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For many people, one of the
most important considerations when choosing binoculars is the price. So what
is the difference between a pair of binoculars costing, say, £50,
and a pair with the same designation costing £100 or even £1000 ?
Generally speaking, you get what you pay for, and the more expensive
binoculars will be made from higher quality materials, giving higher quality
images – sharper and brighter.
Is the ability to see in a dark environment.
Whether by biological or technological means, night vision is made possible by
a combination of two approaches: enhanced spectral range, and enhanced
intensity range. Image intensifiers were invented by Vladimir Zworykin, an
employee of U.S. company RCA during World War II. His work and creation of the
first generation 0 device became the basis for the sniperscope and
snooperscope. Parallel development in Germany occurred by AEG in 1936,
producing a prototype for the Pak anti-tank gun in 1939, which were later
mounted on panzer tanks, and the 'Vampir' man-portable system for infantry
with MP44 rifles.
The image intensifier is a vacuum-tube based device that
converts visible light from an image so that a dimly lit scene can be viewed
by a camera or the naked eye. While many believe the light is 'amplified,' it
is not. When IR light strikes a charged photocathode plate electrons are
emitted through a vacuum tube that strike the microchannel plate that cause
the image screen to illuminate with a picture in the same pattern as the IR
light that strikes the photocathode, and is on a frequency that the human eye
can see. This is much like a CRT television, but instead of colour guns the
photocathode does the emitting. The image is said to become 'intensified'
because the output visible light is brighter than the incoming IR light, and
this effect directly relates to the difference in passive and active night
vision goggles. Amplification of light is the result of LASERs (Light
Amplification through Stimulated Emission of Radiation). Lasers are quite
intense and can destroy an imagetube in a fraction of a second. Currently, the
most popular image intensifier is the drop-in ANVIS module.
Night vision goggles refer to an IR or thermal night vision
device with dual eyepieces; the device can utilize either one intensifier tube
with the same image sent to both eyes, or binocular eyepieces with an image
intensifier tube for each eye. This is opposed to a monocular night vision
device which uses only one eyepiece, such as a weapon mounted sight glass.
Binocular night vision has the advantage of individually focusing the image to
each eye to maintain depth perception, which is not possible with monocular
night vision.
An image intensifier is a device that amplifies
visible and near-infrared light from an image so that a dimly lit scene can be
viewed by a camera or by eye. Unlike a thermographic camera, an image
intensifier does not work in the total absence of visible (or near infra-red)
light. It does, however, create a more realistic image, because the
intensities it shows are related to true optical intensity and not to
temperature. This realism makes it more suitable for use by untrained
operators and can be used to view objects not visible by a difference in
temperature alone. Image intensifiers are also much less expensive than
thermal imaging.
HOW IT WORKS
Image intensifiers work by having an objective lens focusing an image into a
vacuum tube with a photocathode at one end that releases electrons by the
photoelectric effect on the incidence of incoming photons. From there, the
photoelectron is accelerated through around 5000 volts into a tilted
microchannel plate. The high energy electron releases some micro channel plate
electrons, which further release other electrons, in a process called
secondary cascaded emission. The MCP is tilted to encourage more electron
collisions, thus increasing the rate of emission of secondary electrons.
The electrons all move together due to the potential difference across the
tube, and where one or two electrons entered, thousands may emerge. A separate
(lower) charge differential in the tube accelerates the secondary electrons
until they hit a phosphor screen at the other end, releasing a photon for
every electron. Provided the electric field is uniform, the electrons have a
linear path, so correspond to the exact incident image. The image is focused
by conventional optics using an ocular lens. The only multiplicative stage is
in the secondary cascaded emission. The phosphor is usually green, as the
human eye
is more sensitive to green than other colours (hence the soldiers'
nickname 'green TV' for image intensification devices).
UNDERSTANDING
BINOCULAR TERMINOLOGY
A binocular consists of two optical systems that are joined by a hinge and
(typically) share a common focusing mechanism. The
ability to create an image for both eyes simultaneously provides a realistic
perception of depth. Binoculars are available in a great variety of sizes,
magnifying powers and features to suit any purpose or preference.
The best binocular technology has always been developed and used by the
military. With the end of the cold war, much of
the top binocular technology is now available in the market place at falling
prices. High power magnification scopes and micro
binoculars once the domain of the military establishment and spy agencies can
now be obtained. As much better binoculars at less costly pricing is in
the marketplace, it's important for you to understand the optic technology
behind this professional surveillance gear and understand the difference
between cheap low-grade optics and the optics that you want for professional
use.
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Monocular: A monocular is a one piece viewing
scope whereas a binocular is a two piece viewing scope for both eyes. The
monocular
certainly has advantages in covert surveillance in that they can be stored
in pants pockets and easily palmed in your hand for quick covert viewing.
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Binoculars: Binoculars are identified by simple
code. It is usually two numbers separated by an 'X', for example, 10X30. The
first number represents the power of magnification of the binocular which
means that the object or subject appears to be this much closer than when
viewed with the naked eye. The second number is the diameter of the
objective (front) lens. The larger the number, the more light the binocular
gathers and the brighter the image will be. Good binoculars are always a
combination of magnification, objective lens diameter, image quality, size,
weight, durability and quality. The quality of a binocular is not always
apparent at first sight. Its robustness, such as extreme heat, cold,
moisture and jarring.
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Central Focusing Wheel: The central focusing
wheel of a binocular enables you to focus both eyepieces of a binocular at the
same
time. At the same time it is possible to compensate for unequal vision by
adjusting of dioptric correction on the right-hand eyepiece.
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Coating: Done in order to reduce reflections and
increase light transmission by evaporation of extremely thin layers of
substances onto polished lenses and prisms of optical instruments.
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Exit Pupil: The exit pupil is that circle of
light which can be seen i the eyepiece from a distance of about 25 cm. It is a
mathematical value which size is determined by dividing the diameter of
the objective lens by the magnification. The larger the exit pupil, the
brighter
the image will be in low light conditions.
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Field Of View: The field of view is a
measurement of the viewing field or subject area which is expressed by width
in the feet of
the area you can see at 1000 yards. The wider the angle, the wider the
area seen. A wide field of view is better for following fast moving action or
scanning for wildlife.
Focusing: The adjustment of an optical
instrument in order to make the image sharply visible varying distances. Inversion System: If you look through an
astronomical telescope you will see everything not only in reverse, but also
upside down. The inversion systems in binoculars make it possible to see
things 'normally'. Magnification: It indicates how many items
closer an object or subject appears compared to naked eye viewing. Objectives: These are the large lenses at the
front end of the unit that comprise a system of two or more individual
lenses made of different types of glass. Resolving Power: Optical's's instrument
capability to sharply reproduce, point by point, details of the object
viewed.
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MAGNIFICATION (POWER)
Binoculars are often referred to by two numbers
separated with an 'X'. For example; 8x32. The first number is the
power or magnification of the binocular. With an 8x32 binocular, the object
being viewed appears to be eight times closer than you would see it with the
unaided eye.
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OBJECTIVE LENS SIZE
The second number in the formula (8x32) is the
diameter of the objective or front lens. The larger the objective lens, the
more light that enters the
binocular, and the brighter the image. |
Newcomers to astronomy
often rush to buy the first telescope they see in a department store, and this
is perfectly understandable. All the
beautiful pictures of celestial objects seen in magazines or in books have
this effect on most people.
Unfortunately, after setting the telescope in the backyard and taking the
first look at the night sky all the magic is gone. The instrument's narrow
field of view and confusing image orientation will soon cause more frustration
than fun.
The obvious alternative is to invest in binoculars, which have the advantages
of being relatively inexpensive, highly portable and easy to use.
The only disadvantage is that the magnification is generally fixed. Zoom
models with variable magnification are also available, but for reasons you
will find later in this article it is probably better to accept the fixed-
power limitation.
Binoculars magnifying 5 to 8 times are ideal for finding the planets, scanning
Milky Way star fields, studying star clusters such as the Pleiades
and Hyades, and viewing bright comets. The Moon will also prove a constant
source of enjoyment; the mountains, valleys and craters are beautifully
brought out, and
it takes only a few minutes to learn the most prominent features.
How Binoculars Work
To understand the working principle of binoculars, first
you need to know a little about telescopes. In fact, this is exactly what
binoculars are, two
identical telescopes placed next to each other.
At the front of each telescope is a lens, called the objective. Its role is to
gather light from whatever it is you're looking at and bring it to a focus
in the eyepiece, where the light is formed into a visible image and magnified
to take up a large portion of the retina. The magnification depends on the
focal length of the eyepiece, and for binoculars it is usually between 5x and
10x.
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The image produced by this telescope
will be upside down and backwards, but for astronomical viewing this is not a
major inconvenient. In space there is no up and down or left and right.
However, for watching birds or following the action at a baseball game a
right-side-up picture is essential. This is why binoculars use
corrective elements between the objective and the eyepiece, called prisms.
Prisms used in binoculars are blocks of glass that function as mirrors,
but without a mirror's reflective backing. They come in two models and use
different types of glass, and we will discuss about this later in the
article. For now let's just mention their role, that is to bring the light
beams.
From the objective closer together by means of
internal reflection, and also turn the image right-side up and
orient the view properly left to right
To better understand the working principles take a look at the image above.
It shows the path of the light as it enters the objectives, passes through a
set of prisms that turn the image right side up, and finally leaves the
eyepieces to enter the observer's eyes. This applies to all binoculars, no
matter what model or size. |
Magnification
All binoculars are described by using a pair of
numbers, such as 7x50 or
8x30. The first number, including the x, represents magnification
or 'power'. This tells the degree to which the object observed is enlarged.
For example, a 7x binocular makes an object appear seven times closer
than when viewed by the naked eye.
There are some models of binoculars that offer variable magnification,
usually in the range of 5x to 8x. They are called zoom binoculars, and
in most cases are not very suited for astronomical observations because of the
inferior optical quality and
fragile mechanics. The best thing to do is to avoid them and stick with the
usual fixed-power binoculars.
Magnification is not that important, and in most cases comes within 7x to
12x.
If the magnification exceeds these figures, most likely you won't be able to
hold the binoculars steady enough and the images will be blurry and in
constant movement. This is especially frustrating when observing faint
objects like galaxies and nebulae. A tripod mount or image-stabilized
binoculars
will get you rid of this problem, but we will talk about this later in the
article. Aperture
The second number in the two-
number code is aperture (from the Latin aperire -
'to open'), the most important specification of binoculars if you plan to
use them for astronomical observations. It represents the diameter of each of
the objective lenses (the lenses furthest from your eye), given in
millimetres.
Therefore, 7x50 binoculars have objective lenses 50 mm in
diameter.
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Aperture is so important because it
determines the light gathering ability of your binoculars. Most
celestial
objects glow very dimly, so a large aperture becomes much more important
in low light conditions. For example, 35 mm binoculars will do great
when
you watch a baseball game on a sunny day, but when used to observe the
night sky you will find that they are pretty useless compared to typical
50 mm binoculars.
The 15 millimetres difference in lens diameter does not sound that huge,
but we need to compare the surface area, not the diameter. The area of a
circle is proportional to the length of the diameter times itself, so a
small increase in diameter makes a big difference in area. This is why
our
7x50 binocular has twice the light gathering ability of the 7x35
binocular.
After all these said, it is clear that when it comes to aperture bigger
is
better. The larger the light gathering area, the brighter the images
will
appear. Compared to the naked-eye a 50 mm binocular gathers from 50 to
100 times as much light, translating into a
difference of five stellar magnitudes. Therefore, if from your observing
site
you can see stars to magnitude 5.5 with the naked eye, the binocular will
show many more stars down to magnitude 9.5 or even dimmer. |
Exit Pupil
If you divide the objective
lens diameter by the magnification, you will get a
number approximately between 4 and 8. This number is called the exit pupil,
and represents the diameter of the beam of light that leaves the eyepiece
when
you hold a binocular with the objective pointed towards a light source. For
example, a 7x50 binocular has an exit pupil of 50 divided by 7, just over 7
millimetres in diameter.
Ideally, the exit pupil of your binocular should be equal or slightly
smaller
than the pupil of your dark-adapted eye. In this way the binocular delivers
the maximum amount of light and produces the brightest possible images for
its
aperture.
Average young adults under dark night conditions have pupils that are open
to
about 7 millimetres. This means that any instrument with an exit pupil
larger
than 7 millimetres will only waste light, as only the centre of the light
beam could enter the eyes. As we get older our eyes dilate less, so the exit
pupil
size we need decreases to around 5 millimetres.
Field of View
The field of view is the area of sky or land seen through
your binoculars,
determined by the design of the instrument's optics. It is expressed in two
ways; as the width in feet at 1,000 yards, or in degrees of field. When
expressed in feet the field is called linear, and when expressed in degrees
it
is called angular. Don't let the terms confuse you, the conversion is easy -
divide the liner field by 52.35 and you get the angular field.
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In most cases the field
is indicated on the outside of the binocular, in degrees. Average values are
between 5 and 10
degrees, or roughly 260 to 520 feet. To get an actual idea of how wide
this field is, think that in five degrees you can fit almost 10 Full
Moon diameters!
For astronomy, a wide field of view is desirable because if offers a
more
pleasant viewing experience, and you can see more of the sky at a better
edge performance compared to a narrower field. However, when increasing
the field beyond a
certain point images start to exhibit signs of distortion, especially near
the
edges of the field. Also remember that field of view is related to
magnification; the higher the power of your binoculars, the smaller the field
will be. |
Eye Relief
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Eye relief is the
distance behind the eyepiece lenses at which the image is in focus, and
indicates how far the
binoculars can be held from your eyes and still allow you to see the
entire apparent field of view. In general, the longer the focal length
of an eyepiece, the greater is the eye relief. Standard binoculars have eye
relief ranging from only a few millimetres to 25 millimetres or more.
Long eye relief is especially necessary for eyeglass wearers, because
glasses increase the distance between the lens and your eye. In case
your eyeglasses correct only for near or farsightedness, you can simply take
them off and refocus the binocular to compensate. |
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Interpupillary Distance
Interpupillary distance is
the distance between each of the adult person's
eyes. Most binoculars are adjustable to accommodate different interpupillary
distances, typically within a range of 60 mm to 72 mm. Many children and some
women have interpupillary distances too short for standard binoculars, so
the only solution is to choose compact binoculars.
Antireflection coatings
Because when using
binoculars to observe the night sky you want the
biggest possible amount of light to reach your eyes and not be reflected
back into space from the objectives, modern lenses have antireflection
optical coatings on at least one of the air-to-glass surfaces. Very good
models will have all glass surfaces coated, but tend to be more expensive.
The most used and least expensive coating is a single-layer of magnesium
fluoride (MgF), but there are also modern broadband multicoatings. To save
money, some optics manufacturers coat only some of
the air-to-glass surfaces, and basically we can talk about four levels of
coatings used on binoculars.
Coated lenses are the lowest quality, with a single-layer MgF coating on
some of the optical surfaces. Fully coated means that all air-to-glass
surfaces are
coated with a single-layer MgF coating. Multicoated lenses have multi-layer
coatings applied on some surfaces, and finally, fully multicoated lenses have
multi-layer coatings applied to all of the surfaces.
Recently the market has been invaded with binoculars that have so-
called 'ruby' coatings intended to reduce glare in bright light and improve
the
contrast between brown and green objects. Avoid any binocular that uses
these coatings, it will perform poorly for astronomical
use. Prisms
All binoculars are built with prisms that serve as
mirrors to reflect the incoming light between the widely spaced objectives and
the narrowly spaced
eyepieces. They also have the role of inverting the image that the objective
lenses project, in a right-side up and not reversed left to right view. Prisms
come in two types: roof and porro prisms.
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Roof prisms are in line
inside the optical
tubes, allowing binoculars to be made small and light. This is a great
advantage for hikers and birders, but for astronomical use roof-prism
binoculars prove to be poor performers.
In roof prism binocular design the light beam is split in two parts,
then recombined. This process leads to 'phase shifting', meaning that less
light is transmitted in the eyepiece and contrast is decreased. Besides,
roof-prism binoculars are generally much more expensive than porro-prism
binoculars of equal quality.
Porro-prism binoculars align the objective lenses and eyepieces in an
offset arrangement, with the objective lenses farther apart than the
eyepieces. They offer a wide field of view and are very affordable.
However, porro prisms have a minor drawback: they are easier to knock out of
alignment than roof prisms. |
Prisms are made of two types of glass, BK-7 borosilicate
flint glass and BaK-4 barium crown glass. For most designs, prisms made of BaK-
4 are preferred over the standard BK-7 because they have a higher refractive
index and give brighter and well defined images.
To check for yourself the type of prisms in your binocular, hold it pointed
towards a light source and take a look at the exit pupils. If the prisms are
made of BaK-4 glass the exit pupils will be round and evenly illuminated. If
the prisms are of BK-7 glass you will notice squarish, gray edges in the exit
pupils.
Focusing Mechanism
Binoculars come with two
types of focusing mechanisms. Most people opt for
the centre-focus model, which uses a centrally mounted wheel to adjust
both eyepieces at once. There is also a separate adjustment for the right
eyepiece, which helps to correct for any difference in near or
farsightedness between your eyes.
Because focusing requires only one quick adjustment, this model is the
most popular, and best suited if you want to use your binoculars for other
purposes except astronomy.
The second focusing system uses individually focused eyepieces and has no
centrally located focusing mechanism. Even though focusing is slower
compared to the previous model, binoculars that use individual focus tend to
be more rugged and less prone to moisture infiltrations.
These models are the the best choice if you plan to use your binoculars for
night sky observing, because celestial objects don't change their distance and
frequent refocusing is not required.
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Waterproofing
Because cleaning
binoculars can be very expensive, you might consider
buying waterproof models. They are designed to resist repeated changes
in temperature and humidity, making it impossible for condensation to damage
internal parts of the instrument. While waterproofing might not seem
very useful for astronomy, it is a must if you plan to use your binoculars for
boating, hunting, hiking, or other outdoor uses.
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Baffling
Baffling within the
binocular has the role of shielding against stray light and internal
reflections, and can dramatically improve image contrast. To check if your
binocular has quality internal baffling,
point it at a bright surface and examine the filed of view. It should be
surrounded by a black background, without additional light or shiny
reflections. Collimation
Because binoculars are basically two small telescopes
mounted side by side, an
error in collimation (optical and mechanical alignment) can lead to numerous
problems including eyestrain and double-images.
Most cheap binoculars are shipped out from the factory with collimation
problems, and even quality models come out of alignment as they age and get
bumped around. The only solution to this problem is to buy binoculars with
quality mechanics, which will last longer before collimation errors become a
nuisance.
Stabilising the View
For any binoculars to give
their best, they need to be held steady. This
eliminates the constant jiggling associated with hand-holding, and allows
you to see small details and objects fainter than you might think possible.
The traditional and least expensive mount for binoculars is a simple camera
tripod. Most binoculars come with a threaded mounting hole and an L-shaped
adapter that screws into this hole and onto the tripod. If you don't receive
the adapter when you buy your binoculars you can purchase it separately, or
even make your own.
Unfortunately,
this type of mounting is difficult to use and tiring for the neck, especially
when observing objects near the zenith. The solution is to purchase a
commercial mount especially designed for astronomical use, or go for image-
stabilised binoculars if your budget allows it.
Image-stabilized binoculars have an active built in optical system that
compensates for the movement associated with hand-holding. All you need to
do is press a button on the top of the binocular housing, and you will get
tripod-like stability with the
convenience of hand-holding. The only draw-
backs of these models are their small apertures, and prices several times
greater than their non stabilized equivalents. |
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