Sky-Watcher Emerald 9mm 82° Eyepiece

The Emerald 9mm Eyepiece: High-Power 82° Views

Sky-Watcher's Emerald 9 mm eyepiece is designed for amateur astronomers seeking high-power views in an immersive, wide-field format. Sky-Watcher offers the Emerald line in 9 mm, 16 mm and 22 mm for everything from high-power lunar detail to sweeping deep-sky vistas.

Within this lineup, the 9mm is the high-magnification option, for the Moon, planets, double stars and small, bright deep-sky objects.

A 7-Element System with Broadband Anti-Reflection Coatings

At the heart of the Emerald 9mm is a 7-element optical design engineered to deliver its signature 82° apparent field of view.

To maximize light throughput and image contrast, all lenses within the eyepiece are treated with broadband anti-reflection coatings. These specialized films reduce the amount of light lost to reflection at each glass-to-air surface, preventing internal scattering that can reduce contrast and wash out faint details in celestial objects.

The 82° Apparent Field

The defining feature of the Emerald series is its 82° apparent field of view (AFOV). This ultra-wide field of view gives an expansive, immersive view of the night sky.

This wide field offers a tangible practical benefit, especially on non-tracking telescopes like Dobsonians. At a given magnification, a wider apparent field shows a wider true field, which keeps your target in view for a longer period before it drifts out due to Earth's rotation. This reduces the need for constant nudging and allows for more relaxed, continuous observation of your target.

Understanding the 8mm Eye Relief

The Emerald 9 mm eyepiece has an eye relief of 8 mm. Eye relief is the fixed distance from the eyepiece's top lens to the point where your eye must be positioned to see the full 82° field of view. An 8 mm distance is considered short and requires you to place your eye very close to the lens.

This is a common design trade-off in short focal length, wide-field eyepieces. For observers who do not wear glasses, this close distance is typically manageable. However, if you rely on glasses at the eyepiece to correct for astigmatism, the 8 mm distance may not be sufficient to see the entire field. For comparison, the 18 mm eye relief on the 22mm Emerald model is ideal for spectacle wearers.

Calculating Magnification for Your Telescope

An eyepiece's magnification is not fixed; it is a function of the telescope it is paired with. To calculate the magnification of the Emerald 9 mm, simply divide your telescope's focal length by the eyepiece's 9 mm focal length.

This high-power eyepiece will produce significantly different magnifications depending on your instrument's optical design and focal ratio. Here are a few examples:

• In an 8-inch f/6 Dobsonian (1200mm focal length): 1200 mm ÷ 9 mm = 133x magnification.
• In a telescope with a 700mm focal length: 700 mm ÷ 9 mm = 78x magnification.
• In an 11-inch f/10 SCT (2800mm focal length): 2800 mm ÷ 9 mm = 311x magnification.

How the 9mm Focal Length Affects Image Brightness

Beyond magnification, the eyepiece also determines the exit pupil, which controls the brightness of the extended objects you observe. The exit pupil is the diameter of the beam of light leaving the eyepiece. You can calculate it by dividing the eyepiece focal length (9 mm) by your telescope's focal ratio (f-number).

A smaller exit pupil results in a dimmer image background, which can increase contrast on bright objects but make faint objects harder to see. The 9mm Emerald will produce small exit pupils suitable for high-power viewing:

• In an f/10 telescope: 9 mm ÷ 10 = 0.9 mm exit pupil.
• In an f/6 telescope: 9 mm ÷ 6 = 1.5 mm exit pupil.
• In an f/4.5 telescope: 9 mm ÷ 4.5 = 2.0 mm exit pupil.

Standard 1.25" Barrel with Filter Threads

The Emerald 9 mm features a standard 1.25" barrel, for 1.25" focusers and diagonals. In a 2" focuser it needs a 2" to 1.25" adapter.

The barrel is also threaded to accept standard 1.25" astronomy filters. This allows you to directly attach light pollution filters, color filters for planetary observing, or specialized nebula filters to the eyepiece, tailoring your view to specific targets and sky conditions.

Using the Emerald 9mm with Fast and Slow Telescopes

The performance of a wide-field eyepiece can vary with a telescope's focal ratio. In "slow" telescopes with long focal ratios (f/8 and above), like Schmidt-Cassegrains or Maksutov-Cassegrains, the eyepiece will deliver very high magnification and a dark sky background, ideal for resolving fine planetary detail or splitting tight double stars.

In "fast" telescopes with short focal ratios (f/4 to f/6), such as many Dobsonians and astrographs, the eyepiece will provide moderate-to-high magnification with a brighter field of view. This makes it useful for viewing smaller, brighter deep-sky objects like planetary nebulae and globular clusters within an immersive 82° field.

CNC-Machined All-Metal Body and 165g Weight

The eyepiece is constructed from a CNC-machined all-metal body. The machined metal housing protects the optics and is built to withstand years of observing in the field.

Despite its solid metal build, the 9mm model is the lightest and most compact in the Emerald series, weighing just 165 g (0.36 lb). It ships with a protective carry case with a durable exterior and custom-fit interior to shield the eyepiece from dust, moisture, and impacts during transport.

Choosing Your Focal Length: 9mm vs. 16mm and 22mm

The Emerald 9 mm is the highest-magnification eyepiece in its series. While all three models share the same 82° apparent field of view and 7-element optical design, they differ in key specifications that make them suited for different purposes.

• Emerald 9mm: The highest-power option in a 1.25" barrel. Its 8 mm eye relief is short, and its 165 g weight is minimal. This is the eyepiece for pushing magnification on smaller targets.
• Emerald 16mm: A mid-power eyepiece with a 2" barrel and a more comfortable 14 mm of eye relief. It is suited for general deep-sky observing of medium-sized objects like galaxies and larger nebulae.
• Emerald 22mm: The lowest-power option, providing the widest true field of view in the series. It uses a 2" barrel and offers a generous 18 mm of eye relief, which is ideal for eyeglass wearers.

Care and Cleaning for Longevity

There are no specific cleaning instructions for the Emerald eyepieces. Store the eyepiece in its case between sessions and avoid touching the lens surfaces.