While Galileo’s profound discoveries with his first telescope in 1609 are rightly celebrated in the annals of science, the optical design of his first telescope is not. Based on a simple convex objective lens and a concave eyepiece, Galileo’s early telescopes gave drinking-straw-narrow fields of view and and image brightness that dropped off drastically at the edge. His design was quickly replaced by the so-called Keplerian telescope which we all use today. So imagine my surprise when I discovered a relatively new set of binoculars from Vixen Optics that are based on a modern version of Galileo’s original telescope design. These Vixen SG 2.1×42 binoculars, which magnify just 2.1 times and have objective lenses 42 mm diameter, give extraordinary wide-field views of entire constellations, and some say that observing with these binoculars is like having ‘super vision’.
A bit of background…
Galileo’s first astronomical telescopes were based on a design invented by Dutch opticians in the late 1500s. The design used a convex objective lens to collect light and a concave eyepiece placed before the focal point of the objective (see below). Now called a ‘Galilean’ telescope, this design produced an upright image, which was useful for terrestrial observation. But there were disadvantages aplenty. The field of view was very small, so it was hard to see, for example, the entire full Moon even at low magnifications of 10x-20x. The brightness dropped off drastically at the edge of the field of view. And it was impossible in this design to use a micrometer, a device for measuring the angular diameter of objects. These days, only inexpensive and low-power binoculars, which were once called ‘opera glasses’ use the Galilean design.
That’s why nearly all modern astronomy telescopes are based on the ‘Keplerian’ design, a scheme named after the great Johannes Kepler who came up with it in 1611. In a Keplerian telescope, the eyepiece is a convex lens placed behind the focal point of the objective. This design produces an inverted image, which is not a big deal for astronomers, and it gives a relatively wide and bright field of view at higher magnifications.
So what has Vixen done with these SG binoculars? They have essentially made a set of super-premium opera glasses that allows an observer to get very wide-angle views of the night sky, especially a dark night sky, at very low magnification, only twice that of our unaided eyes. While these binoculars use a Galilean design, they have some modern enhancements to minimize optical aberrations. But they perform much like a Galilean telescope of centuries ago.
If you are experienced in these matters, you might conclude these binoculars are designed by a mad man. A 42 mm objective and only 2.1x magnification? This gives an exit pupil, you might think, of 20 mm. (The exit pupil is the diameter of the objective divided by the magnification). Since the pupil of the human eye can only open to 7 mm at best, it seems as if most of the light from these big lenses won’t even enter your eye. It seems like a crazy waste of light!
But things are a little different with Galilean telescopes. The exit pupil lies inside the optical system, so there is no fixed point at which to place your eye to capture the light. In fact, in these Galilean optics, you have to place your eye as close as possible to the eyepiece lens to maximize the field of view. If you can get your eye close enough to the lens, you get a huge field of view of up to 25° compared to 5º to 7º FOV for most binoculars. That’s big enough to take in entire constellations like Cygnus and Sagittarius and cruise along the rich star fields of the Milky Way to see dozens of star clusters, diffuse nebulae, star clouds, and inky-black archipelagos of dark nebulae set against the background stars.
If you move your eye away from the eyepiece, the field of view drops. At the specified eye relief of 8.4 mm, for example, the true field of view is about 12º. And if you wear eyeglasses, you might experience an even smaller field of view, so be advised. The closer you can get your eye to the eyepiece, the better.
As for image brightness, again, in a Galilean telescope, things are a bit different. The gain in image brightness on point sources (that is, stars) in a Galilean telescope scales as the square of the magnification. So for 2.1x, you get an increase in image brightness of 4.41x. That works out to 1.6 magnitudes, assuming the lenses are perfectly transmitting. So if you have nice dark sky in which you can see stars of magnitude 6.5, these binoculars will show you stars of 8th magnitude, again, with a field of view of up to 25°.
That’s the great benefit in observing with these binoculars. These are very compact and light, so you simply hold them up to your eye to see entire constellations with thousands more stars than you could otherwise. That’s the ‘super vision’ effect. These binoculars will not replace your standard 7x50s or 8x42s, and they are not for everybody. But if you wish to cruise the constellations Milky Way with something approaching a naked-eye experience while pulling in thousands more stars and dozens more deep-sky objects, the Vixen SG 2.1×42 binoculars might be for you.
In the U.S., these optics are available for a little under $300 from several astronomy equipment vendors. Despite their ‘opera glass’ heritage, they are a quality instrument with well crafted Japanese-made optics and mechanics that include matte-black interior and baffling to reduce stray reflections and maximize contrast. They only weigh about 410 grams.
If you are curious about these instruments and wish to consider acquiring a pair, here’s a link to an excellent and detailed review.
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