Sky-Watcher BK 120/600 OTA astronomical telescope
Are you looking for a telescope that will show you the Milky Way in all its glory, sprawling open clusters, or bright comets with their long tails? The Sky-Watcher BK 120/600 OTA is an achromatic refractor with a 120mm objective lens and a 600mm focal length - a combination that delivers a focal ratio of f/5 and an exceptionally wide field of view. This telescope appeals to both beginners and experienced astronomers who want a compact and versatile instrument for the night sky.
Optics and image quality
The telescope uses a classic two-element achromatic lens that provides excellent light-gathering ability and a pleasantly sharp image across the entire field of view. The 120mm aperture collects approximately 290 times more light than the dark-adapted human eye, allowing observation of objects down to approximately magnitude 12.5 (compared to magnitude 6 with the naked eye). The optical resolution reaches 1.2″, so the telescope will easily split close double stars and reveal fine details on the surface of the Moon.
As an achromatic refractor, it produces mild chromatic aberration typical of this type of optics when viewing bright objects such as the Moon and planets - particularly at the short focal ratio of f/5. Experienced users can easily suppress this by using a reducing aperture stop on the objective, or by fitting a dedicated minus-violet filter.
Focuser and accessories
The telescope is equipped with a 2″ rack-and-pinion focuser compatible with Sky-Watcher electronic focusers as well as GSO-standard replacements. The package includes a 1.25″ reducer with a 1.25″ 45-degree diagonal fitted. To use 2″ eyepieces and achieve the maximum field of view, a 2″ diagonal needs to be purchased separately - a dielectric version is recommended for the best light transmission. The full range of standard 1.25″ eyepieces and filters can be used directly. The package also includes tube rings with a dovetail bar for mounting, and a Star Pointer finderscope.
Planetary and lunar observation
The 120mm aperture is sufficient for basic planetary and lunar observation. With a suitable short-focal-length eyepiece you will be able to see:
- Lunar surface features larger than 2 km
- Sunspots (a dedicated solar filter for the objective is required)
- Jupiter's cloud belts and the four Galilean moons
- Saturn's rings and its largest moon Titan
- Phases of Mercury and Venus
- Polar caps of Mars at opposition
- Uranus and Neptune as star-like discs
Note: for detailed planetary observation, Maksutov-Cassegrain designs or refractors with a longer focal length are more suitable. This telescope excels primarily when observing large extended objects.
Deep-sky observation - where this telescope truly shines
This is where the Sky-Watcher BK 120/600 is truly at home. Thanks to the f/5 focal ratio and large field of view, it is ideal for observing open clusters, extensive nebulae and star fields along the Milky Way. The telescope can reach approximately 1.1 million stars down to magnitude 12.8, is well suited for visual variable star observation using the comparison method, and will reveal spiral arms of brighter galaxies under a dark sky. Nearly all objects in the Messier catalogue are within its reach - hundreds of open and globular clusters, and bright emission and reflection nebulae with visible internal structure.
Comets, nature observation and photography
The combination of large aperture and short focal length makes this telescope an excellent instrument for comet observation, especially for comets with extensive tails where a wide field of view is essential. With suitable wide-angle eyepieces you get a view comparable to high-powered binoculars, but with significantly greater light-gathering power.
Thanks to the 45-degree diagonal, the telescope is also comfortable to use for observing nature and landscapes - it works as a high-quality spotting scope with the option of photographic documentation.
OTA - freedom of choice
The telescope is supplied as an OTA (optical tube assembly without a mount), giving you complete freedom of choice. Its relatively low weight and compact dimensions make transport and storage straightforward - simply load it into your car and head out to a dark-sky site. Sky-Watcher recommends a sturdy alt-azimuth mount such as the Sky-Watcher AZ4, which matches the tube in terms of weight capacity and rigidity. The dovetail rings included are standard format, so compatibility with most mounts is not an issue.
Technical specifications
| Parameter |
Value |
| Optical design |
Achromatic refractor |
| Objective diameter (aperture) |
120 mm |
| Focal length |
600 mm |
| Focal ratio |
f/5 |
| Optical resolution |
1.2″ |
| Limiting magnitude |
approx. 12.5 mag |
| Light gain vs. naked eye |
approx. 290x |
| Focuser |
2″ rack-and-pinion |
| Included diagonal |
1.25″, 45 degrees |
| Finderscope |
Star Pointer |
| Supplied as |
OTA (without mount) |
| Dovetail standard |
Standard (compatible with most mounts) |
Why choose Sky-Watcher?
Sky-Watcher is one of the most respected manufacturers of astronomical optics in the world - with decades of experience, extensive service support and a huge user community. Every telescope in the BK series goes through strict quality control of both optics and mechanics. A major advantage is full compatibility with the rich Sky-Watcher accessory ecosystem - from electronic focusers and goto mounts to focal reducers and field flatteners.
Whether you are a beginner looking for your first serious telescope, or an experienced observer adding a compact wide-field instrument to your collection, the Sky-Watcher BK 120/600 OTA is a choice you will not regret.
