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Home / Review: Sky-Watche​r Pro 120ED
by Joe Renzetti 03/26/12

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Review: Sky-Watcher Pro 120ED

By Joe Renzetti

image001.jpg
Figure 1.

After many amateur astronomers make the serious plunge, buy a large telescope and enjoy its use for a while, they realize a smaller secondary telescope would be very useful on those nights when you don’t want to drag out your heavy equipment and set it up. This in the hobby is called the “grab-and-go” telescope. My objective for a grab-and go was one that was relatively small and simple to use with optics that would really excel at lunar and planetary views. The options were an apochromatic refractor or a maksutov-cassegrain. The latter is cheap and very compact but suffers from very long cool down times, especially in our climate, and a narrow field of view. The choice I made was an apo refractor. After researching for one that was inexpensive, and with a large enough aperture for planets and deep sky objects, I settled on the Sky-Watcher Pro 120ED.

Sky-Watcher telescopes are made by the parent company Synta. The are based in Richmond, British Columbia and manufactured in China. Their scopes are primarily distributed in Europe and Canada while the app refractors are distributed in the US through Celestron. My Sky-Watcher Pro 120ED was purchased through Amazon.com and arrived quickly in its own very nice aluminum case, which was included free! It is bundled with a 2” Crayford-style focuser, a 9X50 RA finder, a 2” dielectric diagonal, two 1.25” eyepieces with long eye relief (20mm & 5mm), and mounting hardware with a Vixen style dovetail plate. The OTA weight is 11.3 lbs. This was important as I mounted it on an Astro-Tech Voyager alt-az mount which has a payload capacity of 20 lbs. A comparable triple apo refractor made by Explore Scientific is 22 lbs and would be too much for this mount.

The Optics

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Figure 2. Lens

The optics are made with a Schott glass positive front Crown element and a second lens element fabricated of fluorite. The ED glass used is FPL-53. All lenses are uniformly air-spaced in their cell housings. Sky-Watcher’s Metallic High Transmission Coatings are then applied to each air-to-glass lens surface. The lenses are collimated and aligned at the factory so no adjustments are needed by the observer. The doublet optics produce well-corrected, high contrast images with virtually no false color. The 120mm aperture has a 900mm focal length and a f/7.5 focal ratio. After I quickly set it up I went right to the star Vega and viewed with an Explore Scientific 18mm 82° eyepiece. There was no violet fringing detected. The star had a crisp blue color and tack sharp. As the view steadied the concentric rings were tight and solid around the pinpoint star. These increasingly faint light circles are unavoidable in refractors due to diffraction but being well concentric around the central light dot they are a guarantee of good optical alignment of the lenses. Next stop was everyone’s favorite double, Albireo. The colors were perfect and the split was clean and well defined even at 50x magnification. Rocking the focuser the stars again show no false color and they snap to focus with ease. Around it the Milky Way stars looked tack sharp to the edge and the optics give a nice flat field. The refractor’s advantage is it can give a fully illuminated field of view even at the lowest powers. I took it down to 30x using the Explore Scientific 30mm eyepiece and the views around the Milky Way in Cygnus were magnificent.

The Planets

My first view of the Moon was when it was about 85-90% waxing gibbous. It was bright enough to wash out many of the surface details. Despite that, the craters along the terminator easily snapped to focus and delivered excellent contrast. When the seeing is steady the details emerged and craterlets started to reveal themselves at 180x, which is about Specifications for Pro 120ED APO Refractor Telescope the maximum magnification permitted with an average steady atmosphere. At lower powers I was getting a yellow fringe on the outer edge. This was being caused by the Explore Scientific eyepiece. I switched to the stock Sky-Watcher 20mm LET eyepiece and it was virtually color free with a very slight fringe of purple on the edge. Saturn was already far in the west, so it suffered from the usual elements that low in the horizon. Still I could see a little cloud band detail and Cassini’s division was apparent. Later Jupiter was high enough to deliver an amazing view! The disc was solid and cloud bands were all visible with excellent color and contrast. Two dark storm spots were in the NEB and easily stood out. Using a Badder lunar & skyglow filter sharpened the view more and reduced the glare on the outer edge. Even blue cloud details in the equatorial zone could be seen. No false colors were found anywhere along the planet’s edge and the satin-black background really accented the solid disc. The four Galilean moons were also solid looking disc-like instead of resembling nearby stars.

Deep Sky

The 120mm aperture is not exactly ideal for hunting the faint fuzzies but the views it can show are surprisingly good due to the sharp optics and the absence of any secondary obstruction. M31 was up in the east and had a faint but easily visible large core and just a hint of the outer edges. It’s companion M32 could also be seen in the same field of view. Out in the southwest the Butterfly Cluster was shimmering like jewels in the sky against a dark background. At 30x it was easily framed and had bright blue colors in the stars. Same with NGC 457 Owl Cluster. The ‘eyes” stars were impressive in the cluster. The Double Cluster was fully framed and sharp full star fields to the edge of the view at 50x. This one really gives the spacewalk feel. M15 globular had a fair amount of stars resolved. As I pushed the magnification the contrast was lost and the globular looked better at lower powers. M27 & M57 showed impressive brightness and details comparable to an 8” reflector or SCT. Using an Ultrablock filter improved the contrast of the nebulosity. M8 Lagoon Nebula was closer to the city light dome but still was viewable. It is usually advised to only use an OIII filter with apertures at 8” and above but the Sky-Watcher gave a surprisingly great view of the Lagoon’s nebulosity with the OIII filter.

Mechanical Performance

The 2 speed crayford focuser is smooth and solid with no backlash or image shift. This is crucial for holding a steady view at high magnifications and keeping planets in the field. The 2” dielectric diagonal is sturdy and holds the large, heavy 2” eyepieces, and the focuser still performs well with the added weight. The apo’s length is shorter than comparable achromat refractors making it easier to hold a steady view on the alt-az mount and the slow motion controls kept the movements smooth when centering objects. The 8x50 right angle finder is good quality and useful on centering views of planets and bright stars. Hovering for deep sky hunting I found it to be impractical so I mounted a red dot finder for quick and easy star hopping. The supplied dew shield keeps the optics clear through most conditions. Observing on a humid night in a grassy field the optics dew up fairly soon and would benefit from a dew heater.

Overall the Sky-Watcher Pro 120ED delivered performance and views above and beyond my expectations. Sky & Telescope’s review proclaimed “Its performance as a visual telescope was on par with similar-size apo refractors costing three times as much." I do not doubt it is in the same performance class as premium triplet apos. As a doublet it is not 100% color free and maximum performance is dependent on what eyepieces are used. I have not tried any 4-element plossls or orthoscopics I am sure the planetary contrast will be further enhanced. Still you’d have to really look for any false color on the brightest stars to see it. At $1599.00 MSRP with all the accessories included you would be hard pressed to find a better deal for an apo refractor of this quality. This scope is definitely a keeper.

Specifications for Pro 120ED APO Refractor Telescope

Optical Design

ED APO Refractor

Aperture

120mm

Focal Length

900mm

Focal Ratio

7.5

Highest Practical Power

240x

Faintest Stellar Magnitude

13.1

Resolving Power

0.96

Finderscope

9x50 RACI

Focuser

2" 2-speed Crayford style

Diagonal

2" 90º Dielectric Diagonal

Eyepiece(s)

20mm & 5mm 1.25" LET

Piggyback Bracket

Yes

OTA Weight

11.3 lbs

OTA Dimensions

37.8 in x 4.72 in

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