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 the 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
Low to medium priced binoculars can be used for terrestrial as well as astral observation, rather than binoculars dedicated to astronomy.
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. 7×35 or 10×50. 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.
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 of 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.
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.
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 has slightly sharper images and give a better performance for the 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.
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 seriously 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 12-36×70 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 25/40×100, 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.