For anyone interested in exploring the night skies, an
excellent initiation tool is a pair of binoculars. Binoculars gather more light than the naked eye, allowing observation of
fainter objects such as the outer planets Uranus and Neptune. Binoculars also magnify a view, allowing more detail to be
seen – try looking at detailed lunar features, the phases of Venus or the four larger moons around
Jupiter (Io, Europa, Ganymede and Callisto). Binoculars have a field of view wider than typical telescopes allowing, for
example, the length of a comet from head to tail to be seen at once, or a star cluster such as the Pleiades in Taurus.
The initial aim of binocular observation is to learn about astronomical
objects. It is a major step in seeing additional astronomical objects and detail that cannot be seen by eye. Binoculars are
used, even if a telescope is owned, as a complimentary observation tool. For beginners to astronomy, binoculars provide an
effortless option for observation without the necessity of purchasing a telescope and setting one up each time for use, and
binoculars also provide images the right way up.
Low to medium priced binoculars can be used for terrestrial as well as
astral observation, rather than binoculars dedicated to astronomy.
Aperture
When purchasing binoculars take note of the aperture (diameter of the front objective lenses).
Generally, the larger the aperture, the brighter the images observed and the better the resolution. The disadvantage to a
large aperture is increased size and weight. To gather sufficient light to see night-time objects, the aperture needs to be
at least 40mm. Aperture can be determined from the two numbers printed on every pair of binoculars e.g. 7x35 or 10x50. The
second number refers to the aperture in mm of each of the front lenses. 7x35s have an aperture of 35mm and 10x50s have an
aperture of 50mm. Doubling the size of the objective lenses quadruples the light gathering ability. 7x50s gather almost
twice as much light as 7x35s and four times that of 7x25s. However, the size of objective lenses is just one factor to be
considered in determining which binoculars are most suitable for the job.
Magnification
The first number refers to magnification. For astronomy, magnification needs to be a minimum of
7 times, but higher than 10x binoculars are difficult to hold steady and require a tripod thereby losing the versatility and
advantage of this particular tool.
Age and Pupil Size
A further factor to be taken into consideration is individual eyesight, often affected by age. For
the most detailed images, we should try to match the exit pupil (the width of a beam of light exiting the eyepiece)
of the binoculars to the dilated pupil size of our own dark-adapted eye. The maximum dilation for the human eye is 9mm. As
we age, our eyes´ ability to adapt to changing light levels decreases. Most people younger than 30, have pupils that will
dilate to at least 7mm. For 30 to 40 year olds that drops to 6mm and for the over-forties it decreases to 5mm. To determine
the exit pupil of binoculars, divide the aperture by the magnification e.g. 10x50s = 5mm, 7x35s = 5mm and 7x50s = approx.
7mm. If using binoculars with an exit pupil greater than your own pupil size, some of the light will not reach your eyes and
images will be fainter.
Field of View
The field of view is how wide an area is encompassed in the binoculars´ image. This can be
expressed as degrees of an angle (e.g. 8º) or as the width in feet of the image at 1,000 yards (e.g. 325 feet at 1,000
yards). To convert feet at 1,000 yards into degrees, divide the number of feet by 52.5 (e.g. 325 feet divided by 52.5 =
6.2º). Field of view might also be expressed as the width in metres at 1,000 metres. The larger the degree, the wider the
angle and the larger the field of view. For the type of binocular being discussed, 6 to 7 degrees is average, whilst 8 to 10
is wide-angled.
Field of view is related to magnification, with greater magnification creating
a smaller field of view in general. A large field of view is not so essential in astronomy as it is when observing, say, a
terrestrial moving object.
Focusing
The hardier type of binoculars have eyepieces that can be focused independently. However, it is
more usual to find a central dial that moves both eyepieces together and is then refined by the ability to adjust just one
eyepiece.
Optics
There are two basic configurations of binoculars – those where the eyepieces are in line with the
objective lenses (roof prism), and those where the eyepieces are offset from the lenses (Porro prism). Although heavier and
bulkier, Porro prism generally have slightly sharper images and give a better performance for money, especially in medium
and low priced binoculars, making them more suitable for beginners to astronomical observation.
Another consideration is the quality of glass and anti-reflective coatings.
BAK-4 is a high density glass of better optical quality than BK-7. With BAK-4, if you hold binoculars away from your eyes
and up to the light you can see the circular exit pupils in the eyepieces. The less expensive BK-7 prisms have squared-off,
non-circular exit pupils.
The optical elements of binoculars should be coated to reduce internal light
loss and glare. The more surfaces that have had special coatings applied, the less light will be lost. Some of the symbols
used to describe coatings are: (C) = Coated Optics – 1 or more surfaces coated; (FC) = Fully Coated – all air-to-glass
surfaces coated but any plastic lenses may not be; (SLC) = Single Layer Coated; (MC) = Multicoated – 1 or more surfaces are
multi-layer coated; (PMC) = Partially Multicoated – some lenses, not all, have been treated; (FMC) = Fully Multicoated – all
air-to-glass surfaces are multi-layer coated. Coated lenses are the lowest quality, with fully coated at the next level;
basically neither of these will result in a satisfying product for astronomy. Multicoated or fully multicoated lenses are
both good choices with fully multicoated lenses giving the best light transmission and brightest, sharpest images.
Eye Relief
This refers to the distance, in millimetres, that binoculars can be held from the eye and the full
field of view still be comfortably observed. Glasses wearers in particular benefit from longer eye relief.
Medium to High Specification Binoculars
Medium to high specification binoculars cannot be hand-held for any length of time. Even the
lighter models, discussed above, require a very steady hand to prevent wiggly worms when observing planets. Giant binoculars
are too large and too heavy to be held still at all. We are getting into the realms of serious committed observation with
the necessity to add at least a tripod to your equipment. A sturdy and tall tripod is recommended (a stable, shorter
version is fine if you are able to sit whilst observing) and the ability of the mount to look straight up at the zenith
should be tested.
The higher specification binocular might have an additional number in the
level of magnification. If denoted as 12-36x70 there is a zoom capacity from the 1st to 2nd number, usually adjusted by a
lever. Zoom binoculars typically have drawbacks, in the quality of optics and a reduced field of view, and are not generally
used by astronomers.
Alternatively, the numbers may be denoted as 25/40x100, for example. These
have multiple eyepieces enabling you to switch easily between magnification of 25x and 40x. An additional expense but this
is a tool you can play, and observe, with all night.
Planets: