Giant Binoculars: The New Explore Scientific BT-120

Looking down the barrels of the double 120mm aperture, f/5.5 focal length tubes. Credit: Explore Scientific

Two eyes are always better than one. For years, we’ve espoused the virtues of using binoculars, as an intermediary between naked eye observing and learning the sky, versus a telescope. It’s strange to think that I’ve had a set of Canon IS15x45 image-stabilized binoculars for going on two decades now, and I use them for casual observing about 90 percent of the time.  

We recently had a chance to try out a unit that takes this axiom to an extreme. We’re talking about Explore Scientific’s new tripod mounted, BT-120 SF Large Binoculars.  We looked at the 120mm version, but it’s also available in 70mm, 82mm and 100mm versions.

The optional U mount accessory allows maximum slewing from zenith to horizon. Credit: Explore Scientific

These beasts feature a pair of porro achromatic refractors, each with a 120mm aperture and a 660mm f/5.5 focal length, yielding a generous field of view nearly two degrees (four Full Moons) across. This is like having two five-inch refractor telescopes mounted side-by-side. The binoculars come with 2×20 mm ‘argon purged’ eyepieces yielding a 3.6mm exit pupil distance and a wide 62-degree apparent field of view with 33x magnification. The achromatic lenses give bright stars like Sirius a slight but not objectionable deep-violet tinge, standard with these sorts of refracting optics.

We used the binoculars on a alt/azimuth yoke mount atop an Explore Scientific Twilight II tripod. We’ve had a similar Twilight I mount for years; it’s a very sturdy mount. Explore Scientific also makes a simple quarter-inch U-Mount adapter to mate the yoke with a standard large tripod. During our standard vibration test, we found that the view settles down in less than a second after knocking the side of the tube. And while there are no fine tuners, the large altitude locking knobs are a joy to use, even in the cold with gloves on.

Using the Giant Binoculars in the Field

Despite the size, the central handle located between the two tubes makes the 20lb (9 kg) binoculars easy to set up in the field. Just be sure that the dovetail bracket is engaged and the forward and aft stop-screws are in place before letting go. The unit is also equipped with built-in dew shields for each aperture, and a gunsight-style, steeple-shaped finder mounted on the handle between the two tubes. This also worked great with the supplied prototype Thousand Oaks film solar filter mask, supplied with the binoculars (the company states that they will offer the solar filter separately soon as an optional accessory).

Two eyes are better than one. Credit: David Dickinson

The diopter spacing varies from 60mm to 65mm from center-to-center.  That’s pretty generous, as our eyes are on the wide end of that scale, almost 65 mm apart.

What can you see with giant binoculars? The huge advantage with binoculars is that they offer a true (non-inverted) view of the sky; no having to mentally retranslate what you’re seeing at the eyepiece versus looking at a star-map. Prepare to be amazed with your first views through the BT-120s; star-hopping is a snap, and even the familiar Moon takes on a sharp, 3D appearance. We were even able to track the International Space Station on a bright pass and easily pick out the Star Wars ‘TIE fighter’ structure of its solar panels! We could also easily split the 5.3” double star Castor.

But where the BT-120s really shines is when viewing globular and open clusters. We were amazed simply turning them toward familiar sights such as Praesepe (M44) and open clusters (M36, M37, and M38) in Auriga; the glorious Pleiades filled the view from a dark sky site, with hints of the fabled Merope nebulosity visible.

Other applications: I could easily see the BT-120s as versatile for other pursuits as well, such as aircraft spotting, or the ultimate bird-watching station.

Pros: The big advantage of large binoculars is that they add depth and dimension to the time honored art of star-hopping. The BT-120s are simple and effortless to use; you simply aim, focus and explore the sky, no troubleshooting required.

Cons: We found the adjustment to change the interpupillary distance from one viewer to the next to be slightly stiff. Ordinarily, this would not be a problem for a solitary observer, and only really comes into play during public star parties. Also, the steeple finder is tough to see in the dark, but this can be remedied easily with the addition of a Telrad or red dot finder.

Binoculars have always been the hallmark of portability in astronomy. Here the author uses a large solar filter for safe solar viewing Credit: David Dickinson

Plus, the BT-120s were a hit a public events. If you love binoculars and you’re a die-hard visual observer, you’ll love the view through the Explore Scientific BT-120s.  They’re the largest binoculars this side of the Large Binocular Telescope!

MSRP: $2,349.99 ($749.99 for the Twilight II tripod)


About David Dickinson

David is a freelance science writer, frequent contributor to Sky & Telescope and Universe Today, author of several astronomy books and long-time amateur astronomer. He lives with his wife Myscha in Norfolk, Virginia.

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