William Optics ZenithStar 81 Review

written by Zane Landers
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9

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The William Optics ZenithStar 81 is a great telescope for beginner astrophotographers and provides excellent value for the money, though it’s a little more demanding than smaller aperture instruments and slower in focal ratio.

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Review

The William Optics ZenithStar 81 is a well-designed scope for beginner astrophotographers or those on a tight budget, offering sharp images with its doublet optics (though slightly inferior to a quality triplet like the GT81) and plenty of capability for imaging larger deep-sky objects, while still fitting on medium-duty mounts of the EQ5 class range which are sold for slightly more than the cost of the ZenithStar 81 itself. Other 80mm f/7 doublets may offer identical optical performance to the ZenithStar 81 at a lower price but have a lesser quality fit and finish, along with fewer mounting options.

If you want to get started in deep-sky astrophotography without requiring autoguiding or a beefy mount, the ZenithStar 73 and ZenithStar 61, also offered by William Optics, are great scopes, either of which offer a wider field of view and less stringent tracking or guiding requirements than the larger ZenithStar 81 but the same great features and optics.

Optics

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81mm (3.2”) Aperture

559mm Focal Length

f/6.9

Doublet FPL-53 optics

The William Optics ZenithStar 81 is an ED doublet refractor using an FPL-53 lens element, with an aperture of 81mm (3.2”) and a focal length of 559mm, resulting in an f/6.9 system overall. The ZenithStar 81’s optics are similar to those of other 80mm f/7 FPL-53 ED doublet refractors, with the extra mm of aperture being little more than a marketing gimmick to bring the telescope to a “faster” f/6.9 speed and claim an aperture advantage. There is no actual difference between pretty much any of these telescopes.

The ZenithStar 81’s 35mm illuminated imaging circle makes it suitable for use at its native focal length with 35mm sensors like full-frame DSLR or mirrorless cameras or large-format cooled astrophotography cameras, with minimal vignetting at the corners of the frame.

A 559mm focal length is a little long for imaging the largest nebulae and star clusters, but still allows you to get great shots, particularly with a reducer-flattener or larger sensor camera. You also have sufficient focal length for decent, if rather undersampled, photos of larger galaxies or globular star clusters, unlike with a smaller and shorter focal length instrument.

At f/6.9, you can get away without a flattener with smaller sensors or cropped images, but a flattener is ideal for sharp stars all the way to the edges of the ZenithStar 81’s field of view. William Optics recommends using their dedicated Flat81 field flattener for full-frame astrophotography cameras and offers the ZenithStar 81 as a package with one. Smaller sensors will allow you to use a 0.8x reducer-flattener to bring the telescope to f/5.5 and a 447mm focal length, increasing your field of view and reducing required exposure time, but this will vignette too much with smaller sensors to be useful; those looking for a native f/5.5 scope with a full-frame image circle will need a dedicated Petzval refractor.

Mechanics

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The ZenithStar 81 comes with a 2.5” rack-and-pinion focuser for attaching to a camera, field flattener, or eyepieces. The focuser has a 10:1 dual-speed reduction knob for precise focusing, and ample travel and back focus to accommodate most imaging setups. You also get a Bahtinov mask provided to attach to the front of the telescope for assistance in focusing. The ZenithStar 81 has a pair of tube rings with a Vixen/Arca Swiss convertible dual dovetail rail attached to the bottom. The top of the ZenithStar 81 sports William Optics’ signature carry handle/Vixen dovetail/guide scope saddle combination. 

In addition to making the tube rings more rigid and serving as a grip when transporting the scope, the ZenithStar 81’s handle fits the rails of many smaller guide scopes, gripping them with thumb screws. Larger guide scopes or other accessories can attach to the outside of the handle, which is sized the same as a Vixen-style dovetail, for an even sturdier connection. You can also attach an additional standard Vixen/Synta-style finder or guide scope bracket to each side of the ZenithStar 81’s focuser assembly.

The dew shield at the front of the ZenithStar 81 retracts to reduce the size of the telescope when stored, though you will probably have to remove the telescope from its tube rings or slide it forward to be able to retract the dew shield all the way. As with other William Optics scopes, the ZenithStar 81 is available in several different anodized colors, and accessories are offered with identical color combinations for a beautiful setup.

Mounting Options

The ZenithStar 81 weighs 7.8 lbs by itself; equipping it with a full complement of guiding accessories and a suitable camera/flattener can add up to close to 15 lbs, or around the limit of mounts like the Sky-Watcher HEQ5 Pro or Celestron Advanced VX and far too much for smaller equatorial mounts like the EQM-35 or star trackers. Autoguiding is, of course, a must at a focal length of over 500mm.

What can you see if you look through it?

The ZenithStar 81 may not be the best choice for visual astronomy, given its very high cost and small size, but it can still provide enjoyable views with the right eyepieces and a star diagonal, atop a lightweight alt-azimuth mount or whatever equatorial you are using for imaging. Its short focal length is ideal for wide views of open clusters, large emission nebulae, and the Milky Way, especially with a 2” diagonal and eyepiece – though most nebulae are best observed with a filter and/or dark skies. You can also split plenty of colorful double stars at high magnification with the ZenithStar 81. Globular clusters and most planetary nebulae remain unresolved, however, and only the brightest galaxies show up, with a few revealing hints of detail, such as dust lanes, under the best viewing conditions.

An 81mm aperture is enough for interesting views of the planets at ample magnifications. The ZenithStar 81 can show you the phases of Mercury and Venus, and a wealth of exciting detail on the Moon, along with ice caps and a few dark markings on Mars. You can also make out Jupiter’s moons as tiny disks when they transit in front of the planet and resolve the Great Red Spot amidst the planet’s vivid cloud bands on a clear and steady night. Saturn’s rings show the razor-thin Cassini Division with the ZenithStar 81, along with a few moons surrounding the planet and its cream-colored cloud bands. Uranus and Neptune are featureless bluish dots identical in appearance to surrounding stars of similar brightness. 

Value

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Drawbacks

The ZenithStar 81 is a little long in focal length for shooting the largest nebulae and using the smallest mounts, while a little small and short in focal length for smaller targets or to necessitate upgrading to a larger and higher-capacity mount. However, it’s still within the realm of EQ5-class mounts and easy for a beginner to use, provided you invest in autoguiding capabilities and take time to learn how to set up for a night of imaging. You can also get great wide-field shots if you either use a 0.8x reducer or a large sensor, and it’s great for dipping your toes into imaging galaxies. 

A Petzval design is superior to the ZenithStar 81’s simple doublet optics for wide-field imaging – particularly with full-frame sensors and if you want a faster focal ratio – and a triplet refractor, also preferably of larger aperture, is better suited for shooting small targets. However, neither of these options is cheap, and the latter is beyond what is usually recommended for a beginner setup.

Conclusion

While on the large end for a beginner setup and a bit below the demands of more experienced astrophotographers, the William Optics ZenithStar 81 provides excellent capabilities for the price and is a high-quality imaging telescope. 

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TTB score

9

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