Agena's Guide to Choosing Astronomy Eyepieces for Binoviewers

By: Brian Ventrudo
April 20, 2017

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Choosing Eyepieces for Binoviewers
Figure 1 - Binoviewer and eyepieces in a refractor telescope. Credit: William Optics.

1. Introduction

“Two eyes are better than one", the old saying goes, but telescopes aren't exactly designed with this in mind. Humans are built for two-eyed viewing, and looking through a telescope with one eye can be surprisingly tiring. That's why many amateur astronomers turn to binoviewers. These devices can provide quite stunning views of the Moon, planets, and brighter deep-sky sights without causing fatigue and eyestrain. But to use a pair of binoviewers, you will need at least one pair of eyepieces. Choosing the best eyepieces for your binoviewers involves a few additional considerations that “one-eyed" observers don't need to think about. This guide will help you sort through the many options so you can select the most suitable eyepieces for your binoviewers, telescopes, and observing interests.

2. A Quick Overview of Binoviewers for Astronomy

Binoviewers are a packaged set of prisms and other optical elements that split the light from the objective lens or mirror of a telescope into two paths and direct each beam to a separate eyepiece. That means each eye gets, at most, only half the light collected by the telescope's objective, so you see dimmer images compared to observing without binoviewers. But the effect is not as pronounced as you might think because the brain more clearly perceives images from both eyes together rather than a single eye. So you may hardly notice the loss in brightness at all unless you are observing faint objects near the limit of your telescope.

Choosing Eyepieces for Binoviewers
Figure 2 - Denkmeier Binotron-27 binoviewer with added Powerswitch (left) and William Optics binoviewer (right). The optical corrector assemblies (OCA) are not shown. Credit: Agena AstroProducts.

There are two main types of binoviewers for astronomy (see Figure 2). Less expensive binoviewers are based on the optical heads of laboratory microscopes that are repackaged for use with a telescope. These ‘entry level' binoviewers tend to use smaller prisms, although the prisms themselves, and their anti-reflection coatings, are often of high quality. But to minimize cost, these binoviewers often use smaller internal apertures to reduce scattered light. The clear aperture at the rear of these binoviewers (nearest the eyepieces) might be about 20mm or 21mm. As you will learn below, this limits your choice of eyepieces, especially for wide-field viewing at low magnifications. Low-cost ‘microscope head' binoviewers are available from many vendors including Orion, Celestron, Stellarvue, and William Optics. They cost a little less than US$300 in North America. Baader also makes a lower-end binoviewer, the MaxBright, in this price range. The binoviewers from William Optics have been particularly well reviewed.

More expensive binoviewers like the Denkmeier Binotron-27 and the Baader Mark V binos were designed from scratch for astronomy. They use larger prisms and clear apertures of up to 26mm or 27mm which allows them to accommodate wide-field, low-power eyepieces. That means they have the potential to offer more expansive views than less expensive binoviewers. Higher-end binoviewers also work better at higher magnifications because they have better mechanical alignment and eyepiece centering, both of which are critical for merging images from each eyepiece into a single image. The performance of many lower-end binoviewers can suffer at higher magnification.

All binoviewers add considerable distance through which the light from the objective lens must travel. This additional length means that, to achieve focus, you need to adjust the focuser to move the eyepieces in a binoviewer closer to the objective of the telescope. In many cases, especially with Newtonians and refractors, this is impossible because the focuser does not have enough in-travel. So most binoviewers come equipped with an additional optical element, called by various manufacturers the optical corrector assembly (OCA), the optical corrector system (OCS), or the glass path corrector (GPC) to allow you to bring your binoviewers to focus. These optical elements might be as simple as a Barlow lens, or they may be a combination of a Barlow-type lens and additional optics to combat coma and other aberrations. An OCA or OCS increases the effective focal length of the telescope's objective and therefore increases the magnification provided by the eyepieces. With smaller binoviewers, the OCA typically has a magnification of 1.6x or 2.0x.

When used with Schmidt-Cassegrain telescopes (SCT), an OCA/OCS/GPC is often not required because the telescope is focused by moving the primary mirror, and the mirror has enough travel to accommodate the additional path length of the binoviewer. Moving the mirror of an SCT, however, changes the effective focal ratio of the telescope to something slower than the nominal f/10. This has the effect of increasing the magnification of eyepieces when used in binoviewers.

Some higher-end binoviewers have an OCA/OCS/GPC with magnification as low as 1.3x. Some also have adjustable assemblies that provide several magnification factors such as 1.3x, 2x, and 3x for refractors and Newtonians, for example, and 0.66x, 1x, and 2x for Schmidt-Cassegrain scopes.

When using high-end binoviewers at higher magnifications, the OCA/OCS/GPC also compensates for the spherical aberration (SA) and chromatic aberration (CA) induced by the prisms in the binoviewers.  These aberrations are usually not noticeable at low magnifications, but at higher magnifications, particularly when viewing planets, they can soften the view without the presence of an OCA/OCS/GPC.  So you will generally get a crisper view of planets in a binoviewer if you use, for example, a 3x OCA with 24mm eyepieces than with 8mm eyepieces and no OCA.

With some binoviewers, you can also use your own Barlow lens in place of an OCA. When doing so, you need to account for the extra light path of the binoviewer prisms to calculate the actual magnification the Barlow will achieve when used.  As an example, a Tele Vue 2x Barlow, when used with the William Optics binoviewer will result in 3.5x magnification.

3. Basic Eyepiece Considerations for Binoviewing

Many of the same specifications of eyepieces for single-eyed viewing are also important for binoviewing. These specifications include:

  • Focal length, which governs the magnification provided by the eyepiece with a particular telescope (magnification is the focal length of the telescope divided by the focal length of the eyepiece).
  • Eye Relief, which is the distance you need to hold your eye from the outer lens of an eyepiece to see its full field of view. Short eye relief means you need to get your eye close to the glass to see the entire field. Eyeglass wearers, in particular, need a longer eye relief of 17-20mm
  • Apparent field of view (AFOV) is the angular diameter of the field of view when you look through the eyepiece itself without a telescope. It can range from 43° for planetary eyepieces to 68°, 82°, and even 120° for wide-field eyepieces
  • Exit Pupil, while not strictly a specification of an eyepiece, is simply the focal length of the eyepiece divided by the focal ratio of the telescope; the exit pupil affects the perceived brightness of the image
  • Barrel Size. Most astronomy eyepieces have 1.25" or 2" barrels, but most astronomy binoviewers use only 1.25" barrels. However, the arguments below apply for 2" eyepieces when used in larger binoviewers that accommodate them.

Perhaps the most important thing to consider is that both eyepieces used in the binoviewer must match, that is, they must have matching optical characteristics including focal length, exit pupil, and apparent field of view. Ideally, both eyepieces should be of the same design and focal length and come from the same manufacturer. The closer each image matches the other, the easier it will be for your brain to “merge" the images into one.

There are three other key considerations and specifications for eyepieces for binoviewers. The first is the weight of each eyepiece. A binoviewer itself weighs 1-2 lbs, so when you add in a couple of eyepieces, the total weight may tax your mount, especially if it's already near capacity. If you use a Dobsonian, you may need extra weights to balance the back end of the telescope. The extra weight of two eyepieces and a binoviewer can also be a strain on a stock telescope focuser. Many serious binoviewer enthusiasts upgrade to a more robust focuser.

Choosing Eyepieces for Binoviewers
Figure 3 – Two 1.25" eyepieces, a Tele Vue 9mm Nagler eyepiece (left) and a Tele Vue 24mm Panoptic eyepiece (right). The field stop of the 24mm eyepiece is effectively the aperture of the eyepiece barrel, about 27mm, while the field stop of the 9mm Nagler eyepiece is much smaller (in this design, it lies within the eyepiece itself). Credit: Agena AstroProducts.

Another important consideration is the maximum diameter of each eyepiece: not the barrel size, but the maximum diameter of the eyepiece body itself. This measurement is of little importance when you use a single eyepiece, but when you have two eyepieces side by side in a binoviewer, there is an upper limit to their diameter. That upper limit is your interpupillary distance (IPD), which is the measure of the separation of your pupils. Your IPD is fixed, and it lies in the range of 57mm to 74mm for adult men and 55mm to 71mm for adult women. Some larger 1.25" eyepieces have a diameter that falls in this range. If the diameter exceeds your IPD, you won't be able to get the eyepieces close enough together to look through both at once, even when the binoviewer eyepiece holders are set to minimum separation. You can estimate your IPD using a ruler while looking in the mirror. When using the binoviewers, you also need a little extra room, over and above your IPD, to fit the bridge of your nose between the eyepieces.

There's one more eyepiece specification to consider: field stop. The field stop is the limiting aperture of the eyepiece (see Figure 3). It sharply defines the edge of the field of view and prevents a gradual decrease in image quality across the field. For moderate to short-focal length eyepieces, the field stop is usually defined by a ring inside the barrel at the entrance of the eyepiece. A Tele Vue DeLite eyepiece with 7mm focal length, for example, has an aperture within the optical train with a field stop of 7.3mm. For wide-angle long-focal-length eyepieces, the field stop has an upper limit defined by the barrel size. In eyepieces with a barrel of 1.25" (31.7mm) outer diameter, the maximum field stop is defined by the inner diameter of the barrel. That's usually about 27mm. For eyepieces with a 2" (50.8mm) barrel, the maximum field stop is about 46mm. To get the widest possible true field of view with a 1.25" eyepiece and a given telescope, you need to choose an eyepiece with the largest possible field stop, something approaching 27mm. If you want a wider field, you must move to a 2" eyepiece.

Why is the field stop important when choosing eyepieces for binoviewers? Let's turn to that question in the next section.

Important Tip: When seating your eyepieces in your binoviewers, you may encounter problems merging the two images. The most common cause is that the eyepieces are not sitting square in the eyepiece holders of the binoviewer. This can often can happen because each eyepiece can tilt a little as you tighten them in the binoviewer using the set screw or compression ring. The undercut or tapered barrels of the eyepieces are often the culprit because the compression ring may engage only a portion of the undercut or it may engage on the slope of the taper. If you notice merging problems, you may need to play with the seating and tightness as you secure the eyepiece in the binoviewer. Just a small adjustment can make a big difference.

4. Eyepieces for Low-Power and Wide-Angle Binoviewing

4.1 Eyepieces for Smaller Binoviewers

Many observers with binoviewers look forward to seeing sweeping two-eyed views of the Milky Way and wide-angle views of the Moon and larger deep-sky objects like the Double Cluster or Orion Nebula. Wide-field observing can, however, be a challenge with binoviewers. This is true because, as mentioned in Section 2, the OCA assemblies required to bring binoviewers to focus result in higher magnifications and smaller true fields of view compared to single-eyed views.

Choosing Eyepieces for Binoviewers
Figure 4 - The clear aperture of binoviewers with 20mm clear aperture (left) and 27mm clear aperture (right). Credit: Agena AstroProducts.

But there's another complication when trying to get wide-angle views, especially for smaller ‘microscope-head' binoviewers - the limited clear aperture. These binoviewers have clear apertures at the back end of the optical train (nearest the eyepieces) of only 20mm or 21mm (see Figure 4). So if you try to use a 1.25" eyepiece with a large 27mm field stop to get a wide field of view, you will see significant reduction of the field of view of the eyepiece. This is called vignetting. It depends somewhat on the distance between the clear aperture of the binoviewer closest to the eyepiece and the field stop of the eyepiece itself. In the worst case, as an example, if a binoviewer has a clear aperture of 20mm right up against the 27mm field stop of the eyepiece, the effective field stop of the eyepiece would be only 20mm. So a lot of the true field of view of the eyepiece would be blocked. The situation is never that extreme, because there is some separation between the two, but as a rule of thumb you don't want to have your eyepiece field stop more than 5-10% larger than the clear aperture of the binoviewer.

So for smaller binoviewers with a clear aperture of 20-21mm, to get the maximum field of view without vignetting, you need to select eyepieces with a field stop less than 20-22mm or so. Wide-angle eyepieces with a 27mm field stop such as the Tele Vue 24mm Panoptic or the Explore Scientific 24mm 68° are overkill for these binoviewers; you can use them, but you won't get the full field of view. The upshot is that smaller field stops of some binoviewers, along with the effect of the OCA, result in a higher magnification and smaller field of view than can be achieved with single eyepiece viewing.


Example 1 – Field of View of a Small Binoviewer with a 10" f/5 Dobsonian and an 8" SCT

Let's do an example. With a 10" (250mm) f/5 Dobsonian and a single 24mm Tele Vue Panoptic eyepiece with a 68° apparent field of view, and with no binoviewer, you can get a magnification of 250 x 5 / 24 = 52x. The true field of view is 68° / 52 = 1.3°, or well over twice the diameter of the full Moon. Because the 24mm Panoptic has a field stop of 27mm, this is about the maximum field of view you can get with a 1.25" eyepiece in this telescope.

Now let's say you want to use a small binoviewer with this telescope to get a wide field. The binoviewer has a clear aperture of 20mm and it comes with a 1.6x OCA to help you bring the binoviewer to focus. So instead of the 24mm Panoptic, you choose the 19mm Panoptic with a 21mm field stop. That gives you a magnification of 250mm x 5 x 1.6 / 19 = 105x and a true field of view of 68° / 105 = 0.65°, half as wide as with single-eyed viewing. That's the maximum field of view you can get with this telescope and binoviewer with a 1.25" eyepiece with a 68° apparent field of view. Or you could choose, for example, a less expensive Tele Vue 25mm Plossl eyepiece, or a similar Plossl, with a 50° AFOV. This gives a magnification in the binoviewers of 250mm x 5 x 1.6 / 25 = 80x and a true field of view of 0.63°. So this eyepiece gives the same field of view as the Panoptic but at a lower magnification.

With an 8" (200mm) f/10 Schmidt-Cassegrain telescope, the same small binoviewers, and the 19mm Panoptic, there's no need for a OCA, but focusing the telescope with the binoviewer increases the focal ratio of the telescope slightly, say, to about f/12. So the magnification comes in at 200 x 12 / 19 = 126x, which gives a true field of view of 68° / 126 = 0.54°, which is about the width of the full Moon. That's the widest view you can get with a 1.25" eyepiece and these small binoviewers with this telescope.


Which longer-focal-length eyepieces have a 20-21mm clear aperture suitable for use in small binoviewers? Not all eyepieces have field stops listed in their specifications, but for eyepieces with a 68°-70° AFOV, this implies sticking to focal lengths under 19mm approximately. For eyepieces with a 60°-65° AFOV, look for focal lengths of 19mm to 21mm, approximately. And for eyepieces with a 50° AFOV, you should stick with focal lengths under 25mm or so. Some good eyepiece options are listed in Table 1, and you can select the best eyepiece for your needs based on price, eye relief, and focal length.

Eyepiece ModelFocal Length(mm)Apparent FOV(°)Link
Tele Vue Panoptic 19 68
GSO SuperView 20 68
Explore Scientific 68-Degree Series 20 68
Agena Enhanced Wide-Angle 20 66
William Optics Wide-Angle* 20 66
Agena Wide-Angle 20 66
BST Flat Field 20 65
Explore Scientific 62-Degree Series 20 62
Tele Vue DeLite 18.2 62
GSO Plossl 25 52
Tele Vue Plossl 25 50
Vixen SLV Lanthanum 25 50
Table 1 – Eyepieces suitable for lowest-power, widest-field use in small binoviewers with a clear aperture of 20-21mm

(*A pair of these eyepieces is included with William Optics binoviewers).

The eyepieces listed in Table 1 have a price of between $40 and $250, approximately. More expensive eyepieces are generally better constructed with higher quality materials. But they also tend to provide better optical performance and sharper images, especially near the edge of the field of view and especially in telescopes faster than f/7 or so. If budget is not a consideration, some excellent eyepieces for wide-field observation with smaller binoviewers with 20-21mm clear aperture are the Tele Vue 19mm Panoptics, the Explore Scientific 20mm Argon-Purged eyepieces with 62° field of view and 68° field of view, the Tele Vue 25mm Plossl and the Vixen SLV 25mm Lanthanum.

As you choose your eyepieces, also keep in mind your preference and restrictions for eye relief and weight. And remember… you need two eyepieces of the same focal length, AFOV, and design.

Choosing Eyepieces for Binoviewers
Figure 5 – The Tele Vue 24mm Panoptic and the Explore Scientific 68-degree-series 24mm eyepieces give maximum field of view in large binoviewers with 27mm clear apertures.

4.2 Eyepieces for Larger Binoviewers

Larger astronomy binoviewers like Binotrons do have a full 27mm clear aperture (or close to it), so they can use eyepieces with a field stop of up to 27mm. So when it comes to larger binoviewers, you can choose the widest-possible field 1.25" eyepieces. These binoviewers are expensive, with a price of 2x to 3x that of smaller devices, but the prices of the eyepieces are more or less the same as for lower-cost binoviewers.

Which 1.25" eyepieces have the maximum possible field stop? The Televue 24mm Panoptic, the Explore Scientific 24mm 68° EP, for example (see Figure 5), both have field stops of approximately 27mm and are popular with experienced binoviewing enthusiasts, as is the Tele Vue 32mm Plossl. Eyepieces with similar AFOV and focal lengths will also have similar field stops. Table 2 gives a summary of some good eyepieces for wide-angle viewing with large binoviewers. Some of these longer-focal-length eyepieces tend to be a little larger, so it's important to make sure you select a pair that are not too wide for your IPD (see Section 3). Some of these eyepieces may vignette slightly, even in large binoviewers, but the effect is not pronounced. 

The Baader Morpheus 17.5mm eyepiece with 76° AFOV and 19mm eye relief is another good choice for binoviewing. While it has a substantially larger AFOV than the Televue Panoptic or the Explore Scientifc 68° eyepieces, the shorter focal length renders a slightly smaller true field of view than those eyepieces. 

Eyepiece ModelFocal Length (mm)Apparent FOV (°)Product Link
Baader Morpheus 17.5 76
Tele Vue Panoptic 24 68
GSO SuperView 20 68
Explore Scientific 68-Degree Series 24 68
Baader Hyperion 24 68
Vixen LVW 22 65
GSO Plossl 32 52
Meade Series 400 Plossl 32 52
Celestron Omni Plossl 32 52
Tele Vue Plossl 32 50
Baader Classic Plossl 32 50
Brandon 32 45
Table 2 – 1.25" eyepieces suitable for lowest-power, widest-field use in larger binoviewers with a clear aperture of 27mm

Example 2 – Field of View of a Large Binoviewer with a 10" f/5 Dobsonian and an 8" SCT

Now to another example with larger binoviewers using the same telescopes as in Example 1, namely a 10" f/5 Dobsonian and an 8" SCT. And let's assume we have a Binotron-27 binoviewer with a clear aperture of 27mm and a Powerswitch (including OCA) set to 1.3x. As in Example 1, with a 10" (250mm) f/5 Dobsonian and a single 24mm Tele Vue Panoptic eyepiece with a 68° apparent field of view, and with no binoviewers, you can get a magnification of 250 x 5 / 24 = 52x. The true field of view is 68° / 52 = 1.3°, or well over twice the diameter of the full Moon. We can stick with a pair of 24mm Panoptics with this binoviewer because of the big field stop. In the binoviewer, these eyepieces give a magnification of 250 x 5 x 1.3 / 24 = 67x and they have a true field of view of 1° compared to the field of 0.65°. That means you see 240% more sky than with the smaller binoviewers and shorter-focal-length eyepiece in Example 1.

NOTE: You should also keep in mind that some eyepieces are designed to work with a specialty Barlow-type lens to perform well. For example, the Panoptic eyepiece is designed to be used with the Tele Vue Powermate amplifier instead of a Barlow. If you use a Panoptic with some OCA/GPC/OCS devices, you may notice a slightly fuzzy field stop, not because of the lack of clear aperture in the binoviewer, but because of the wide-field design of the Panoptic needs a Powermate to work best. The effect may be small and it may vary from one design of binoviewer/OCS combination to another.

4.3 Very-Wide AFOV Eyepieces with Binoviewers

So far we've looked at eyepieces with a 68° AFOV or less. What about eyepieces with a very wide apparent field of view, such as 82° or 100° or more? These eyepieces will work in binoviewers, but there are two reasons they are not favored. For one, the argument of field stop holds. You cannot get a 1.25" eyepiece with a field stop larger than 27mm, which means you cannot get a wider true field of view with these ultra-wide-field eyepieces in this barrel size.

Also, the wide apparent fields provided by these eyepieces tend to be too much for binoviewing because they require you to move your head slightly to take in the full field of view. If you turn your head to optimize your view in one eyepiece, you will lose the view in the other eyepiece. It's not that 82°+ eyepieces don't work in binoviewers, but their wide fields tend to be wasted. Also, the immersive effect of using two eyes with binoviewers gives you the impression of looking at a very wide and pleasing field of view even with eyepieces of 50°-70° AFOV.

5. Other Eyepieces for Binoviewing

5.1 Eyepieces for Moderate to High Magnification

While looking through binoviewers with lowest magnification and widest field is a joy when observing many larger celestial objects, there are some sights such as the Moon and planets, globular clusters, double stars, and planetary nebulae where more magnification is needed. Because shorter-focal-length eyepieces have smaller field stops, smaller than the 20mm clear apertures of small binoviewers, there are fewer restrictions for selecting a pair of eyepieces for this application. If you have a 1.25" eyepiece that you prefer for single-eyed viewing, chances are it will work well in binoviewers if you get a second eyepiece of the same focal length and design from the same manufacturer.

There is only one practical restriction to the upper limit of magnification with most binoviewers. With short-focal-length eyepieces, the alignment of the prisms in the binoviewer becomes critical. Many observers report that it is more difficult to merge images with eyepieces shorter than 9mm or 10mm focal length. But since most binoviewers require a Barlow-like OCA to come to focus, eyepieces with moderate focal lengths can still provide a sufficiently high magnification for most interests.

5.2 Zoom Eyepieces

To get a wide range of magnification with your binoviewers, you may wish to get several pairs of eyepieces. Of course, it doesn't take long before the number of eyepieces in the collection of a binoviewing enthusiast becomes cumbersome. To get four magnifications with a particular OCA, for example, you need eight eyepieces. One solution is to get a good matched pair of zoom eyepieces for your binoviewers to provide multiple magnifications. Several suppliers offer eyepieces with 7-21mm or 8-24mm zoom action that involves rotating the body of the eyepiece until it clicks into place for each of several predetermined focal lengths. These eyepieces work well enough for one-eyed viewing, but some can be frustrating to use in binoviewers. That's because it's essential that each eyepiece “clicks" into place to give exactly the same magnification and field of view. If they don't, most observers find it difficult to merge the two images into a single image without eyestrain. Some zoom eyepieces, which do not use click stops at all, are generally unsuitable for binoviewers.

Illuminated Reticle
Figure 6 - The Baader Hyperion 8-24mm Mark IV zoom eyepiece works well with binoviewers.

Zoom eyepieces with precise and repeatable clickstops, however, do work well in binoviewers. The Baader Hyperion 8-24mm zoom eyepiece has a huge following among binoviewing observers. The newer Mark IV model of this popular eyepiece, shown above in Figure 6, was designed specifically with binoviewers in mind. This ocular provide highly precise and smooth click stops and has excellent optics. It provides focal lengths of 8mm, 12mm, 16mm, 20mm, and 24mm. The apparent field of view ranges from 68° at 8mm to 50° at 24mm. While this eyepiece costs almost $300 each, you only need two to get five distinct magnifications and you save yourself the expense and trouble of amassing multiple pairs of eyepieces.

5.3 3D L-O-A Eyepieces

We're accustomed to looking with two eyes at terrestrial objects in our everyday life, and if an object and its surroundings are close, we can perceive depth in a scene as well as its dimensions. Although celestial objects are too far away for our eyes to perceive three dimensions, when we look with binoviewers at the night sky, we can sometimes perceive that we're seeing objects in 3D. This effect is illusory, but many experienced observers have remarked upon it.

Choosing Eyepieces for Binoviewers
Figure 7 – Lederman Optical Array (L-O-A) 21mm eyepieces for binoviewers. The array is visible in the field lens in the right eyepiece.

The company 3D Astronomy has taken this perception to a new level by designing a pair of eyepieces that induce a very remarkable 3D effect. These eyepiece pairs are based on the Lederman Optical Array (L-O-A). One eyepiece in the pair is a standard high-quality astronomical eyepiece. The other has mounted on its field lens a tiny array of five glass mirrors. This is the L-O-A. When used together in binoviewers, the two eyepieces create an apparent parallax between the images. This gives the perception of six layers of depth within the field of view, somewhat like the effect seen with 3D movie glasses. This renders for deep-sky objects a dramatic 3D appearance with high contrast and sharp detail. Dark lanes and bright knots in nebulae and galaxies appear more prominent. Open stars clusters appear to have depth and texture, and globular star clusters appear as spheres of stars hovering in the field of view. While the 3D perspective is simulated and bears no relation to the actual physical distances between objects in the field of view, the visual effect is real, dramatic, and remarkable.

L-O-A eyepieces come in two versions. One pair has a focal length of 21 mm and an apparent field of view of 65°. A more economical pair has a focal length of 32mm and an apparent field of view of 50°.

Both L-O-A eyepiece sets are designed to work in any kind of astronomy binoviewer, either the smaller microscope-head designs or the larger sets with wider clear apertures. These eyepieces are not intended for use on the Moon or planets.

6. Summing It Up

This guide has gone through the basics of selecting eyepieces for astronomical binoviewers. You've learned about the two main types of binoviewers used in astronomy and how they can influence your choice of eyepieces. You've seen how to determine the best eyepieces for wide-field observation with both types of binoviewers. And you've had a look at specialty eyepieces - zoom and 3D - that make binoviewing more convenient and interesting. Not everyone takes to binoviewing, but if you select a good binoviewer for your telescope, and most importantly, if you choose the correct set of eyepieces for your equipment, interests, and budget, you will find binoviewing a rewarding pastime that will help you maintain your interest in visual astronomy for many years to come.

Note: Agena AstroProducts would like to thank William Paolini, author of Choosing and Using Astronomical Eyepieces, for his suggestions in preparing this article.