A
lot of ink has already been spent on this subject since many
astronomy enthusiasts are wondering what the actual gain is observing
with both eyes instead of only one. Let me begin by saying that this
whole discussion is fairly pointless because observing with both eyes
is a completely different experience than observing with only one.
The feeling of total immersion that not even a 150° eyepiece can
ever offer, the strange 3D-effect, the joy and relaxation of using
both eyes… Personally, even if there were no light gathering gain
at all I’d opt for a binoscope, regardless the expense. On the
other hand there are people who’re having difficulties observing
with both eyes. And finally there’s the big unknown factor: the
human brain, which is both unpredictable and personal.
So
what’s the use of me writing this article? Because we astronomy
enthusiasts have the unstoppable need to quantify everything. How
much more can you see with a 14” telescope compared to a 10”? How
does a refractor compare to a Newtonian (please, no, not again…)?
Or… how much more can you see with both eyes? So here I go…
explaining my 2 cents on this, for what they’re worth.
The misconception
The misconception
Unfortunately
very little real scientific study, if any at all, has been carried
out to shed some light here. That is to say, a study that has been
performed from an astronomical and not a medical point of view. Back
in 1965, Campbell and Green published their renowned study in which
they concluded that detection improves by a factor of 1.41 (√2)
when using both eyes instead of only one. This figure has since also
been adopted by part of the astronomical community. A factor of 1.41,
or an actual detection gain of 41%, implies a telescope diameter gain
of 19%. In other words, an 18" binoscope would be more or less
equal to a 21" monocular telescope. Given that it is generally
considered that you need at least a 30% increase in aperture (or 14%
in diameter) to get any visible difference at the eyepiece, the real
detection gain between an 18" mono or an 18" bino would
be marginal and hence not worth the expense. Compare it to a C8 vs. a C9.25. The awe that one
experiences when observing with a binoscope would therefore merely be
an illusion, created by the larger perceived field of view and
the beguiling 3D-effect.
But
is this really so?
After
having observed with a big binoscope for 2 years and a half and
having done direct comparisons between mono and bino several times
during every observing night (a binoscope is put in focus one
eyepiece at a time), I daresay that all of this is pure nonsense.
First
of all, the Campbell and Green study was a medical study, not an
astronomical one, and it was intended for "normal" eye use.
Citing it for binocular detection under astronomical conditions is
therefore absurd, to say the least. Pirenne already concluded in 1949
that there is no such thing as a single binocular summation factor,
but that binocular summation greatly increases with deteriorating
conditions. The darker it gets, the better you see with both eyes.
I’ve
also read an article of someone who claims to have performed a
limiting magnitude test with a pair of binoculars and found an actual
detection gain of 0.2 magnitudes. This is , if you ask me, a very
dubious statement because there is no clarification as regards to the
method, nor the reference stars used. An 18” telescope has a
theoretical magnitude limit of 17. A 25” (twice the aperture) has a
limit of 17.7. Therefore, if you’re going to do such a test with an
18” binoscope, you need a field of view which contains stars of
mag. 17, 17.1, 17.2 and so on. You also need a perfect sky with a
perfect atmosphere. When sky conditions are less than perfect, or
worse, average, the limiting magnitude difference diminishes somewhat
due to a brighter background and hence reduced contrast. So what is
the real difference between
an 18” and a 25” under a certain sky? You’d have to put both
telescopes next to each other in order to find out and point to a
star field which contains the right reference stars with the right
magnitudes. And even then, we all know how tricky limiting
magnitude tests can be with the naked eye, let alone with a
telescope, or two scopes with a different exit pupil.
What
I can already state with great certainty, is that when closing one
eye many of the fainter stars simply disappear, or become very hard
to see. I’ll also give an example in my observation of M76 (see
below).
What
does real science tell us?
-
The extent of summation depends on stimulus contrast and duration
(Bearse and Freeman, 1994)
-
There is significant summation at low contrast (Banton and Levi,
1991)
-
At low contrast, the level of summation is greater than could be
expected by probability summation alone (Simmons and Kingdom, 1988)
-
Summation depends on the complexity of the task, with simple tasks
(detection) displaying far greater summation than complicated ones
(pattern recognition) (Frisen and Lindblom, 1988)
Here’s
the link to the article where I got this information and which also
contains the links to the various studies I mentioned:
When
you read them more carefully, you come to the conclusion that there’s
still a great deal of uncertainty about the extent of binocular
summation. Still, these studies lead
us to believe that the binocular summation factor in the total dark
(greatly) exceeds the 1.41 factor under "normal"
conditions.
What
does experience in the field tell us?
Drop
a binoscope at a star party and see where all the people are
flocking.
Seriously
though. I've made a few direct comparisons between my 18"
binoscope and big monoscopes. On the picture below you can see the
18" bino with in the background two high-quality 20"
scopes, one of which was an f/5, coincidentally the same focal ratio
as the bino. According to the 1.41 theory, these scopes should yield
more or less similar performances. In reality, however, the 20"
were no match at all for the binoscope, which brought out details in
faint objects which you could only dream of in the monocular
telescopes. This was confirmed by all present (about 20 people - all
experienced observers).
I've
also had the opportunity to compare the 18" bino to a 27"
mono and this turned out to be a clash between more or less equals in
terms of light gathering power and the perception of detail.
Elaborating on this experience would be inappropriate, however,
because there were no neutral observers present.
I’d
also like to add some notes about a very intensive comparison I've made
on various objects, observing with both and with only one eye. These
observations were done by me,
with my eyes and with
my sky conditions. I
do not wish to generalise these results in any way but suppose that
they give an indication
of what one might expect.
Overall,
a nice way of putting it is that with binocular vision I can easily
see with direct vision what I can only perceive with averted vision
in mono. Closing one eye,
not only a nebulous object becomes fainter and loses detail, but
several faint stars disappear as well or become hard to see.
-
NGC246 (104x with OIII filters): In mono the nebula shows an opening towards the south-east. With both eyes, the nebula not only becomes significantly brighter, but the opening fills with nebulosity, making the overall aspect of the nebula decisively round.
-
NGC7009 (285x without filters): In mono the ansae are faintly visible. In bino they become evident and even show hint of detail. The internal structures of the nebula look much more pronounced and the overall blue-greenish hue becomes striking. A noteworthy difference.
-
NGC604 (285x without filters): In mono there were maybe three or four stars within the nebula that could be identified with absolute certainty. In bino an entire cluster appeared.
-
NGC1491 (104x with OIII filters): The difference was not that large, in a sense that what was there in bino was also there in mono, albeit fainter and with averted vision.
-
M76 (285x without filters): Here’s an interesting observation. In bino I noticed a tiny little star in the northern arch of the eastern wing, next to the more obvious mag. 14 star. It was there and easily visible, also with direct vision. When I closed one eye it simply disappeared. Well, I still thought I saw it but am pretty sure that this was an illusion because I had just seen it with both eyes open. After taking some time with one eye closed, looking around and then trying to spot it again, I couldn’t see it. Also the mag. 14 star was difficult with one eye, by the way.Coordinates: 01423079 +5134386, mag. 15.264
I'd
like to present this sketch which is a direct comparison between mono
and bino of planetary nebula NGC1501, just to give you an idea:
Conclusion?
Over
time I've grown to the conviction that detection with a binoscope
increases by a factor nearing 2, or an aperture increase of 41%.
Hence an 18" bino would have the light gathering power of a 25"
mono. It's difficult to put a real number on it because the binoscope
offers vastly enhanced contrast, whereas the bigger monocular scope
offers better resolution (but suffers more from bad seeing). It may
even be possible that this factor increases somewhat beyond a
factor of 2 because, when observing at the limit as we astronomers
do, it is far less likely that a "false" light signal is
accepted as "true" by both eyes simultaneously.
This
figure should not be taken as an absolute. I’m not a scientist, nor
do I wish to present my findings as a general guideline. It’s just
an impression after years of experience with making direct
comparisons and hearing comments from people who’ve actually
observed with binoscopes.
Surface
brightness
It
may be that a binoscope brings objects with an extremely low surface
brightness within reach, which remain impossible with any telescope,
not even a 30”. An 18” binoscope offers the theoretical
light gathering power of a 25” mono but with the exit pupil of an
18”. This means that with an 18” binoscope you can go as low as
65x and still retain a 7mm exit pupil. With a 30” scope, you’re
limited at 108x. Therefore it may be possible that very large and
extremely faint objects are visible in the 18” bino and not even in
a 30”! With my bino I’ve observed planetary nebula
Purgathofer-Weinberger 1, under an SQM20.9 sky. I’m not sure
whether this would be possible with monocular telescopes… It’s
probably an interesting point for discussion.
Making
binoscopes… or not?
But
if all of this is true, why aren't there more big binoscopes around?
Why is there only one manufacturer left (for as far as I know
and apart from a few home-builders) that offers them commercially?
The answer's very simple: it's not about cost as such because, even
though the expense for a quality bino's significant, my binoscope
cost me less than a 25" Obsession. The real problem is that a
big binoscope's awefully difficult to make. Alignment needs to be
precise up to a nanometer and stability needs to be such that both
tubes remain absolutely motionless regards to one another, no matter
how you move the scope. Every single binoscope is a new adventure in
which you're never sure which problems may arise. Even with all of
his experience, Mr. Otte spent 15 months building mine whereas he had
stated 3-4 months initially. Furthermore, big binoscopes are
incredibly bulky (mine's 145cm wide and weighs in at about 150kg) and
are impossible to assemble by yourself, whereas you can easily assemble a
25" Dob on your own. I've given up on hauling my bino to star
parties because it's simply too much of a hassle.
In
brief, a (big) binoscope is commercially unfeasible.
Disclaimer
I
have no commercial interests (actually, Mr. Otte has recently
retired) and I live happily on my hilltop under the stars. Therefore
I have no reason to boast the results because I couldn't care less
about what other people think. These are simply my impressions and
thoughts, with my scope and eyes. Hopefully they may be useful to
others.
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