William Optics ZenithStar 73 Review
The William Optics ZenithStar 73 III is an ideal telescope for wide-field deep-sky astrophotographers, regardless of whether you’re a beginner or an experienced imager.
The William Optics ZenithStar 73 III is a popular choice among beginner astrophotographers thanks to its compact size, ease of use, and value for the money. It is an ED doublet refractor with mounting requirements less stringent than larger scopes, though it does require a true equatorial mount in the range of a 15-20 lb or higher stated weight capacity as opposed to a lightweight “star tracker” equatorial mount like smaller instruments.
Other ED doublet refractors with identical optics and seemingly similar features to the ZenithStar 73 are plentiful, but the ZenithStar 73’s mechanical design and included features, while making it more expensive than competitors, save you a lot of money and hassle in shopping for better rings and various dovetail/bracket parts to rigidly attach the telescope to a suitable mount and guide scope.
73mm (2.8”) Aperture
430mm Focal Length
Doublet FPL-53 optics
The William Optics ZenithStar 73 is an ED doublet refractor with an aperture of 73mm (2.8”) and a focal length of 430mm, making it an f/5.9 system. The ZenithStar 73 uses the same optics as most 72mm ED doublet refractors, but William Optics uses a slightly different mechanical design which exposes more of the outer edges of the lens to claim an aperture and speed advantage over competitors, though there is basically no real-world effect of a 1mm aperture increase and f/0.1 gain in speed.
Being an ED doublet, the ZenithStar 73 is equipped with an FPL-53 ED lens element, which helps reduce chromatic aberration to levels similar to that of a similar diameter f/15 achromat, though it is not completely eliminated. The ZenithStar 73 is sold only as an optical tube, without a mount, eyepieces, camera adapter, or other accessories, which must be purchased separately.
The ZenithStar 73’s large 35mm imaging circle means you won’t have to worry about vignetting at the edges of your frame. This means the ZenithStar 73 is perfect for DSLR cameras with APS-C or 35mm sensors, along with the larger color and monochrome camera sensors in dedicated astronomy cameras.
With a focal length of 430mm, this telescope is well-suited for capturing large deep-sky objects such as nebulae and open star clusters. You’ll need an aftermarket field flattener, such as the one offered by William Optics, or a 0.8x reducer-flattener to increase speed to f/4.72 and shorten the focal length to 344mm.
The ZenithStar 73 uses a fairly standard 2.5” all-metal rack-and-pinion focuser to hold eyepieces or cameras and is equipped with a 10:1 dual-speed reduction knob for fine focus adjustments. The focuser also has a generous amount of travel and back focus to accommodate most cameras, flatteners, and filters. Cheaper scopes may only use a 2” focuser, which can cause issues with certain camera setups when it comes to stiffness or avoiding vignetting.
Unlike competing instruments, the ZenithStar 73 features a Vixen-style dovetail bar attached to a pair of tube rings gripping the tube. This makes for a stiff connection and also allows you to rotate and slide the tube for balance, either by sliding the dovetail in your mount’s saddle or moving the tube within the rings. The ZenithStar 73’s dovetail is also compatible with Arca Swiss mount saddles or ¼ 20 photo tripod screws. Many of the other 72mm ED doublets on the market use a cheap Vixen-style “mounting foot” attached to a simple rotating ring that grips the tube, hardly a versatile or sturdy mounting option.
The ZenithStar 73 also features a carry handle attached to the top of the rings. In addition to making the rings even stiffer by joining them at the top and making it easier to pick the scope up, the handle’s slot fits Vixen/Synta style guide scope rails or finder brackets, allowing you to attach a guide scope such as the 50mm unit offered by William Optics. Removing the thumb screws from the handle turns it into a second Vixen-style dovetail bar which can be clamped by larger guide scope brackets and other accessories. This is much better than a simple side-mounted finder shoe which is less sturdy and introduces off-axis weight and torque to your mount, which could affect tracking or guiding accuracy.
Lastly, William Optics provides a Bahtinov focusing mask with the ZenithStar 73 to aid in focusing, along with a soft carry case which is included by default.
At 5.5 lbs, which will easily be nearly doubled by the addition of a camera, field flattener, and guiding setup, the ZenithStar 73 is outgrowing “star tracker” mounts like the iOptron SkyGuider Pro. You’ll want something like the Sky-Watcher EQM-35, HEQ5, or Celestron Advanced VX to hold this telescope for deep-sky imaging, and autoguiding is highly recommended for consistent results with long exposures.
What can you see if you look through it?
The ZenithStar 73 is not an ideal telescope to purchase for visual observation due to its price and small aperture, but it can still be fun to use with suitable high-quality eyepieces and a star diagonal. The short focal length allows for expansive views of open star clusters and the Milky Way, particularly with a 2” diagonal and eyepiece. With the help of a 2-3x Barlow lens or very short focal length eyepieces, it is possible to use the ZenithStar 73 to observe the most obvious planetary features such as the phases of Venus and Mercury, lunar craters and mountains, Mars’ ice caps, Saturn’s rings, and the moons and cloud belts of Jupiter. Uranus and Neptune will be hard to tell apart from stars with such a small aperture. While a few globular clusters and galaxies can be seen, the small aperture of the ZenithStar 73 means they’ll be little more than featureless fuzzy balls; the fun is mainly in viewing open star clusters, or large nebulae with the aid of dark skies and a high-quality nebula filter.
The short focal length of the ZenithStar 73 lessens mounting and overall skill requirements to use it effectively for imaging, but it does mean it’s a poor choice for shots of smaller star clusters or galaxies. However, it is generally not recommended to start out with a very large and long focal length scope for deep-sky imaging anyway.
Compared to more expensive triplet and Petzval designs, the ZenithStar 73 is not as well-corrected for chromatic aberration or a flattened field by default; an aftermarket flattener cleans up the image fairly well, however, and chromatic aberration is hardly a concern at such a short focal length anyways.
The William Optics ZenithStar 73 is a great telescope for deep-sky imaging with smaller equatorial mounts and features plenty of provided mounting options and high-quality parts. The ability to choose your own color and pick from William Optics’ suite of aftermarket accessories is a nice bonus.