Resolution and field of view are set by the lenses used and can be determined easily by looking through the optics. If you look at a print-out of the picture at the end of this chapter from a distance of 10 yards (meters), you will be able to separate some of the line series in your binoculars, while others merge into a solid bar. The bars which you can resolve correspond to the resolution of your binocular. The bars have a length of 0.5°, which equals approximately the diameter of the Sun and Moon. The circle above the bars also has a diameter of 0.5°. In it the separation distances of some double stars are shown, which give you a feeling of the units arc minutes and arc seconds when observed from a distance of 10 yards or meters.
You can also measure the field of view with the stars of the Big Dipper: The upper stars are about 10° apart, the lower 7°. The stars at the end are separated by about 5°.
The mechanics should be as robust as possible, and the binoculars must not be misaligned – if you see double images, both tubes point into different directions. A slight adjustment can indeed be compensated for by the eyes for some time, so it is not too obvious, but it quickly leads to headaches. If you observe a distant object alternately with one eye and then with two eyes (you can cover one lens with your hand) and notice double images for a short time or if the image “jumps“, the unit may have taken a hit by a fall and is out of adjustment.
There are two different kinds of housings for binoculars – German and American style. In German housings the lenses and prisms are in separate components, while American style binoculars combine everything in one single piece and are therefore less susceptible to dirt and bumps. There are also binoculars that can easily be focused using a central wheel, while other models have to be focused individually on each eyepiece.
Color aberration and edge blur can be seen clearly when you look at sharp edges at some distance (antennas, building edges or roofs). Some distortion at the edge can be tolerated but it should not extend too far into the field.
A binocular should definitely have a photo thread to attach it to a ball head or an L-adapter and a tripod. Only when it is in a fixed position, the image is steady and you can fully use its power. You can use a pair of binoculars with up to 7x magnification reasonably stable without a tripod, but most people need a tripod for magnifications more than 10x.
In principle, any photo tripod can also be used for a pair of binoculars, but it should be possible to extend it at least at head height – as most of the time you will be looking up. Wooden tripods dampen vibrations better than aluminum tripods. The matching ball heads are also available at photographic stores.
In addition to these simple solutions, there are also full-size binocular mounts available. Very comfortable are parallelogram mounts which work much like an old, height-adjustable desk lamp. In this case, the binoculars are mounted on two parallel rods. It is adjustable in height so that you can stand not only directly under the binoculars (so that the tripod is out of the way when you look at or near the zenith), but you can also change the height without losing an object. The latter is especially helpful when you want to show an object to different people.
Tinkerers can find many instructions and tips on the internet and can build, for example, a Sky-Scanner. In this case, the binoculars look down on a mirror, so that you can look comfortably down into the binocular and don‘t have to worry about a stiff neck. However, finding objects is no longer quite so simple, and you can‘t use just any cheap mirror.
Other accessories are neither readily available nor necessary. You might think about some dew shields which you can make yourself easily. These are small tubes whose length should be about twice as long as the lens diameter. They are put over the tubes and protect the lenses from fogging. When it cools at night, the humidity without dew shields can not only condense on the housing, but also on the lenses, so that you may look through a film of water after some time. Lens shades can be easily prepared from tubes, cardboard or flexible plastic. To avoid reflections, they should be painted matte black on the inside.
For safe solar viewing, you can easily build a suitable filter which is mounted on the front lenses. Baader Planetarium, Mammendorf, Germany, provides foil material including instructions for this – Check out protecsolar.com for their AstroSolar Safety Film. Do not use other, self-made filters (space blankets, shooters glasses, CDs) or even filters mounted in the eyepieces, they provide no protection!
Very good binoculars – which may cost more than a comparable telescope – offer comfortable angled viewing and interchangeable eyepieces for different magnifications. Here, you can also use deep-sky filters for the observation of nebulae.