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A Review of the Vixen 2.1x42 Super Wide Constellation and Kasai Trading 2.3x40 Wide Field Binoculars

By: Brian Ventrudo
May 31, 2018

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Review of the Vixen 2.1x42 Super Wide Constellation and Kasai Trading 2.3x40 Wide Field Binoculars
Figure 1 - Vixen 2.1x42 Super Wide Constellation Binoculars (left) and Kasai Trading 2.3x40 Wide Field Binoculars (right). Image Credit: Vixen Optics and Kasai Trading

1. Overview

Even experienced stargazers enjoy stepping away from the telescope occasionally to take in the night sky without optics. The wide field of view available with our eyes, especially in very dark locations where the Milky Way is clearly visible,offers a very different experience than looking at the narrow field of view of a telescope, or even a standard pair of binoculars.

But the eye can bring in only so much light, and many celestial sights and features remain just out of sight to the unaided eye, even in the darkest sky. To bridge the gap between purely visual observing and seeing the sky with standard binoculars or a telescope, a few manufacturers have developed small, low power (2-3x), wide-field binoculars that can pull in fainter stars while still showing entire large constellations. These somewhat quirky binoculars are a specialty item, to be sure, and not intended for use as a primary instrument, or even as a primary pair of binoculars. Once you get used to their operation and limitations, however, they can be a lot of fun and help you see the sky with what some observers describe as 'super vision'.

There are several of these wide-field binoculars on the market, but two of the most popular and commonly available are the Vixen 2.1x42 Super Wide Constellation Binoculars and Kasai Trading 2.3x40 Wide Field Binoculars (made in 2016 and later). This article takes a side-by-side look at these devices to help you figure out which one might be right for your budget and interests.

2. Optical Background

To understand the performance of these binoculars, it helps to review a bit of optics.

Vixen Kasai Review
Figure 2 - The optical path of a Galilean telescope (credit: Museo Galileo)

The Vixen and Kasai wide-field binoculars are based on Galileo's telescope design of more than 400 years ago, a design that used a convex objective lens to collect light and a concave eyepiece placed before the focal point of the objective. This 'Galilean' design produced an upright image, which was useful for terrestrial observation. But there were many disadvantages. The field of view is 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.

Vixen Kasai Review
Figure 3 - The optical path of a Keplerian telescope (credit: Museo Galileo)

These days, only inexpensive and low-power binoculars, so-called 'opera glasses', use the Galilean design. Nearly all modern astronomy telescopes are based on the 'Keplerian' design, a scheme named after the great Johannes Kepler who developed 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.

The Vixen and Kasai wide-field binoculars are based on the Galilean design, but they include additional lens elements in the objective and eyepiece to reduce aberrations and increase the field of view. They are essentially 'super' opera glasses that allow an observer to get very wide-angle views of the night skyat very low magnification. These Galilean binoculars have a few practical differences from the standard Keplerian telescope design that most amateur astronomers are familiar with. Here are the most important:

Non-Inverted Image. In contrast to the Keplerian design, the light from the objective lenses is not brought to an intermediate focus, which means the image at the eyepiece is upright. While an upright image is not normally an advantage for most astronomical applications, it's useful for exploring large constellations because they appear just as they do with your unaided eye. You can simply see more stars.

Light-collecting Ability. In a Galilean optical system, the light collecting ability scales as the square of the magnification. So the 2.1x Vixen binoculars, for example, collect (2.1)2= 4.41x as much light as the human eye. Allowing for 5% transmission loss and making the logarithmic conversion to magnitude, these binoculars provide a boost of about 1.5 magnitudes. So if you can see magnitude 6.0 stars with your eyes, you can see magnitude 7.5 stars with the Vixens. The Kasai binoculars, with a slightly higher magnification of 2.3x, should theoretically render images about 20% brighter than the Vixens.

Field of View. In a Galilean system, the field of view is directly proportional to the aperture of the objective lenses. That's why these devices have relatively large lenses for their modest magnifications.

Exit pupil and Eye Position. In a Keplerian telescope, the exit pupil is the aperture of the objective divided by the magnification. So for a 2.1x42 optic, you might think the exit pupil is an enormous 20mm, some three times the maximum exit pupil of a dark-adapted human eye. But in a Galilean telescope, the exit pupil is virtual and located inside the optical train. There is no fixed position to place your eye to get the maximum field of view. The closer you get your eye, the wider the field of view you can see. Those who must wear glasses while observing will not be able to get their eye close enough to get the maximum possible field of view.

Vixen Kasai Review
Figure 4 – A schematic of the optical layout of the Kasai Trading 2.3x40 binoculars (credit: Kasai Trading)

3. Specifications and Accessories

Table 1 summarizes the particulars of the Kasai and Vixen wide-field binoculars. On paper, both binoculars are similar. The Vixens have a slightly larger objective size, which implies a slightly wider field of view. The Kasais have a slightly higher magnification, which implies they produce slightly brighter star images.

The apparent field of view of the Vixens and Kasai binoculars are specified as 25° and 28o respectively, although these numbers should be regarded as approximations only. The apparent field of view varies strongly with the observer's eye position. The numbers in Table 1 are estimates of what an experienced observer can achieve with the eye as close as possible to the eyepiece lens and without eyeglasses. With the eye positioned 8.4mm from the eyepiece lens, the field of view is approximately 12° with both instruments.

Both binoculars come with lens covers and a carrying case with belt loop. The Kasais include a small microfiber cloth for cleaning the housing and a neck strap for the binoculars themselves, and an additional strap for the carrying case. The Vixens include a carrying strap for the binoculars themselves but not for the carrying case, and they come with a set of simple instructions.

Both units are approximately the same size, although the Vixens are about twice the weight of the Kasais.

Table 1: List of Cameras Currently Offered by ZWO
SpecificationVixen 2.1x42 Super Wide Constellation BinocularsKasai Trading 2.3x40 Wide Field Binoculars
Objective lens diameter (mm) 42 40
Magnification 2.1x 2.3x
Max. Apparent Field of View* 25° 28°
Coatings Fully multi-coated Fully multi-coated
Eye Relief** 8.4 8.4
Interpupillary Distance 53.34mm - 76.2mm 52mm - 73mm
Objective Lens Cover Yes Yes
Eyepiece Lens Cover Yes Yes
Carrying Strap No Neck Strap
Case Yes, with belt loop Yes, with belt loop and strap
Measured Dimensions (LxWxD in mm) 127 x 55 x 45, approx. 123 x 50 x 45, approx.
Weight (g) 450 (approx.) 255

* Nominal, with the eye at the minimum distance from the eyepiece lens.
** This eye relief is chosen as a representative observing distance for observers with eyeglasses.

Review of the Vixen 2.1x42 Super Wide Constellation and Kasai Trading 2.3x40 Wide Field Binoculars
Figure 5 – A side view of the Kasai (left) and Vixen (right) binoculars.

The inside of the Vixen objective holders are threaded, but they are not a standard M48x0.75 thread for astronomy filters, so there is no means to attach such filters.

The Kasai binoculars have optional M48 filter adapters so you can attach OIII, UHC, or even H-beta filters for observing large emission nebulae. There's also an optional set of goggles that will hold the Kasai binoculars so you can wear them like glasses for fatigue-free observing.

The most striking difference between the two binoculars is the price. The Vixen 2.1x42 Super Wide Constellation binoculars have a regular price of $299, while the Kasai Trading 2.3x40 Wide Field Binoculars have a regular price of $165 (both prices as of early 2018).

Why such a difference? Are the Vixen binoculars worth a premium price? The next two sections compare and contrast the fit, finish, and performance of both instruments to help you answer this question for yourself.

4. Side-by-Side Comparison: Mechanics

Both the Kasai 2.3x40 and Vixen 2.1x42 wide-field binoculars do indeed have the size and appearance of opera glasses, although they clearly have a premium build quality. They are both small and relatively shallow compared to standard binoculars, but both have large and hefty optical elements and mechanics. The all-metal construction of the bodies in these binoculars makes for robust instruments, but both will feel uncomfortably cold in bare hands on chilly nights.

Both instruments feature a metal frame, barrels, and eyepiece lens housings. The Kasai have dual arms that extend from a central pivot out to the eyepiece holders and objective holders. The Vixens have a single thick arm that holds a uniform cylindrical barrel near its midpoint. They are simple to operate: each has a pivot for adjusting the interpupillary distance (IPD) and for independently adjusting the focus of each barrel. Adjustment of the IPD is smooth in both instruments and the barrels hold their position solidly during use.

Review of the Vixen 2.1x42 Super Wide Constellation and Kasai Trading 2.3x40 Wide Field Binoculars
Figure 6 – The barrels and eyepiece lens of the Kasai 2.3x40 binoculars (top) and the Vixen 2.1x42 binoculars (bottom)

In both instruments, each eyepiece is focused separately by rotating the eyepiece lens holder. Both instruments have good mechanics in this regard. The Kasais are a little stiffer to turn but still smooth, while the Vixen focusing mechanisms are easier to turn and buttery smooth.

The Kasai binoculars also have a narrower opening for the eyepiece lens compared to the larger opening for the Vixens. This is likely because the curvature of the eyepiece lens is quite significant, so an eyepiece aperture is necessary to help the observer place the eyes in the optimum position. The reduced eyepiece opening in the Kasais was neither a help or hindrance in observing: the full field of view was still accessible.

When it comes to mechanics and overall physical configuration, the Vixens have two advantages over the Kasai. For one, they have a higher-quality feel and heft. Their extra weight is a result of their more solid construction and thicker metal housing (there may also be additional glass as well, though this is not known for sure).

More importantly, the Vixen binoculars are easier to hold securely while observing. That's because they have a deep and uniform cylindrical shape compared to the Kasai, which makes it easy to wrap two fingers and a thumb around each barrel. Holding the Kasai is not difficult, just a little more tenuous compared to the Vixens. For both instruments, it's a good idea to use the neck straps as a safeguard in case they slip out of your hand, especially if you're using gloves.

5. Side-by-Side Comparison: Optical Performance and Field Test

Field testing of the Vixen 2.1x42 Super Wide Constellation and Kasai Trading 2.3x40 Wide Field Binoculars took place in mid-March in a yellow zone in rural Virginia, about an hour west of Washington, D.C. on a moonless night. Outdoor temperatures during field testing was about 40°F and sky transparency was about 4/5. In the darker parts of the sky, away from the urban light dome, stars of magnitude 5 to 5.5 were easily visible. These instruments are ideal for sweeping the summer Milky Way, but at the time of testing, the richest section of the Milky Way in Scorpius and Sagittarius was not visible. But there were plenty of good star fields around Orion, Gemini, Cassiopeia, and Perseus to do a side-by-side comparison of both instruments.

5.1 How to Observe with These Instruments

First, a word about observing technique. Observing with these wide-field binoculars requires a slightly different approach than observing with standard binoculars or a telescope and eyepiece. In a wide field eyepiece and a telescope, for example, observers can move their eye all around the apparent field, as if they are looking out the window of a spaceship, and still see sharp star images to the edge. But in these wide-field Galilean binoculars, the off-axis aberrations areenormous compared to most other astronomical systems, especially when you look more than about 10-12° off axis. If you move your direct vision to the edge of the field, star images are far from sharp. However, you can see the edge of the field with your peripheral vision and so enjoy the effect of the wide field, and the aberrations are not as bothersome because your eyes do not see as much detail off axis (for the same reason you cannot read using your peripheral vision).

The upshot is this: when you observe with these binoculars, you should keep your direct vision at or near the center of the field at all times. If you want to see something interesting near the edge of the field, keep your eyes fixed at center and move your head and the binoculars together to center the object of interest in the field of view.

When you raise these binoculars to your eyes, there is indeed the sense that you are seeing the sky with enhanced vision. Constellations outlined by fainter 4th and 5th magnitude stars, like Eridanus for example, pop into view, the V-shape of the head of Taurus takes on a Z-shape as another linear branch of the Hyades star cluster becomes obvious. You may be inclined to grab a star map that goes down to magnitude 7 or 7.5 and start learning a few new stars and asterisms as you examine the sky.

Both these binoculars also have one built-in advantage. Their low magnification means that hand or arm shake is hardly a problem as it is with 7x or 10x binoculars. As long as your arms don't tire, you get a fairly steady view.

5.2 Apparent Field of View

To get an estimate of this parameter for each instrument, pairs of bright stars were selected, and each placed at opposite ends of the field of view with the observer's eye as close as physically possible to the eyepiece glass. With a little effort, Rigel and Sirius fit into the apparent field with a little room to spare, which suggests a field of view of about 25° in both instruments. The entire constellation of Orion fit nicely into the field.

When trying to maximize your field, be prepared to get your eyes as close as possible to the eyepiece lens, and be further prepared to contaminate the lens with oil from your eyelashes. Frequent cleaning is required. On cold nights, there is also the risk of fogging the eyepieces with the warm moisture from the observer's eyes and face. This is especially true with the Vixen's eyepieces because they have a larger exposed glass surface compared to the Kasai.

5.3 Image Brightness and Quality On and Off Axis

Neither instrument gave the type of tack-sharp on-axis image that might be expected in a good pair of Keplerian binoculars or a telescope. But when observing the brighter stars of Orion, as well as Sirius, in the center of the field of view, it became clear the Vixens gave slightly sharper images at optimum focus. They also gave slightly richer rendering of color in the stars Betelgeuse and Rigel than the Kasai. At this magnification, no significant chromatic aberration was observed in either instrument.

When observing off-axis, it was clear the Vixen binoculars have a much larger degree of aberration at the extreme edge of the field of view compared to the Kasais. It wasn't a problem or distraction because this part of the field was observed with peripheral vision only, but it was curious to see this difference. Eye position had some influence of this effect.

While inspecting Collinder 70, the collection of stars that surround and encompass Orion's Belt, it became clear the 2.3x Kasai binoculars showed slightly more and fainter stars than the 2.1x Vixens. Moving over to Melotte 20, the star cluster around Mirfak in Perseus, confirmed this observation. The effect was not pronounced, but it was obvious. This is not a surprise because the light gathering ability scales as the square of the magnification.

While the Kasais shows slightly more stars, the Vixens excelled in providing better contrast, darker background, and better overall image quality across the center of the field of view. The '3D' perception that a good pair of binoculars provides to an observer was much more evident in the Vixens.

Review of the Vixen 2.1x42 Super Wide Constellation and Kasai Trading 2.3x40 Wide Field Binoculars
Figure 8 – Sky conditions during testing of the Vixen 2.1x42 and Kasai Trading 2.3x40 binoculars. These instruments are well suited for viewing regions around Orion's Belt and Sword, the Pleiades and Hyades star clusters and the Milky Way through Cassiopeia and Perseus. Image credit: Brian Ventrudo

5.4 Deep-Sky Observing Impressions

Examining stars in the bowl of the Big Dipper and comparing them to star maps did confirm a roughly 1.5 magnitude extension over naked-eye observing for both instruments. Under the test conditions, stars of magnitude 7 were easily seen throughout the sky, which gives the remarkable impression of seeing the sky anew. The familiar constellations were there, but the normally dark patches of sky were filled with many hundreds of stars which made for an entirely pleasing experience, especially when viewing already star-rich constellations such as Cassiopeia, Perseus, Orion, Canis Major, Auriga, and the lower half of Gemini.

What did deep-sky objects look like with these instruments?

With so little magnification and relatively modest light-collecting ability, the Vixen and Kasai binoculars did not provide viewsavailable with 7x50 or 8x42 binoculars, for example, and these little instruments are better suited for taking in entire areas of the sky rather than getting close-ups of particular objects, with the exception of large open clusters. The Pleiades and Hyades, for example, were spectacular in these binoculars, and the stars of Collinder 285, which makes up much of the Big Dipper, were also impressive. Following the band of the winter Milky Way through Perseus and Auriga wasa splendid experience, one that suggests these instruments are well suited to cruising the much richer summer Milky Way.

The Orion Nebula looked a little brighter with these binoculars, but it was only slightly enlarged and little of its bat-wing shape was evident. The rest of the prominent stars in the Sword of Orion were well resolved. The Beehive (M44) was easily visible and gave some indication of granularity in these little Galilean binoculars. Same story for the superb Double Cluster in Perseus.

Smaller clusters such as M35 in Gemini and M41 in Canis Major were easily visible as unresolved smudges. Ursa Major was situated towards a patch of urban light pollution, and the galaxies M81 and M82 were not visible in either instrument. But it would be worth trying to see this pair in dark and pristine sky, along with brighter galaxies like M31 and M33.

6. Summary

The Vixen 2.1x42 and Kasai Trading 2.3x40 binoculars bridge the gap between purely visual observing and seeing the with standard binoculars or a telescope. They offer about 1.5 magnitudes of brightness over the naked eye while still allowing most observers to take in entire large constellations such as Orion, Cygnus, and Ursa Major. Because of their relatively simple Galilean design, these instruments have significant aberrations, especially near the edge of their fields of view, but the central 10-12 degrees offers sufficiently sharp images for pleasant viewing. These binoculars are not intended for use as a primary instrument, or even as a primary pair of binoculars. But once you get used to their operation and limitations, they can be a lot of fun and help you see the sky with an 'enhanced vision'.

Vixen 2.1x42 Super Wide Constellation Binoculars

  • Excellent build quality fit, and finish
  • Relatively easy to hold
  • Excellent color rendition and on-axis image quality
  • Very good image contrast and '3D' effect


  • Large off-axis aberrations at the edge of the field
  • Large eyepiece lenses susceptible to fogging
  • Relatively expensive

Kasai Trading 2.3x40 Wide Field Binoculars (made in 2016 and later)

  • Shows slightly more stars compared to Vixens
  • Adaptable to use with nebula filters and other accessories
  • Relatively affordable
  • Easy to position eye in correct location


  • Slightly less image contrast
  • Somewhat 'flat' image compared to Vixens
  • Barrel size and shape makes them harder to hold securely

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