Equipment review Vixen’s giant binoculars Specifications among largest sold Vixen BT-125-A Use the BT-125-A, and you’ll know why many observers love large binoculars. by Phil Harrington : 4.9 inches Prism type: BaK-4 Focal length: 760 millimeters Focal ratio: f/6 Eye relief: 20mm o question about it: Binocular the tripod. Although the optical unit True field of view: 1.6° (with 30mm astronomy is growing in popu- weighs 24 pounds (10.9 kg) — certainly eyepieces) Nlarity. But merely looking on the heavy side for some people — a Weight: 24 pounds (10.9 kilograms) through small, handheld models doesn’t convenient handle between the barrels Price: $3,699 (binoculars only) satisfy today’s stargazers. We’re just as makes setup easy. $3,999 (with two 22mm aperture-hungry as observers. Two Teflon-wrapped metal axles eyepieces [28x]) And with good reason. Giant binoculars, extending from the binoculars’ center of $4,399 (complete package) with triple-digit and magnifica- gravity nest into matching hinged clamps tions of 20x or more, are among the most on the fork mount. Once the binoculars exciting instruments for viewing the are in place, adjust the clamp pressure (each a 4.9-inch f/5 doublet) and heavens because they collect enough light until the unit moves up and down you know the BT-125-A means business. to make objects easy to see, and they smoothly (without swinging uncontrol- The greenish coatings in the test pair offer a wide field of view. lably). Azimuth (left-right) motion was appeared flawless. When I shined a flash- equally smooth after I turned a tension- light into the barrels, I saw no indication Setup and operation adjustment wheel at the half-pier’s base. of manufacturing flaws, such as errant Among the largest giant binoculars sold The binoculars are too long to swing metal flakes or other quality-control today are Vixen’s BT-125-A. Recently, I through the fork without hitting its base. issues. Dew caps extend 3.2 inches in had the opportunity to put them through To aim them higher than about 45°, you front of each objective to guard against their paces. I was not disappointed. first have to tilt the fork by loosening a stray light and fogging. Assembly takes about 5 minutes. A setscrew at the center of the fork base and Both eyepieces showed circular, fully captive bolt secures the fork-style alt- pivoting the arms. Angling them at 45° illuminated exit pupils. This indicates azimuth mount/half-pier combination to makes it possible to point at the zenith that the Vixen prisms, made of superior without hitting the mount. Although this BaK-4 glass, are the proper size for the Phil Harrington is a contributing editor of shifts the center of gravity away from the optical design. If the exit pupils were Astronomy. He is currently working on a new tripod’s center, it does not compromise asymmetric or showed a grayed, dia- observing guide for challenging objects. the mount’s stability. Because celestial mond-shaped edge, the prisms would not objects look best when viewed through be able to deliver the full potential of the less atmosphere, I did most of my huge objective lenses. observing above the 45° threshold. Mechanics and engineering As with all giant binoculars, the eyepieces Optics focus individually. Focusing was simple One look at the fully to achieve and smooth. Neither helical multicoated objective focuser showed any hint of binding. The 45° angled eyepieces sit in cylin- Each front lens of the drical prism assemblies that pivot, allow- BT-125-A measures 4.9 ing observers to adjust interocular inches (125mm) across. This distance (the spacing between the two allows a lot of light to reach your eyes, brightening objects that eyepieces) until both fields of view normally appear faint through bin- merge. The range of interocular adjust- oculars. All photos: William Zuback: Astronomy ment varies from 57 to 73 millimeters. I

© 2009 Kalmbach Publishing Co. This material may not be reproduced in any form without permission from the publisher. www.Astronomy.com found viewing to be comfortable even while wearing eyeglasses, thanks to each eyepiece’s generous 20mm of eye relief. With 30mm eyepieces, the BT-125-A boasts a true field of view of 1.6°, which I confirmed during testing. Stars were pin- points across all but the outermost edge of the field. Aiming the BT-125-A accu- Jupiter’s equatorial rately proved tricky at first. Although the belts and Saturn’s rings binoculars do not come with a finder, were also clear, although they do have a standard finder mounting 30x is not enough oomph base. Once I put my own finder scope to show fine planetary detail. into place, aiming became a breeze. That’s why I’m pleased Vix- en’s new version of these binocu- Under the sky lars has interchangeable I clearly resolved stars in the bright glob- eyepieces. All BT-125-A models ular clusters M3 and the Hercules Cluster now work with any 11⁄4" eye- (M13), while the distinctive disks of two piece. That’s a plus because some planetary nebulae — the Dumbbell Neb- other binoculars work only with ula (M27) and the Ring Nebula (M57) — special eyepieces. I can only imagine were both readily visible. what the views through these will be I also spotted the nebulous bridge like with a pair of premium wide-field connecting the Whirlpool Galaxy (M51) eyepieces in place. Wow! and its irregular companion, NGC 5195. Vixen’s BT-125-A binoculars weigh I could even make out a hint of M51’s Contact information 24 pounds (10.9 kg). You can set the distinctive spiral structure. unit up in only a few minutes. Targets closer to home also put on fine Vixen Optics shows. Although I noted some residual 1010 Calle Cordillera, Suite 106 in the form of a San Clemente, CA 92673 purple fringe to the Moon, the impact of [t] 949.429.6363 the faux 3-D effect that the Vixens cre- [w] www.vixenoptics.com ated overshadowed that small defect.

Eyepieces are interchangeable, and each focuses separately. Observers also can change the inter- pupillary distance, which is the separation between your eyes.

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