Why binoculars are better

Introduction

Richard Berry¹ — a prolific contributor to the ATM literature — described his observing experience with Lee Cain's 17½" binocular telescope at the 1984 Texas Star Party as "...defying description: I discovered that the quality and sheer realism, for lack of a better term, of an image viewed with two eyes greatly exceeds any monocular image I've seen...on closing one eye, I still had a splendid view, but the subtle impression of the nebular glow as grainless, smooth, silky, and fluid, and the faint western extensions of the [Lagoon] nebula, were lost with one eye." (Telescope Making 23, 1984, p. 34).

When attending a Sacramento Valley Astronomical Society² star party I had a chance to see the Whirlpool Galaxy through another member's Fujinon³ 25x150 binoculars. It was my first encounter with giant binoculars. M51's spiral structure was easily visible, but I couldn't detect it later that night using my 6" richest-field telescope, an instrument with the same aperture size and magnification.

Phil Harrington⁴ — author of Touring the Universe through Binoculars — says "...when it comes to viewing the heavens, two eyes are definitely better than one. Our power of resolution and ability to detect faint objects are dramatically improved by using both eyes. In addition, color perception and contrast enhancement also benefit."
 

Inspired by the Fujinon's surprising performance and encouraged by reports that binocular vision enhances nearly every aspect of visual perception for amateur astronomers, I decided to make a large binocular telescope.

I first built a 12½" binocular; it compelled me to build the 22".  The following topics are covered here:

Visual performance — some testimonials from experience amateur astronomers
Why two eyes are better than one — what the brain does with dual visual input
Binoculars and binoviewers — a comparison
 

Visual performance

Views of all kinds of deep space objects and planets through a binocular telescope are breathtaking. Nothing prepared me for the realism, contrast and detail afforded by two eyes. I revisit many Messier and NGC objects, just to see what they really look like — using both eyes.

Experienced amateur observers usually provide interesting feedback after looking through the 22" binoculars. I often attend the Shingletown Star Party, which has been organized by TAC-SAC¹² — amateur astronomers from the Sacramento area. They're a group of dedicated observers bound by a Web site and a listserv, which they use to organize remote observing trips and file observing reports. Here are impressions of three TAC-SAC observers.

Randy Muller, June 26, 2006, Shingletown

The best view I had [of Markarian's chain] was scanning it with Bruce Sayre's amazing 22" binocular telescope. I asked him if I could look at it, and he was surprised it was still up. It was, though it was heading into the Redding light dome. I wanted to look anyway.

At first I had a lot of trouble recognizing the field, but then I realized (with Bruce's help) that the field was mirror-reversed, due to the tertiary mirror (bringing the images from each scope to the eyepieces in the center of the whole assembly). Once I realized that, I adjusted my search pattern and found the mighty ellipticals M84 and M86 immediately.

The effort was worth it! Drenched with photons in both eyes, it was a thrilling experience to see all those galaxies around M84 and M86 so bright and clear. Moving along down Markarian's chain, and then hopping over in a giant arc to M87, passing by the giant Messiers 88, 91, 90 and 89 was breathtaking. It really made me feel like I was sailing in a sea of "Island Universes".

Matt Tarlach, July 13, 2005, Shingletown

Another highlight was M27 in Bruce Sayre's 22" binocular. Awesome!!! I don't think I've ever seen so much color in an object.

Shneor Sherman, December 12, 2004, Fiddletown

I continued on to M31, which was near the zenith. The view was pleasing, and I now inserted the Denks with 30mm Rini eyepieces [into my 22"]. I wanted to see if I could replicate my experience of the previous week, when I viewed M31 through a pair of 22" binoculars and noticed that the globulars in M31 appeared disk-like rather than stellar (only when using both eyes). Unfortunately, I was unable to replicate the view, as a couple of globulars I looked at maintained their stellar appearance.

Randy Muller, June 22, 2004, Shingletown

(Responding to the question, "What was your favorite [view] at SSP this year?")

The Lagoon Nebula, (M8) (in OIII) at around 100x through Bruce Sayre's 22" binocular. I visited Bruce near the end of the evening (actually I kept him up longer than he wanted to be up, but he graciously allowed me to look through his scope), so there was no line, and I was able to adjust precisely the inter-ocular distance and focus of each OTA.

Never have I viewed such bright and overwhelming detail in this object. My brain interpreted the stars as being in the foreground, bright billows and wisps of nebulosity being in the middle-ground and the fainter parts of the nebula as being in the background.

The wide-angle eyepieces, plus the illusion of depth made me feel like I was inside the nebula. As I looked, the whole thing had an almost windswept feel as if I was standing on a hill with fog billowing and blowing around me. But the image was still, so that gave it a peaceful, calm and serene tone, rather than stormy and violent.

The color was that unearthly fluorescent blue-green typical of all the bright emission nebulae, but instead of being present only in the brightest central area, it was suffused throughout the whole nebula, with varying intensity, giving rise to a sense of varying density and thickness.

It was an incredible view that I will never forget.

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Why two eyes are better than one

There is, in fact, a reason for this — it's more than just hype from a community of like-minded Newtonian binocular buffs. While the major benefit of binocular vision is depth perception, or stereopsis, there is another, more subtle benefit called binocular summation. Stereopsis plays no role whatsoever in telescopic observations: it's all about summation.

Binocular summation has been investigated by vision scientists^{5, 6, 11}, and studies have shown that visual perception using two eyes is enhanced over one eye in several ways:

1. Visual acuity
2. Visual detection at threshold levels, including both absolute light detection and contrast sensitivity
3. Brightness perception
4. Flicker perception
5. Form recognition

Experiments show that the binocular detection threshold of dim light is 1.4 times better than for monocular viewing. This has been attributed to two theories, probability summation and neural summation.

Probability summation holds that chances are statistically better using two receptors instead of one. Neural summation hypothesizes that some connection between monocular neural pathways in the brain might produce a physiological "sum" of the images. Studies have shown that binocular sensitivity is well in excess of any gain that would be expected from probability summation, thus supporting the presence of neural summation.

Binocular summation is especially applicable when looking at deep space objects and their features that are at the thresholds of detection. Obviously, the benefits binocular summation are totally dependent on either a binocular telescope or binocular viewer.

I was interested in determining for myself whether binocular summation improved visual acuity. During an appointment with my ophthalmologist, I tested both eyes without my glasses, both independently and then together. My right eye tested 20/40 and my left eye tested 20/30. Using both eyes, however, I tested 20/15 — a definite improvement.

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Binoculars and binoviewers

A binocular viewer — such as the Denkmeier II⁷ — also improves visual observing. Newtonian binoculars and binocular viewers share these characteristics:

1. Natural, comfortable viewing with both eyes
2. A sense of depth (not parallax, and maybe not spacewalk viewing, but definitely spaceship window viewing)

I have no doubt that anyone who invests in a binocular viewer for a large-aperture monocular telescope will definitely get their money's worth. There is a small amount of light loss due to the beam splitter, prisms and other lenses, perhaps a half magnitude, but the benefits of using both eyes outweighs this disadvantage.

I suspect the magic of the brain's signal processing actually improves the monocular view for brighter objects; some argue it helps at all magnitudes. More information on binocular viewers can be found in Sky & Telescope, Vol. 104, No. 3, September 2002, p. 46 and Sky & Telescope, Vol. 109, No. 3, March 2005⁸ and in a Cloudy Nights review⁹.

A large binocular, however, has these advantages over a larger viewer-equipped monocular:

1. No light is lost, as is the case with the viewer's extra optical surfaces and prisms (however well-coated)
2. No Barlow lens — also called a corrector lens — is needed, so there is no image magnification (usually at least 20% in a viewer)
3. A binocular can use 2" focusers, so any pair of long focal length 2" eyepieces can be used, making magnifications below 100x possible — constrained only by the maximum diameter of the eyepiece and the viewer's interpupillary requirements (e.g., with my interpupillary separation I could use a Tele Vue 41mm Panoptic or 31mm Nagler eyepiece, but many could not)
4. A binocular does not constrain clear aperture, which means that large aperture-stop eyepieces suffer no vignetting. Most binocular viewers restrict have clear apertures under 30 mm (the field stop of a BW Optik 30 mm 2" eyepiece is 41 mm).

These factors point to significantly better low-power viewing with a binocular that can exploit modern wide angle eyepieces. I regularly use 30mm, 80° apparent field 2" eyepieces with my 22" binocular, which yield 95x magnification (almost as good as a 31mm Nagler). Although there are 1¼" eyepieces made with focal lengths up to 40 mm or more, most are four-element Plossls and have apparent fields of only 50° or so. While quite acceptable with binoviewers, they lack the spectacular views afforded eyepieces in the 80° wide angle class.

Tele Vue's longest focal length Nagler with a 1¼" barrel is 16mm. This eyepiece, used with a 1.2x optical compensator Denk II would yield 209x magnification on the 22" binocular.

Siebert Optics¹⁰ does sell binocular viewers that support 2" eyepieces and have enough clear aperture (45 mm) so minimum vignetting occurs at the eyepiece's field stop. The Siebert Optics Black Knight Elite 45 2" viewer, used with a 1.5x corrector and a BW Optik 30 mm eyepiece would yield 143x.

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Internet links

1. Richard Berry 
2. Sacramento Valley Astronomical Society (SVAS)
3. Fujinon Specialty Products Div. binoculars
4. Phil Harrington 's
   Touring the Universe through Binoculars home page
5. Online lecture and lab notes from Vision Science III course taught by Dr. Thomas Salmon, Assoc. Prof. of Optometry, Northeastern State University, Tahlequah, OK:
   Lecture 11 - Binocular Summation
6. Online lecture notes from Human Factors in Transportation course taught by Dr. Thomas Schnell, University of Iowa, Ames, IO:
   Advantage of Binocular over Monocular Vision

7. Denkmeier Optical

8. Sky & Telescope

9. Jim Gutman's review of bino viewers:
   Cloudy Nights Binoviewer shootout

10. Siebert Optics' Black Knight Elite 45 2" binoviewer

11. WikipediA has a clear and concise explanation of binocular vision and summation

12. TAC-SAC - The Astronomy Connection - Sacramento