. The 2" models are the FE02-020 (2x) and FE03-020 (3x). With a focal extender, 3 eyepieces offer 6 magnification choices.

With the 2x extender the 18mm gives the magnification of a 9mm eyepiece; with the 3x, that of a 6mm.

Explore Scientific 82° 18mm vs. A Standard 50° Plössl Eyepiece

A Plössl of the same focal length shows a much narrower field.

• Field of View: The most significant difference is the 82° apparent field compared to a Plössl's typical 50°. This provides a much larger, more immersive window onto the sky, making it easier to frame large objects and star-hop to new targets.
• Trade-Offs: A Plössl is generally lighter and smaller than this 390 g, 89mm by 56mm eyepiece.