Review: Apertura DT6 Dobsonian Telescope

Metaphysical perfection
5/5

When it comes to commercial Dobsonians, the optics tend to be fairly good, and usually either identical or very similar. Each of the size classes of Dobsonians is represented well by every major telescope brand. Typically they come in 6”, 8”, and 10” varieties, each with the same focal length, as well as a 12” version which is a little longer. When choosing a Dobsonian telescope, there is no need to differentiate based upon optical quality, as for the most part they are identical and typically they are pretty good. Instead, the choice is really between the cost, weight, mount, and accessories.

It is plainly obvious from the price tag that The Apertura DT-series of Dobsonian Telescopes, in particular the DT6, prioritizes cost. The good news is that it does so without really cutting any corners, and a few things are actually superior on the DT series compared to other available dobsonians.

Apertura products, sold by High Point Scientific, are a rebrand of Guan Sheng Optical (GSO) products for the U.S. market.

  • The price tag.
  • Simple, sturdy, easy to use rockerbox mount.
  • Very smooth all-metal 1.25” Crayford focuser.
  • Excellent performance at high power.
  • It is the heaviest among all 6” f/8 commercial Dobsonians.
  • Finderscope could be larger.
  • Includes only one eyepiece, you’d have to fit more eyepieces into your budget.

The Apertura DT6 is very easy to use. The motions of the simplistic mount are very smooth, there is little to no wobble, and there is plenty enough aperture to find and enjoy many deep sky objects as well as the planets. A 6” Dobsonian is probably the ideal first telescope for a teenage or adult beginner (it’s a bit too big for kids), and the Apertura DT6 is arguably the best 6” Dobsonian, especially when factoring in the all-too-crucial cost.

Pretty much any 6” Dobsonian will show you the same beautiful views of planets and deep sky objects. However, there are a few reasons to prefer the DT6.

If you’re on a strict budget, I think you ought to choose the telescope you buy, not necessarily based on the accessories that come with it, but the unchangeable (or difficult to change) aspects like the focuser, mount, even finderscope. If it’s the difference between a smaller telescope with better accessories and a bigger one with fewer accessories, I’d choose the bigger one. You can pick up more accessories in time, but you can’t simply add an extra inch of aperture whenever you want.

The Apertura DT6 provides an excellent basis for a serious observing experience at a very affordable cost. It’s a tough call between it, the Orion XT6, and the SkyWatcher Classic 150P. I’d say that the DT6 wins because it has the best focuser. However, if the telescope must be portable, the very lightweight base of the XT6 will come in handy, as will the handles of the Classic 150P. However I suspect the overall motions of the XT6 and the DT6 will be superior due to the simpler bearings–my experience with the SkyWatcher’s tension adjustment knobs is not great, though I’ve only used them in larger telescopes.

Other non-dobsonian telescopes in the price range have smaller apertures than the DT6, often cutting corners on finicky German Equatorial Mounts, using cheaper accessories and focusers, and generally being less capable overall.

Contents Of This Review

Apertura DT6

Rating: 5/5

Overall Build and Impressions

The Apertura DT6 is built quite sturdy, from the rolled steel optical tube assembly to the solid particle board rockerbox. (Though like all particle board rockerboxes, it would be destroyed if it gets too wet. The paint coatings can help mitigate this for the simple problems of dew and frost, but it wouldn’t tolerate sitting on a wet ground too often). The rockerbox is very sturdy and does not wobble at all. The focuser is all metal and very smooth, with no play in the drawtube that would ruin collimation. Collimation is easy to do with a Cheshire or collimation cap, and no special tools are needed, even the secondary mirror is collimated with an ordinary screwdriver on the very rare occasion when it would need to be adjusted.

The DT6 wasn’t my first telescope, but I sure wish it was. And though I do now have larger telescopes, the DT6 still maintains a great balance between weight and optical power, and I still use it instead of my 10” Dobsonian when I feel too tired to take the 10 out for a brief bit of observing.

Optics of Apertura DT6

The Apertura DT6, like most full-size 6” Dobsonians, has a diffraction-limited parabolic mirror with 6” (150mm) of aperture and a focal length of 3.9’ (1200mm), for a focal ratio of f/8. This focal length matches the other, standard commercial 8” and 10” Dobsonians. However, unlike the fat 10” and 8”, the long thin tube of the 6” dobsonian means that it is, at least for its aperture, an absolute planet buster. It is easier to make long-focus mirrors (and lenses) than it is to make short-focus ones, so it should be no surprise that the optical quality of the 6 inches can surpass that of the larger instruments (however, being at a smaller aperture, that doesn’t mean the overall resolution is better–you should still expect an 8” and 10” Dobsonian to beat the DT6 on planets, but perhaps not by as much as you’d expect from the increase in aperture alone.)

The maximum useful magnification of a 6” telescope is 300x, and the minimum useful would be no less than 21x.

The star test is good, with only a small amount of undercorrection, and stars snap to focus at high powers.

One of the keys to the 6” f/8 telescope’s good performance is a relatively small secondary mirror, which obstructs the front aperture of the telescope by only 22%, very close to the point at which central obstruction ceases to be noticeable. Larger obstructions reduce contrast by spreading out some of the light coming from a star into concentric rings. Therefore long-focus newtonians can typically outperform Cassegrain-type telescopes of the same aperture, as typical Cassegrains (Schmidt and Maksutov) have fairly large central obstructions.

Another feature of the long-focus mirror is that it is easier to collimate–a high alignment precision is much less necessary than it is on the larger, fatter, faster dobs.

I stress, however, that if the optical quality of the DT6 is good, it means that you can count on almost any other modern commercial dobsonian with a 6” f/8 primary mirror to be just as good.

Focuser

Apertura DT6 focuser tubeA lot of telescope manufacturers, especially on the 6” Dobsonians, tend to overlook the focuser. This is a bit of a shame, as the focuser is the main element you use to interact with the telescope, the one moving part you must always use when changing eyepieces or removing your glasses. The focuser on some 6” dobs is a simple plastic rack-and-pinion design. While a well-implemented rack and pinion (particularly an all-metal one) can suffice, the smooth motion of a Crayford-type focuser is desired. The Apertura DT6 has one of the only Crayford focusers on a 6” Dobsonian.

The focuser only accepts 1.25” eyepieces, 2” eyepieces can not be adapted. 2” eyepieces can typically provide much wider fields of view than 1.25” eyepieces, so this may seem like a major drawback. However, the small size of the secondary mirror of the DT6 barely fully illuminates the 1.25” wide field–a 2” field would have severe vignetting at the edges. The DT6 is the only telescope in the DT series with a 1.25” focuser, whereas the other DT-series telescopes have 2” Crayford focusers.

Compared to many of the telescopes I’ve used, I can say that the 1.25” Crayford Focuser used by the Apertura DT6 is buttery-smooth and works great. However, it is a 1-speed focuser, so there can be a lot of vibrations at higher powers, and truly fine focus can be elusive if you don’t have an eye that can focus independently of the telescope.

Mount

A Dobsonian’s rockerbox is composed of a rocker-box that holds the altitude bearings, resting on top of a simplified lazy-susan to move around. It is an “Altitude-Azimuth” or “Up, Down, and All Around” mount. It can not be set up to track the sky automatically. (Though expensive computerized Dobs which can do so do exist)

Like most dobs, the mount comes flat-packed and must be assembled with the included Allen wrench. It’s an hour’s job, and one I did immediately when my DT6 arrived, just in time for darkness.

The mount is much closer to a traditional Dobsonian (powered by yogurt and eggs and held together by gravity) than some other scopes on the market. Rather than the altitude bearings being supported by tension knobs or handles, the DT-series uses a smallish altitude trunnion resting on Teflon glides, with tension increased using a pair of springs that connect the altitude trunnion on the optical tube assembly with the base of the mount. A similar mechanism is used on Orion’s SkyQuest XT-series (but not the XT-plus), and older versions of SkyWatcher’s dobs. Traditionally, Dobsonian telescopes are built with the largest altitude bearing surfaces possible, as this provides the smoothest motion with the least stiction. On the DT-series mount, the springs provide enough additional tension to replicate the effect of larger altitude bearings.

Because the mount lacks altitude tension adjustments, I’ve found that the finderscope can severely unbalance the scope when pointed within a few degrees of the zenith. However, that part of the sky (called “Dobson’s Hole”) is practically inaccessible to Altitude-Azimuth mounted telescopes anyway. Slightly more annoyingly, heavy eyepieces, or perhaps cameras, will unbalance the telescope when it’s pointed very low in the sky, causing it to tend to fall. However, this is only noticeable very close to the horizon, again, not a place where you would want to do much astronomy.

There are tension adjustments on the azimuth axis on the bottom board, in the form of a large knob. You can also vary the number of plastic washers when assembling the telescope to adjust the tension. I believe I put in two out of the five included, and it works great.

You might fear that tracking the sky without an Equatorial mount would be a problem. It most certainly is not. With the DT6, the motions are very smooth right up to the 300x maximum useful power of the telescope. It is effortless to nudge the telescope so the planet is on one edge of the field of view, wait for it to transit across, and then nudge it again. There are some vibrations when doing this, but they die down quickly enough so as not to be a severe problem.

Weight of DT6

The Apertura DT6 is the heaviest of all of the currently available 6” imported Dobsonians, in large part due to the heavy mount, though the optical tube is comparatively beefy as well. In fact, the GSO dobs in general (Apertura DT- and AD-, as well as Zhumell Z- series) are all the heaviest in their size class. Is this a problem? For the larger apertures, that may be. However, the DT6 in particular is lightweight enough that I can carry it fully assembled by holding the handle on the front of the base and grabbing the side board. It is easy to carry the rockerbox alone with one hand, and the optical tube can be carried in your arms.

Portability

Before I moved to my current house, I lived at a place where there was no sky for me to observe–too many trees. So I kept the telescope in the car and drove out to a nearby field every time I wanted to observe. When doing this, it is best to keep the telescope and mount separate. The rockerbox base takes up a car seat and can be buckled in place, while the optical tube can either lie across the backseat of even a relatively small car, or front-to-back in a hatchback/SUV with the back seats collapsed. Travelling on a family vacation with it might be limiting, but if you’re driving yourself and your telescope to a dark site, you can probably fit it in pretty much any car.

The telescope must be stored in a place where you can easily get it outside to the observing space. Carrying the whole thing fully assembled while going up and down stairs is not an option, so if you have a flight of stairs you’ll have to take two trips (one for the optical tube, one for the mount). Keep it in a place with no obstructions or obstacles. If you have to keep it in the living room, then it’s fine, you have an interesting conversation piece!

A Dobsonian may look big and bulky, but it can actually occupy less floor space than a tripod-mounted telescope.

Accessories with DT6

The Apertura DT-series telescopes keep the costs down by providing very few accessories. In fact, there’s only two: A 25mm Plossl eyepiece, and a 6x30mm Straight-Thru finderscope.

DT6 plossl eyepieceThe 25mm Eyepiece is a four-element Plossl, and it is one of my favorites. Plossls have moderately wide fields of view (52 degrees) and tend to be very sharp right up to the edge. Though the GSO/Apertura Plossls don’t quite get the edge perfect, for the cost they are very good. The lenses are blackened on the inside and the all-metal/glass construction makes them very sturdy. I have dropped mine a few times, which you should try not to do, and they have suffered no damage. There’s a good reason that most serious telescopes come with a 25mm Plossl. It’s a really good all-around focal length. It is very comfortable to look through due to generous eye relief and large eye lens, and it provides good low-but-not-too-low power. The 25mm in the DT6 reveals the rings of Saturn, the cloud bands of Jupiter, Jupiter’s Moons, etc, at 48x. However, its wide field of view also makes finding and observing faint objects much easier.

The Moon almost fills up the eyepiece, with enough black space at the edges to keep the limb of the Moon sharp.

The 25mm Plossl doesn’t quite have a wide enough field of view to fit in some of the largest Deep-Sky Objects, such as the Pleiades, Beehive, and Andromeda Galaxy. It also doesn’t offer a high enough magnification to truly take advantage of the resolution of the telescope on planets and double stars.

The 6×30 Straight-Thru Finderscope is… acceptable. It works well enough to get the job done. I think it’s probably better than a red dot finder. In my opinion, red dot finders (like those sold on Orion’s SkyQuest XT-series of telescopes, or like Telrads) are best utilized in small, fast telescopes with wide enough fields of view that the low power eyepiece can itself be a finderscope. But f=1200 Dobs don’t have a wide enough field of view to be their own finderscope, and so a magnifying finder can be very helpful. At only 30mm, the view is pretty dim, but it’s enough to see some of the crucial asterisms needed to find faint deep sky objects, and enough to see some of the brighter deep sky objects on their own.

I had been used to a right-angle finderscope, on the 8” SkyWatcher FlexTube dobs used at the Observatory I volunteer at. Going to a straight-thru upside-down image was annoying, as you have to crane your head back to some weird angles to look through it, especially when pointing high up. However, when I went back to the right-angle finderscope, I realized that the straight-thru finderscope, while uncomfortable, does help you get pointed to where you want in the first place much more easily. You can see the sky and the view through the straight-thru finder at the same time.

Troublingly, I could never get the finderscope to come to focus properly with my glasses on. The focus adjustment is done by unscrewing the objective lens, and if you unscrew it enough to put it into focus it just comes off. With my glasses off, I can get it in focus, but then the crosshairs are out of focus. The crucial aspect of a straight-thru finder’s ability to overlay the finder view and the view with the other eye only works when your eyes are focused at infinity, looking parallel. If the crosshair and the image are focused differently, then it can be tough to mentally align what you see in the finder with what you see in the sky.

Replacing the Accessories

The 25mm Plossl need not be replaced, but rather supplemented. The finderscope meanwhile definitely could stand to be replaced. A good pairing would be a Telrad or Rigel Quikfinder (red dot finder) plus a 9×50 Right-Angle-Correct-Image finderscope. The added weight would likely necessitate the addition of a small magnetic counterweight at the back of the telescope.

In order to see objects at high powers, you’ll need some higher power eyepieces. There’s a lot of different choices you could make. If you got just one, I would recommend a 6mm Goldline eyepiece (66-degree Ultra-Wide-Angle), which would provide 200x. It’s what I personally use for planetary viewing, and its wide field of view means you’ll be nudging the telescope less often. However, for a while I’d use a cheap 6mm Kellner that cost $15 bucks–long focus telescopes can better support cheap, simple eyepiece designs like the three-element Kellner. The Kellner did better on the Moon.

If you can get two eyepieces, get the 6mm Goldline and a 15mm eyepiece. The 15mm Goldline ended up being one of my most-used eyepieces despite its poor edge-correction. However, feel free to choose another eyepiece in the 10-15mm focal length range, such as the Dual ED (Paradigm/StarGuider) design or even a regular Plossl eyepiece. 15mm is enough to split out detail in some of the smaller deep sky objects without making them too dim, such as the interesting barred structure in the Globular Cluster Messier 4, or the wispy filaments in the Great Nebula M42.

Another useful eyepiece is one that maximizes the field of view: a 32mm Plossl. GSO/Apertura makes one that works fine. A 32mm Plossl would fit in most of the Pleiades and renders objects like the Andromeda Galaxy even brighter and broader.

Barlows are less needed in long-focus telescopes like the DT6 compared to short-focus ones. If you get a Barlow, go for a 2x or 2.5x unit, nothing more. You want to be sure you get an achromatic (two-element) or apochromatic (three-or-more-element), fully multicoated Barlow lens.

I rarely observe above 200x, but if you have very good seeing conditions, you might consider getting an eyepiece in the 240x to 300x range–the optics of the telescope are good right up to the 300x limit.

What can you see?

The DT6 has been my observing workhorse for almost a year, and we have together viewed many of the bright and interesting objects. A 6” telescope can absolutely do well on deep sky objects, and some of my best ever views of the Pleiades, Andromeda Galaxy, and Orion Nebula have been through the DT6. If you live in the suburbs or darker skies, you will never run out of things to find. I have had a lot of fun chasing deep sky objects, be they brilliant gray structured blobs, or dim patches of slightly paler sky. However, where the DT6 really excels is the planets. When viewed at a magnification of 150-300x, when the atmosphere is still and the planet is high in the sky, the telescope soaks up detail.

Jupiter always shows its cloud banding. But often you can see subtle structure within the bands, more belts and zones, little white specs (Moon-sized storms), the smooth dark polar hoods. On good nights, the Great Red Spot has shown detail as a dark-rimmed orange oval with a bifurcated white tail coming behind it. When the Galilean satellites pass in front of Jupiter, they cast their black pinpoint shadows and in good conditions, you can see the little disk of the satellite itself.

Saturn shows its rings easily even at low power. At high power, they are sharp and the Cassini division is visible across the whole ring, with the dark inner ring faintly visible in good conditions and always visible as a dark “shadow” in front of Saturn itself. The true shadow of the rings on the planet, as well as the planet on the rings, is visible when they’re aligned properly. Cloud banding can be seen, and on a very good night, I could just see the famous polar hexagon as a little shapeless dark dot on the polar hood. Its moon Titan is easily visible, and in good conditions, you can see five or six of the major Saturnian moons as very dim stars scattered close by. Typically you’ll see Titan, Rhea, and Iapetus if its bright side faces us.

Mars has just gone through a very close opposition and so it will be about a decade before the views get really good again. However, even at a moderate distance, the DT6 shows Mars with noticeable but tough albedo features and the ice cap can be seen. At a close opposition, the view of Mars is stunning, with lots of distinct albedo features that change over days and hours, a pale blue rim of atmospheric haze, and the ice cap as a tiny white dot.

Uranus resembles strongly the many small bluish planetary nebulae which can be seen with the telescope. It is somewhat brighter, but visible as only a soft disk. Neptune would look much the same.

Venus is a large, bright star most of the time. When the power is raised, the glare can be reduced, and it looks like a little Cue Ball. Its phase is visible when it is gibbous, and it is very pretty as a crescent. I had great fun earlier this year watching Venus go from a half-disk to a crescent to a thin sliver. Surface detail is practically impossible to see, though some filters can reveal some faint cloud structures.

To describe what the Moon looks like in detail would take all day. Suffice it to say, it is excellent.

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