Sky-Watcher’s Stargate 18-Inch Dobsonian

By Dennis di Cicco

 The Sky-Watcher Stargate 18-inch with SynScan motor drives for Go To pointing and tracking is a big telescope, standing more than 7 feet tall when pointed at the zenith. While the author set up the telescope by himself, the process would be far easier with two people.

The Stargate truss-tube Dobsonians are billed as the largest mass-market telescopes available. Is the 18-inch model worthy of the hype?

Stargate 18-inch Dobsonian

U.S. Price: $5,999 ($6,999 as tested with Go To motor drives)

What We Like

Excellent optics
Optional drive offers accurate pointing and tracking
Overall performance

What We Don’t Like

Abysmal instruction manual for telescope assembly

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AFTER TESTING SKY-WATCHER’S 18-inch Stargate truss-tube Dobsonian for several months late last year, it’s easy to understand why deep-sky observers proclaim that aperture is king. Objects that appear faint and ill-defined in smaller scopes take on a whole new dimension in an 18-inch. Many globular clusters are transformed from small, fuzzy glows into brilliant spheres of sparkling stars. Lots of planetary nebulae appear large and bright enough to show obvious structure not seen with smaller apertures. And the fields surrounding many familiar deep-sky objects are filled with a multitude of faint background stars lending 3D-like perspectives.

It was apparent how much I liked the Stargate when I quickly found myself looking forward to each clear night as an opportunity to observe rather just one to work on a product review. While the telescope ended up being one I truly enjoyed, and one I can strongly recom-mend, the review process didn’t start out that way. But let’s save that part of the story for later in this review and begin with the good stuff. Although I’m no stranger to using large telescopes, especially at star parties, my own recreational deep-sky observing is usually done with a 12-inch f/5 Dobsonian. As such, the 18-inch Stargate was a signifi cant step up. In addition to having 2¼ times more light grasp than the 12-inch, the Stargate’s longer focal length (1,900 mm versus 1,524 mm) offered notice-ably more magnifi cation for a given set of eyepieces. By itself, this longer focal length would have yielded 25% more magnification. But because the Stargate is an f/4.1 Newtonian and suffers more from coma than the 12-inch f/5, I did most of my Stargate observing with a Tele Vue Paracorr coma corrector that increased the Stargate’s effective focal length by a factor of 1.15×.

And this meant a 43% increase in magnification for the same eyepiece used with the Stargate compared to my 12-inch Dob. The Stargate ships with 2-inch 28-mm and 1¼-inch 10-mm eyepieces of decent quality. They yield 68× with a 49-arc minute field of view and 190× with a 15-arc minute field, respectively. There is also a Cheshire eyepiece collimation tool, which is very good for checking the scope’s optical alignment. Due to the length of the instrument, however, collimating the optics with the included tool is best done with two people — one looking through the Cheshire eyepiece while the other adjusts the collimation screws on the primary-mirror cell. views with a 21-mm Ethos, which gave almost the same fi eld of view as the Panoptic, but at an impressive 103×.

The Stargate comes with a 9×50 straight-through fi nder and a 2-inch, two-speed focuser with extension tubes/adapters for 2- and 1¼-inch eyepieces.

As explained in the accompanying text, the author typically observed with the Tele Vue Paracorr coma corrector pictured here.

The Stargate Dobs have noteworthy primary mirrors made from fused glass. The 18-inch has a front plate (about ¾ inch thick) and back plate separated by a dozen ribs surrounding a central hollow core. As such the 3½-inch-thick mirror weighs only about 30 pounds and ac-climates to ambient temperatures much faster than a solid disk of glass would.

In the workshop, the author’s Foucault test of the primary mirror shows no sign of “print through” from the supporting rib structure. There is a slight hint of a central depression due to the hollow core, but it falls completely within the shadow of the secondary mirror and is of no consequence. The mirror is an overall well-corrected paraboloid with subtle artifacts typical of machine-polished optics. It delivered excellent performance at the eyepiece.

First Night Out My first night under the stars with any telescope is usually reserved for just get-ting a feel for what it’s like to set up and use the equipment — serious observing isn’t typically on the agenda. As such I picked a spot on the walkway outside my garage where the sky is, unfortunately, heavily obscured by the house and surrounding trees. While I set up the scope by myself, it would be far easier with two people. The base with its optional motor drives weighs about 66 pounds (30 kg). But it’s the optical assembly (a.k.a. “the tube”) that presents the biggest challenge. Complete with the side cradles attached to the primary-mirror assembly, the tube weighs almost 100 pounds, and it’s quite awkward for one person to lift and position on the base. But I still found this easier to do than to assemble the tube beginning with the primary-mirror assembly (by itself a 60+ pound unit) placed on the base.

The problem here for one person is trying to get the secondary-mirror cage attached to the top of the six truss poles while they are pointing 7 feet above the ground and fl ailing around.
As twilight receded down the western sky, I pointed the Stargate to brilliant Vega high overhead. Although I’m well over 6 feet tall, I could just reach the eyepiece when standing on a 9-inch-high step stool. At 196× Vega appeared dazzlingly bright with four razor-sharp diffraction spikes extend-ing across my 18-arc minute-wide field of view. Boosting the magnification to 308× and racking the eyepiece from one side of focus to the other revealed diffraction patterns suggesting that the optics in this scope were very good, but Vega was really too bright for a good star test, so I nudged the scope to the northeast to look at Epsilon Lyrae, the famous Double-Double.

Moving my eye from the Stargate’s 9×50 finder to the 308× eyepiece provided me with a dramatic, in-your-face example of the advantages that a big aperture brings to many types of observing. The tightly spaced components of this pair of double stars were cleanly resolved with a wide gap of dark sky between the tight pairs. And all four stars were surrounded with neat sets of diffraction rings. I’ve seen this kind of clearly resolved separation at similar magnifications with smaller, high-quality telescopes, but there were added dimensions to the view in the Stargate. Each of the four stars vividly displayed its own delicate hue — something that is far more subtle with smaller apertures. And there was a multitude of faint background stars that go mostly unnoticed in smaller scopes, giving a truly three-dimensional feel to the scene. It was now obvious that the optics were indeed very good.

With the threat of rain in the forecast, the author spread out the parts of the Stargate on his garage fl oor before beginning the assembly process.
The two sections forming each of the scope’s six truss poles have solid, screw-together connections at their centers. This keeps the length of the individual pieces very manageable for transport and storage.
The truss poles fit into numbered connections on the primary- (seen here) and secondary-mirror assemblies, and lock in place with a captive clamp and hand lever. As such, no tools are needed to assemble the optical tube.

With my appetite whetted for more, I decided to engage the motor drives and attempt the necessary star alignment for Go To pointing and tracking even though I hadn’t yet gone through the manual for Sky-Watcher’s SynScan drive system. Following instructions that scroll across the hand control, and after a few false starts (mainly because of accidentally pushing the wrong buttons on the hand control), I got the drive working, and I grabbed a star chart to see what interesting objects were within my limited view of the sky. Despite my less-than-ideal alignment, the Go To pointing was very good and tracking was likewise good. Thus began a very enjoyable evening of observing.

And It Gets Better If I considered that first night good, then the second night was nothing short of spectacular. By then I’d made a trip to the local outlet for Harbor Freight Tools and picked up a small furniture dolly for $8 and a two-tier step ladder, on sale for $20. With a couple of pieces of scrap wood I sup-ported the assembled Stargate on the dolly and could easily roll it in and out of my garage. Furthermore, as the image on page 62 shows, it was an easy matter to lift each leg on the base about ½ inch and slide a brick under it, putting the scope on a solid footing free of the dolly. After levelling the base and marking which leg belonged on which brick, I marked the location of the bricks with a few pieces of tape on the driveway. On subsequent nights it took only a few minutes to roll the scope out of the garage and have it ready for observing. This may sound stupid, but because I didn’t have to disassemble the scope to transport and store it, it was as easy to use in my driveway as any grab-and-go scope I’ve tested.

By the second night I’d also gone through the SynScan manual and had a better handle on the best way to do star alignments and, in general, use the drive’s features. The hand control has the typical catalogues of objects that are available with modern Go To systems, including the Caldwell catalogue. There is, however, no listing of named deep-sky objects, so if you’re looking for the Ring Nebula, you’ll have to find it by either its Messier number (M57) or its designation in the New General Catalogue (NGC 6720).

From an operational standpoint, the drive worked very well. When slewing the telescope at the higher speeds, there’s a somewhat annoying lag in the response to pressing the slew buttons, but this goes away at slower speeds, making it easy to centre objects in the finder and telescope eyepiece. Since the power jack for the motors moves as the scope turns in azimuth, you have to be mindful of having enough slack in the power cord for the scope to turn. There is a cord-wrap feature that helps by preventing the scope from continuously slewing in one direction, but I never figured out exactly when it would activate. This is no big deal, but it can be a bit surprising when you expect to have the scope slew only a short distance from one object to the next but then find the azimuth motion reverse direction and turn nearly 360° to get back to the general area where you were just looking. There are a few subtle differences between the SynScan system and other Go To scopes I’ve used, but overall I was very pleased with its operation and features. I’ve not used a lot of Dobsonian scopes with Go To pointing, but I found this one to be very accurate, and the tracking excellent. Furthermore, you can disengage the motor clutches on both axes and move the scope manually to any part of the sky and reengage the clutches and resume observing without having to re-initial-ize the drives. A very nice feature.

Why the Rocky Start?With so many positive things going for the Stargate, you’re probably wondering why I said earlier that this review didn’t start off well. The answer can be summed up in three words — the instruction manual. Regardless of what I might intuitively know about setting up telescopes, for the sake of a review I always follow the step-by-step instructions in the manual. I can’t recall assembly instructions worse than those for the Stargate. In addition to easily recognised mistakes such as referring to the secondary-mirror assembly as the primary-mirror assembly, there are misidentified parts mentioned in the text that make the assembly procedure confusing. But even worse are the diagrams, many of which are riddled with errors. There are mislabelled diagrams; diagrams with missing labels; and diagrams mentioned in the text that simply don’t exist. The worst of these errors involved the novel cable system used for the altitude drive, so the manual will be less of a problem for people who don’t purchase a Star-gate model with drives. But I found the Go To pointing and tracking to be so valuable that I would strongly recommend people consider getting the scopes with motor drives.

After working through the scope’s frustrating assembly procedure, I worried that things were going to get even worse. When I initially unpacked the telescope from its four large shipping boxes there was a prominent slip of paper with the bold headline “Attention.” It directed me to the Sky-Watcher website to download the latest version of the SynScan firmware and install it using instructions in the manual. This is rather common for today’s computerised scopes, but I was dismayed to see that the latest version of the firmware was dated more than six months before the Stargate was shipped to me for review, and the version of firmware in my hand control was even older. An update was clearly needed to keep the review accurate, but I certainly wasn’t looking forward to another round of step-by-step instructions in the manual. But here’s the punchline: The whole procedure went precisely as described in the SynScan manual and took about 10 minutes! The only hurdle was the required serial connection between the computer and hand control to do the update.

The Stargate’s ribbed secondary mirror is made of molded glass to help reduce weight and speed temperature acclimation.

Lightweight plastic covers for the primary and secondary mirrors help keep the optics protected and dust-free when the scope is not in use.

Since the hand control needs to be plugged into the scope and powered on for the update, that meant bringing a computer to the scope, and it’s been years since any of my laptop computers has included a serial connection. I had to use a USB-to-serial adaptor, which was no big deal. Memories of the bumpy road travelled to get from opening the telescope’s shipping boxes to first light with the Stargate 18-inch SynScan instantly faded into the background when I had that first look at the Double Double mentioned earlier. And were it not for writing this review, it would have probably stayed that way. The Stargate turned out to be a wonderful instrument to use. As with any big telescope, there are special considerations that go with storing, moving, setting up, and taking down the Stargate. But if you’re prepared for them, then I can strongly recommend this telescope. It really is that good.


Here’s a sneak peek of the new night-vision system we’ll be reviewing in the coming months. It was awarded a Hot Product in last January’s issue (page 35) and is seen here on the Stargate with an optional iPhone adaptor. It was used for the “snapshot” at far right of the planetary nebula NGC 7662 in Pegasus, better known as the Blue Snowball Nebula. Deep-sky astrophotography with an iPhone – we do live in interesting times….

As described in the text, the author found this $8 furniture dolly an excellent way to move the fully assembled scope over hard surfaces, including on a lengthy trek down his street to a neighbour’s yard to view an occultation by Neptune’s moon Triton last October 5th. It was a simple matter to lift each of the scope’s three legs just enough to slip bricks under them, creating a solid base for the scope to sit on.

The altitude drive for the Stargate SynScan models uses a wire-cable system visible in this view of the telescope base. It worked very well but had to be disconnected from the telescope tube in order for the tube assembly to be removed from the base.
Electrical connections for the SynScan drives are straightforward and well-marked. Furthermore, the cable connectors for the azimuth motor and encoder are slightly different, making it impossible to attach them incorrectly, even in the dark.

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