If binoculars will be used for astronomy,there is no substitute for field testing them under the stars. The night sky offers the potential of objective and quantitative testing that is not easily available in daylight without relatively sophisticated optical equipment. In addition, with the exception of distortion and kidney bean, all the optical tests detailed above are easier to perform under the night sky. You will obtain better, and more easily comparable, results if you mount the binoculars. Here is a list of tests to perform at night:
• Overall Optical Performance. You can perform a simple and, to some extent, quantifiable test of the optical quality of your binoculars by determining the closest double stars that you can distinguish. In general, double stars, those with components of approximately the same magnitude, are easier to split than those that have components of significantly different magnitudes. The ability to separate double stars is obviously a function of magnification, so you should therefore expect binoculars with higher magnifications to routinely outperform those with lower magnifications. Because you are observing at low magnification, you should not expect to see the close separations that are discernible with telescopes of similar aperture working at high magnification. You should also be aware that differences between sky conditions at different times and places and differences in the optical acuity and observing experience of different observers restrict the objectivity of this test. It does, however, enable a single observer to compare the general optical performance of different binoculars with a high level of confidence. A table of appropriate double stars is given in Appendix A.
• Limiting Magnitude. The standard way of establishing the limiting magnitude of an instrument is to count the stars in a known region of sky. Some useful regions applicable to binoculars are detailed in Appendix B.
• True Field of View. When you are trying to find objects by star-hopping, it is essential to know what the true field of view of your binoculars is. The field of view that manufacturers state for their binoculars are not always correct, and, when wrong, they tend to err on the optimistic side. By placing stars of known separation at diametrically opposite sides of the binocular field, you can easily determine its true field of view. Similarly, if the field of view is severely degraded toward the periphery, you can determine the size of what you consider to be the usable field. A table of convenient star pairings, with relevant charts, is given in Appendix C.
• Field of View. Field of view is a very personal thing. Most people seem to prefer a wider field of view, such as that from an ultrawide eyepiece, giving 82 degrees or so of apparent field of view and perceive apparent fields of view of less than about 65 degrees to be akin to tunnel vision. Others find that a narrower apparent field of view helps them concentrate on the object under observation and they dislike having to "look around" to find the edges of the field of view.
• Collimation. Poor collimation is more obtrusive, and thus easier to test for, at night. Focus the binoculars on a reasonably bright star, then carefully move the binoculars away, keeping the image in view, until the binoculars are about 15 to 20 cm (6 to 8 in) from your eyes. If you still have a single image, the binoculars are probably collimated within acceptable limits. If the images from each optical tube separate from each other, then the binoculars are miscollimated and will cause eye strain.
You should not expect to find a binocular that is perfect in every respect, but these simple tests should enable you to make a reasonably good assessment of the binocular in hand and to compare it to other binoculars.
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Binoculars are generally robust and require very little maintenance if they are properly cared for. As with any other optical equipment, indeed any equipment, prevention and preemptive maintenance are considerably preferable to curative maintenance and repair. There are five categories of foreign matter that can threaten the well-being of binoculars. In no particular order these are:
• Moisture. This can invade the binocular either by condensation or from direct exposure to water.
• User-originated grime. This includes grease from fingers and eyelashes, spillage of food and drink, hair, and flakes of dead skin.
• Environmental grime. Dust is the usual culprit here, but grit can also enter the binocular in some circumstances.
• Flora. The usual culprits are algae and fungi.
• Fauna. Arthropods, especially insects and arachnids, can find ideal homes in the nooks an crannies of binoculars and their cases. William Gascoyne invented the eyepiece reticle after a spider had spun its web near the focal plane of his telescope!
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