I am planning to travel to Hawaii in a few weeks. Astronomy isn't my main goal, but as long as I'm there, it would be criminal not to take advantage of Mauna Kea, one of the world's foremost sites for astronomy. Morevoer, Hawaii is far enough south to give good views of important parts of the sky that are never visible from the northern United States.
In the past, I have taken my TeleVue Ranger, a 70-mm f/6.9 refractor, on numerous airplane trips. It works quite well on a photo tripod, making a package that's light and compact, and also reasonably capable. But I always wonder if I could do better. So I decided to try the GoScope 80, an 80-mm f/4.4 achromat that I reviewed for Sky & Telescope seven years ago. This scope is lighter than the Ranger, but also has bigger aperture, making it better for deep-sky observing -- at least in theory. I was very impressed by the GoScope when I tested it for that S&T, but that was in the context of cheap beginner scopes -- not a very high bar. I wondered how it would stack up against the Ranger, which was considered top-end in its day.
On a beautifully clear night in mid-March, I compared both scope on an array of objects including all the Messier objects in Orion and Leo, and the double stars Iota Orionis and Gamma Leonis. I also star-tested both on Regulus. Although I don't care much about planetary performance for this particular trip, I also compared both scopes on Jupiter shortly before dawn.
The focal lengths of the scopes are 350 mm for the GoScope and 480 mm for the Ranger. For medium-high power I used a 5.2-mm Pentax XL to achieve 67X in the GoScope and a 7-mm Pentax XL to achieve 69X in the Ranger. Using nearly identical magnifications in eyepieces of very similar design eliminated one important variable in the comparison. For low power, I used a 25-mm Plossl in the GoScope to achieve 14X with a 3.6-degree field of view, and a 30-mm Celeston Ultima in the Ranger to achieve 16X with a 3.1-degree field of view.
Both scopes were mounted as I would take them traveling -- on photo tripods using ballheads with quick-releas plates and the ballhead tilted over sideways, as shown in the photo below. The GoScope's native tabletop mount works quite well, and it too screws into regular photo-tripod legs. But a ballhead is somewhat lighter, considerably more compact, and works even better.
I wish I had more conclusive results about comparing the two scopes on deep-sky objects. The fact is that everything I could see in one, I could also see in the other. That's not altogether surprising. First of all, the difference between 70 and 80 mm aperture isn't huge. And in addition, once you know something is there, it's relatively easy to find it, even if it wasn't apparent at first sight.
The most consistent difference is that faint stars were distinctly easier to see in the GoScope than in the Ranger. That's a testament to both to the GoScope's optical quality and to its focuser, because seeing faint stars requires the ability to focus them to tight pinpoints as well as good light-gathering capability.
As for nebulosity, I would say that it's brighter in the GoScope and more contrasty in the Ranger. But the differences are very slight indeed. As far as the level of detail, or ease of seeing it, I could discern no difference.
Star testing on Regulus was a different story. The Ranger has a near-perfect star test, aside from a good deal of false color. The GoScope has nice tight rings on one side of focus, but is somewhat spiky on the other side. As for false color, the difference is overwhelming. The Ranger comes close to showing Regulus as it really is, the same as my reflectors. The GoScope shows a strong blue halo around any first-magnitude star.
Likewise, on Jupiter the next morning, the Ranger's false color was merely obvious, whereas the GoScope's was overwhelming. Despite that, the GoScope showed the two main belts reasonably crisply at magnifications up to 88X (using a 4-mm eyepiece, the shortest I had handy). But the Ranger's views at corresponding magnifications were distinctly sharper. The Ranger's view at 120X was at least as sharp as the GoScope's at 88X, and showed some nice scalloping along the edges of the belts.
This is all unsurprising; after all the Ranger's optical tube alone cost four times more than the GoScope complete with mount. Moreover, all achromats show an irreducible minimum of false color, which is proportional to aperture divided by focal ratio. The theoretical false color of an achromat at 80-mm f/4.4 is much more than at 70-mm f/6.9. But for people who think that the GoScope is inordinately fast, remember that its false color is only a little stronger than that of a 100-mm f/6 achromat, which is quite a popular format. (100/6 ~= 17, 80/4.4 ~= 18, 70/6.9 ~= 10).
Both scopes have very good, though unconventional, focusers. The Ranger has a drawtube for coarse focus and a helical focuser for fine tuning. The GoScope moves the objective back and forth inside the tube, rather than lengthening the tube as in conventional designs. On the whole, I find the GoScope's focuser easier to use, and it's also less prone to get stiff in cold weather.
The GoScope is considerably lighter than the Ranger (2.7 lb, or 1.2 kg, compared to 3.7 lb, or 1.7 kg), complete with diagonal and red-dot finder for both. It is also somewhat shorter -- 15.5 inches (39 cm) compared to 17.5 inches (45 cm). However, the GoScope is also 25% fatter, making it considerably more voluminous despite its shorter length. The fact that a scope with just 14% more aperture is 25% fatter implies that it has more clearance between the tube and the light path. That, together with the fact that it's more lightly built, probably accounts for the fact that the GoScope cooled down enough to deliver sharp images quite a bit more rapidly than the Ranger. (Outdoor temperatures were around 14F, or -10C.)
The GoScope places considerably less demand on the mount than the Ranger, due to its lighter weight and shorter moment arm. I own two different sets of photo-tripod legs (Bogen 3001 and 3011) and ballheads (Bogen 484 RC2 and 488 RC2), and the GoScope is about as steady and smooth on the lighter legs and head as the Ranger is on the heavier legs and head. Moreover, due to the fine details of the two telescopes' mounting blocks, the quick-release plate attaches solidly to the GoScope, whereas it tends to work loose over time with the Ranger.
The improved performance on the mount is probably the clinching factor that will push me to take the GoScope rather than the Ranger to Hawaii. The Bogen 3001 legs, 484 RC2 head, and GoScope makes a versatile package weighing just 7.3 pounds, and its ergonomics and deep-sky performance are marginally better than the Ranger's. Taken together, that outweighs the Ranger's significant edge in planetary observing. After all, I need to travel to Hawaii to observe Alpha Centauri and the Eta Carinae Nebula, but I can observe Jupiter back home -- albeit not quite as high in the sky.
The GoScope's biggest liability might be the fact that its f/4.4 focal ratio makes it hard to achieve reasonably high magnifications with conventional eyepieces. I usually use an 8-24 zoom with the Ranger. But the Goscope with that zoom at 8 mm achieves just 44X, which is definitely on the low side for deep-sky observing with an 80-mm refractor. I'm not about to use my Pentax XL's which weigh about a pound apiece. That means that a full set of them would outweigh the scope itself by a fair margin.
In some ways, the most interesting fact is that there's any question at all. The fact that the GoScope, at $150 for optical tube and mount complete, can equal or exceed the deep-sky performance of the Ranger, whose optical tube cost more than $600, is mighty impressive.