Sky-Watcher Emerald 16mm 82° Eyepiece

The Emerald 16mm: An 82° Eyepiece for Immersive Viewing

The Sky-Watcher Emerald 16mm eyepiece is designed for amateur astronomers seeking an expansive, high-contrast view of the night sky. With a 16mm focal length, it provides medium magnification in most telescopes, and the ultra-wide field pulls you deeper into the night sky.

This eyepiece is positioned as a mid-power option in the Emerald series, sitting between the low-power 22mm and the high-power 9mm models. It is a visual eyepiece for telescopes with a 2" focuser or diagonal.

7-Element Design with Broadband Anti-Reflection Coatings

At the heart of the Emerald 16mm is a 7-element optical design. The 82-degree design provides a wide field of view and stronger contrast.

To maximize image brightness and minimize internal reflections, all lenses within the optical path are coated with a broadband anti-reflection film. By reducing scattered light, these coatings also improve image contrast, making the background sky appear darker and the object you're observing stand out more clearly.

Exploring the Expansive 82° Apparent Field

The 82° apparent field of view (AFOV) is the defining feature of the Emerald 16mm eyepiece. This ultra-wide field gives an expansive, immersive view.

This expansive view has practical benefits beyond aesthetics. For observers using non-tracking mounts like Dobsonians, a wider apparent field at a given magnification means that objects drift through the eyepiece for a longer period before needing to be re-centered. This allows for more relaxed, sustained observation of details on planets or within deep-sky objects without constant nudging of the telescope.

Observer Comfort and the 14mm Eye Relief

The Emerald 16mm features an eye relief of 14mm, which is the specified distance from the eyepiece's top lens to the point where your eye must be positioned to see the full field of view.

However, for observers who must wear eyeglasses to correct for astigmatism, 14mm of eye relief may not be enough to see the entire 82° field at once. Sky-Watcher's documentation notes that the 18mm eye relief found on the 22mm Emerald model is ideal for spectacle wearers.

Calculating Magnification: 44x to 175x Examples

The magnification provided by the Emerald 16mm depends entirely on the focal length of your telescope. To calculate the power, simply divide your telescope's focal length (in millimeters) by the eyepiece's focal length of 16mm.

Some examples:

• In a 700mm focal length telescope: The eyepiece yields 44x (700 / 16), providing wide, rich-field views.
• In an 8" f/6 Dobsonian (1200mm focal length): The eyepiece provides 75x (1200 / 16).
• In an 11" f/10 SCT (2800mm focal length): It delivers a much higher 175x (2800 / 16), useful for examining smaller targets.

Matching the 16mm Eyepiece to Your Telescope's Exit Pupil

Exit pupil is the diameter of the beam of light that leaves the eyepiece, and it sets how bright extended objects look. You can calculate it by dividing the eyepiece focal length (16mm) by your telescope's focal ratio (f-number).

A smaller exit pupil gives a darker sky background.

• In an f/4.5 telescope: The exit pupil is 3.6mm (16 / 4.5).
• In an f/6 telescope: The exit pupil is 2.7mm (16 / 6).
• In an f/10 telescope: The exit pupil is a small 1.6mm (16 / 10), which darkens the sky background and is better suited for bringing out detail in brighter objects.

Telescope Interface: The 2" Barrel and Filter Threads

The Sky-Watcher Emerald 16mm eyepiece features a 2" diameter barrel. Your telescope must be equipped with a 2" focuser or a 2" diagonal to use this eyepiece.

The barrel is also threaded to accept standard 2" astronomy filters. This allows you to directly attach light pollution, nebula (UHC, OIII), or color filters to the eyepiece, enhancing your views of specific objects without needing a separate filter holder.

Optical Performance with Fast and Slow Telescopes

Sky-Watcher does not rate the Emerald eyepieces by telescope focal ratio. In slower telescopes the 16mm gives higher magnification and smaller exit pupils; in faster ones lower magnification, a wider true field and larger exit pupils.

The 540g CNC-Machined All-Metal Body

The Emerald 16mm is constructed with a precision CNC-machined all-metal body. The machined metal housing protects the optics and is built to withstand years of observing in the field.

Weighing 540 g (1.2 lb), this is a heavy eyepiece. Heavy eyepieces can change a telescope's balance, so check it after swapping eyepieces. The eyepiece's dimensions are 4.75" in length by 2.25" in width.

Emerald Series Comparison: 16mm vs. 9mm and 22mm

The Emerald 16mm is the mid-focal length option in Sky-Watcher's 82° eyepiece lineup. While all three models share the same 7-element design and 82° apparent field, they differ in key specifications that tailor them to different applications.

• Emerald 9mm: The high-power option. It is the only model in the series with a 1.25" barrel. It has a very short 8mm of eye relief.
• Emerald 16mm: The 16mm model uses a 2" barrel and offers 14mm of eye relief. Its magnification sits between the wide-field 22mm and the high-power 9mm.
• Emerald 22mm: As the lowest-power model, the 22mm uses a 2" barrel and provides the longest eye relief of the series at 18mm. This is ideal for spectacle wearers.

Field Use: Care and Cleaning of Your 16mm Eyepiece

Proper care is essential to maintaining the optical performance of your Emerald 16mm eyepiece. There are no specific cleaning instructions for the Emerald eyepieces. Store the eyepiece in its carry case between sessions and avoid touching the lens surfaces.