Why a binoscope?

Experienced observers claim that observing with a binoscope is superior to observing with a traditional 'mono-telescope that has a comparably large, but only one big mirror.

For instance, the noted astronomy writer and columnist Phil Harrington inevitably ends his columns with "And remember that two eyes are better than one". And he is not alone. I paraphrase some remarks made by observers who are lyrical about observing with a binoscope:
...watching the globular cluster almost made me tumble from the ladder...
...it feels as if I approached the doorstep of the cluster with a spaceship...

Well, this sounds like a revolution in the visual observing of deepsky objects, right? But what facts support these claims? What happens when signals are received separately by two eyes and processed by the brains? And 'how much better' is it to observe with two eyes instead of one? It turns out that there are multiple complex processes that play a role.

1) Improved signal to noise ratio
The most important factor is the better signal to noise ratio that leads to the perception of a darker sky background. What does this mean? A stimulus that is not originating from an astronomical object (for instance 'light noise' from light pollution) could be interpreted by one eye as a bona fide signal. But the chance that such random noise signals hit two eyes simultaneously and reach the brains is very small indeed. In other words, when looking with two eyes, the brains do have to suppress much less background noise created by light pollution. And this automatically translates itself into a darker sky background, even in my light-polluted hometown. This phenomenon is in particular relevant for the observation of very faint galaxy halo's or arms. Within these faint, extended objects many more details become visible and the contrast within the objects is greatly enhanced.

Related to this reduction of the 'signal to noise ratio', binoscopes also provide much 'calmer' views. Smooth is another word that has been used to describe the views in a binoscope. When looking with one eye, the image is distorted by phenomena such as floaters, which results in a restless, scintillating view. True, after a long time at the one eyepiece this view also becomes calmer. This can easily be experienced when you change to the other eye after, for instance, an hour. One will then again have a restless view in the 'fresh' eye. Now, when looking through a binoscope this effect is striking. When closing either the left or right eye one is confronted with these (even different) scintillating images. When opening both eyes this instantaneously disappears and a smooth, silk-like image emerges. Mel Bartels considers this the greatest 'wow' factor of a binoscope and I agree. The effect is overwhelming when looking at faint halo's of galaxies etc. Each time I experience this I'm pleased to be looking through a binoscope.

2) Stereopsis
Stereopsis is the ability see depth. Because of the different positions of the eyes, an object is viewed by each eye from a slightly different angle (parallax). This creates a spatial 3D effect. How closer the object is, the larger the angle and how greater the 3D effect. But unfortunately, astronomical objects are so distant that there is no such thing as parallax there and consequently no 'real' stereopsis. But, there is a related phenomenon called chromatic stereopsis or chromostereopsis. This is caused by the slightly different breaking in the eye lens of for instance red versus blue light. As a consequence red and blue light focus in a slightly different place on the retina. This effect is different for each eye and it therefore appears as if red stars stand a bit closer than blue stars. When looking through a binoscope, chromostereopsis hereby creates an illusion of depth, although this is completely artificial.

3) Enlarged field of view
Mel Bartels often stresses that you can achieve larger fields of view with a binoscope in comparison with a mono-telescope. Indeed, looking through a binoscope, this is a beautiful effect that is immediately obvious. But why is that? If you take the example of my 2 x 13 inch binoscope, with f/5.0 mirrors, the use of two 10 mm Televue Ethosses delivers a 165 x magnification and a 0.61 degree true field. Suppose this binoscope has an approximate equivalent of a 18 inch mono-telescope, also being f/5.0. Now the 10 mm Ethos would deliver a 225 x magnification and a 0.45 degree true field, which is only half of the true field you see with the binoscope. To achieve a 165 x magnification and a 0.61 degree true field, the 18" needs to be f/3.7. And since this creates massive coma, a coma-reducing Paracorr will be needed. As a consequence, the mirror needs to be even f/3.2 for these same magnification and true field (with thanks to the Televue Eyepiece Calculator!). Now consider what will be the costs of such a steep mirror!

There is another aspect here that is rarely addressed. In humans, the horizontal binocular visual field is 120 degrees. But there is an additional 45 degrees monocular field on each side of the binocular field. So, there is a total of a 90 degrees field that is non-binocular, but that is seen by looking with two eyes. By looking with one eye only, this is just a mere 45 degrees of the one eye. And even though you cannot encompass the entire 120 plus 90 degrees at one glance, you will observe it peripherally and it adds to the feeling of being present in the picture. Of course, the choice of eyepieces is important here: with two 50 degree apparent field of view eyepieces, you will just see the field stops. But the effects are very obvious when two Ethos eyepieces with a 100 degree apparent field of view are used.

4) Comfort of two eyes
There is also the 'ordinary' effect of the increased comfort by looking with two eyes instead of with one eye, and another squeezed eye. Sustained and concentrated viewing at faint details with two eyes, without the strain of looking with one eye only, is more pleasant and more relaxing.

5) Binocular summation
Binocular summation is the process by which the brain combines the information that they get through incoming signals in the left and right eye. The summation means that the ability to detect faint objects is enhanced. Put differently, the threshold value for the detection of faint objects is lower with two eyes than with one eye. Statistically there is an advantage for the detection of a weak signal when two detectors are used instead of one detector. This advantage can be deduced to a factor sqrt2, or 1.41, called the binocular summation factor.

In addition, there is also a phenomenon called binocular facilitation. This takes place when the visual cortex receives signals from two eyes simultaneously. In that case the activity of the brain is enhanced more than the sum of both brain activities that are provoked by each one eye separately. Theoretically these phenomena result in the better visual detection of faint objects and should also lead to the better detection of details in such objects.

I place 'binocular summation' as beneficial factor last in this section, and indeed it is probably also the least important factor. The gain in contrast, chemostereopsis, the wider fields of views and the comfort factors probably outweigh the relatively small gain in aperture. Still the binocular summation factor dominates the discussion on the advantages of a binoscope versus a large mono-telescope. I devote an extensive note to this subject (see Binocular Summation Factor).

Binoscopes versus binoviewers
One major reason why binoscopes are so impopular is the availability of a number of very high quality binoviewers. Why bother to make a complex binoscope when you can just stick in a binoviewer in an existing, large mirror mono-Dobsonian telescope? After all, this also provides a binocular view of deepsky objects and is probably also a far cheaper option than a binoscope. But is observing with a binoviewer the same as observing with a binoscope. Let's go over the same points as above.
ad 1) The light signal that comes through a binoviewer is principally one signal, it is just split into two halves. Therefore, the improved signal to noise ratio does not apply to a binoviewer. So the higher level of contrast, obtained with a binoscope, can possibly not be achieved with a binoviewer. Binoviewer enthousiasts argue though that this is maybe true, but that the brains still manages to filter out some noise, which results in a higher contrast than when one eye is used only.
ad 2) I don't know whether chromostereopsis applies for a binoviewer, but maybe it does.
ad 3) Instead of an enlarged field of view, the views with a binoviewer are actually reduced due to the use of a Barlow in the binoviewer. This is considered a negative point of binoviewers, even by binoviewer enthousiasts.
ad 4) A binoviewer also provides the comfort of looking with two eyes. This is probably one of the nicest aspect of a binoviewer.
ad 5) The single light signal is split in two, and the effective aperture of the large mono-telescope is lowered by the use of a binoviewer. As said above, this is, however, not a major drawback of using a binoviewer.

Probably due to a smaller increase in the level of contrast, observers who have compared binoviewers with a binoscope confirm that views with a binoscope are much more 'lifely' than with a binoviewer. Care should be take though. These observers mainly talk about deepsky objects such as galaxies. Most often binoviewers are used on low contrast but bright objects such as planets and the moon. Different brightness/ contrast objects may benefit differentially from binoviewer or binoscope.

In conclusion, there is enough to say in favour of binocular vision versus monocular vision, as I try to argue in this section. In a next section I'll describe some observations that I made with the 2 x 13 inch binoscope, in comparison to my 16 inch mono-telescope.