Sky-Watcher Plossl SWA 9mm 58° 1.25″ Eyepiece
The Sky-Watcher Plossl SWA 9mm eyepiece delivers exceptional versatility for astronomical observation, combining the proven optical performance of the Plossl design with a generous 58° apparent field of view. This medium focal length makes it an excellent all-around choice for observing planets, the Moon, bright deep-sky objects, and double stars with sharp, high-contrast images.
Versatile Focal Length
The 9mm focal length occupies a sweet spot in eyepiece selection, providing magnifications that work effectively across a wide range of observing scenarios. In a telescope with 1000mm focal length, this eyepiece delivers approximately 111× magnification - ideal for detailed planetary observation while maintaining enough field of view to keep objects comfortably centered.
This magnification range proves particularly effective for observing Jupiter's cloud bands and Great Red Spot, Saturn's rings and atmospheric features, Mars during favorable oppositions, and intricate lunar crater systems. The moderate power also works well for splitting close double stars and observing the brighter globular clusters and planetary nebulae.
Unlike very short focal length eyepieces that can be challenging to use, the 9mm provides comfortable eye positioning and a more forgiving observing experience while still delivering the magnification needed for planetary detail.
Wide-Field Viewing Experience
The 58° apparent field of view significantly enhances your observing experience compared to narrower-field eyepieces. This generous field creates a more immersive viewing experience, making you feel less like you're looking through a tunnel and more like you have a window to the cosmos.
The wide field proves particularly valuable when observing at moderate to high magnifications. Objects remain in the field of view longer as Earth's rotation carries them across the sky, reducing the need for frequent telescope adjustments. This is especially appreciated when studying planetary features or sketching lunar details - you can concentrate on observation rather than constantly repositioning.
For deep-sky objects like open star clusters or large nebulae, the 58° field provides enough coverage to frame the object attractively while the 9mm focal length delivers sufficient magnification to reveal internal structure and detail.
Sharp, High-Contrast Optics
Sky-Watcher's Plossl SWA design employs a 5-element optical configuration that delivers very sharp images with high contrast across the entire field of view. The multi-element design effectively corrects common aberrations that can degrade image quality, ensuring stars remain pinpoint sharp from center to edge.
All optical surfaces feature Fully Multi-Coated (FMC) anti-reflection treatments. These precision coatings minimize light loss through internal reflections while maximizing light transmission to your eye. The result is brighter images with improved contrast - critical for revealing low-contrast planetary features like Jupiter's subtle belt variations or delicate lunar shadows.
The high contrast performance makes this eyepiece particularly effective for planetary observation, where distinguishing subtle atmospheric features depends on excellent optical quality. Cloud bands, polar regions, and storm systems appear with enhanced definition against their surroundings.
Comfortable for Extended Viewing
The eyepiece provides 16mm of eye relief - the distance between the eyepiece lens and your eye where you can see the full field of view. This generous eye relief accommodates eyeglass wearers comfortably, allowing you to observe without removing your glasses while still seeing the entire 58° field.
Even if you don't wear glasses, the comfortable eye relief means you won't need to press your eye uncomfortably close to the lens. This relaxed viewing position proves especially valuable during long observing sessions, reducing eye fatigue and allowing you to concentrate on what you're seeing rather than fighting to maintain proper eye position.
Fold-down rubber eyecups provide additional viewing comfort and help block stray light from entering your eye. For eyeglass wearers, the eyecups fold down to bring your glasses closer to the eyepiece lens. For observers without glasses, the extended eyecups create a comfortable cushion and light shield.
Practical Design Features
The eyepiece barrel incorporates thoughtful design elements that enhance field usability. A rubberized grip section on the outer body provides secure handling even when wearing gloves during cold-weather observing. The textured surface prevents slipping and makes eyepiece changes quick and positive, even in darkness.
The standard 1.25″ barrel diameter ensures universal compatibility with telescopes, star diagonals, and focusers. Threading on the barrel accepts M28.5×0.6 astronomical filters, allowing you to use planetary color filters, moon filters, or light pollution filters without needing separate filter adapters.
Ideal Applications
Planetary Observation: The 9mm focal length provides excellent magnification for studying planets. On Jupiter, you'll resolve cloud belt structure, the Great Red Spot (when visible), and the four Galilean moons. Saturn reveals the Cassini Division in its rings along with atmospheric banding. Mars shows polar caps and major albedo features during good oppositions.
Lunar Exploration: The Moon offers endless fascination through this eyepiece. The magnification reveals intricate crater detail, mountain ranges, rilles, and the stark beauty of the terminator - the boundary between lunar day and night where shadows create dramatic relief.
Double Stars: The sharp optics and moderate magnification make this eyepiece well-suited for splitting double stars. Many classic doubles separate cleanly at this power, revealing their color contrasts and orbital relationships.
Bright Deep-Sky Objects: While primarily marketed as a planetary eyepiece, the 9mm performs admirably on brighter deep-sky targets. Globular clusters resolve into individual stars around their edges, planetary nebulae show their disk and sometimes central stars, and compact galaxies reveal their cores and overall structure.
Universal Telescope Compatibility
The eyepiece works with all telescope types equipped with 1.25″ focusers: refractors, Newtonian reflectors, Dobsonians, Schmidt-Cassegrains, and Maksutov-Cassegrains. The optical performance remains consistent regardless of telescope design, though the actual magnification will vary based on your telescope's focal length.
To calculate magnification with your telescope, divide your telescope's focal length by 9mm. For example: a 750mm focal length telescope yields 83× magnification, while a 1200mm telescope provides 133×.
Technical Specifications
Focal Length: 9mm
Apparent Field of View: 58°
Eye Relief: 16mm
Optical Design: Plossl SWA (Super Wide Angle)
Optical Elements: 5-element configuration
Coatings: FMC (Fully Multi-Coated) on all air-to-glass surfaces
Barrel Diameter: 1.25″ (31.75mm)
Filter Thread: M28.5×0.6
Eyecup Type: Fold-down rubber eyecups
Body Features: Rubberized grip section
Recommended Use: Planetary, lunar, double stars, bright deep-sky objects
Building Your Eyepiece Collection
The 9mm focal length serves as an excellent mid-range eyepiece in a balanced collection. Pair it with a longer focal length (20-25mm) for wide-field views and object location, and a shorter focal length (5-6mm or use this 9mm with a Barlow lens) for maximum planetary detail. This combination covers most observing needs efficiently.
The Sky-Watcher Plossl SWA series offers consistent optical quality and mechanical design across focal lengths, so adding other focal lengths from the same series ensures a matched set with familiar handling characteristics.
