China’s monopoly allowed it to raise prices by hundreds of percent for various rare-earth materials from 2009 to 2011 and also to impose export quotas on many of these products. This brought about a large change in the dynamics of the rare-earth markets. Mining of bastnasite resumed at Mountain Pass, California, in 2011 after a nine-year hiatus, and mining of monazite began that same year at Mount Weld, Australia. At the same time, loparite was being mined in Russia, while monazite was mined in India, Vietnam, Thailand, and Malaysia. Those and other mining operations brought a new equilibrium between demand and supply in which China was still the major supplier of rare-earth minerals, but companies either sought alternative sources, used less, or recycled more rare earths.

There are complicated design trade-offs, and the best choice can substantially depend on the optical instrument, the operation conditions and the priorities concerning performance parameters. Cost and weight can be additional aspects to consider.

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Further, the actual position of the exit pupil should be close to the position of the eye pupil, so that light which gets through the exit pupil can also enter the eye. The distance between the exit pupil and the physical end of the ocular is called the eye relief. Ideally, the eye relief should be at least about 5 mm, and possibly even 20 mm for observers wearing glasses. However, oculars with small focal length tend to have a smaller eye relief, e.g. only 2 or 3 mm, although there are design methods with which more can be achieved – possibly at the expense of other parameters. A particularly large eye relief is required for riflescopes because the recoil would otherwise push the ocular into the eye.

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There is a wide range of optical designs of oculars. In the simplest case, there is only a biconvex lens or alternatively (for Galilean telescopes) a biconcave (negative) lens. Other designs contain two or more lenses; for example, the Huygens eyepiece and the Ramsden eyepiece contain two planar-convex lenses, namely a field lens and an eye lens. Other designs contain achromatic lenses for reducing chromatic aberrations. Further designs, which are optimized in various respects like a field of view and image distortions, are named after Georg Simon Plössl, Ernst Abbe and others.

The transverse limitations which an ocular imposes on the transmitted light are characterized with the diameter of the exit pupil. That diameter should approximately fit the diameter of the pupil of the observing eye:

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As of 2017, known world reserves of rare-earth minerals amounted to some 120 million metric tons of contained REO. China has the largest fraction (37 percent), followed by Brazil and Vietnam (18 percent each), Russia (15 percent), and the remaining countries (12 percent). With reserves this large, the world would not run out of rare earths for more than 900 years if demand for the minerals would remain at 2017 levels. Historically, however, demand for rare earths has risen at a rate of about 10 percent per year. If demand continued to grow at this rate and no recycling of produced rare earths were undertaken, known world reserves likely would be exhausted sometime after the mid-21st century.

The most frequently used barrel diameter for telescopes is 1.25 inches = 31.75 mm, but consumer devices may use smaller barrels of 0.965 inch = 24.5 mm, while there are also larger sizes such as 2 inch = 50.8 mm or even larger ones up to 4 inches for use in certain professional instruments. Large barrel diameters usually imply a higher price, but allow for a larger field of view for a given magnification.

Rare-earth ore deposits are found all over the world. The major ores are in China, the United States, Australia, and Russia, while other viable ore bodies are found in Canada, India, South Africa, and southeast Asia. The major minerals contained in these ore bodies are bastnasite (fluorocarbonate), monazite (phosphate), loparite [(R,Na,Sr,Ca)(Ti,Nb,Ta,Fe3+)O3], and laterite clays (SiO2, Al2O3, and Fe2O3).

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Considering both the limited reserves and high value of the rare-earth metals, recycling these elements from consumer products that reach the end of their useful life is expected to become more important. At present, only scrap metal, magnet materials, and compounds used in the manufacture of phosphors and catalysts are recycled. However, products that contain relatively large amounts of rare earths could be recycled immediately using existing techniques. These include rechargeable nickel–metal hydride batteries that contain a few grams to a few kilograms of LaNi5-based alloys as a hydrogen absorber as well as large SmCo5- and Nd2Fe14B-based permanent magnets. All of these materials hold 25–30 percent by weight light lanthanides—much more than even the best rare-earth-containing ore (see below). However, the majority of consumer electronic devices contain only small amounts of rare earths. For example, a hard drive’s spindle magnet contains only a few grams of Nd2Fe14B. A speaker magnet of a cellular phone makes up less than 0.1 percent of the total mass of the telephone. A compact fluorescent lamp has only a fraction of a gram of lanthanide metals in the phosphor. Considering the complexity of many modern electronic devices, recycling of rare earths must be done simultaneously with recycling of other valuable resources and potentially dangerous substances. These include precious metals (such as silver, gold, and palladium), nonferrous metals (such as aluminum, cobalt, nickel, copper, gallium, and zinc), carcinogens (such as cadmium), poisons (such as mercury, lead, and beryllium), plastics, glass, and ceramics. Numerous scientific and engineering issues, therefore, must be resolved, first, in order to create consumer products that are easily recyclable at the end of their life and, second, to make recycling of rare earths both meaningful and economical, thus making the best use of the rare earths—an extremely valuable but limited resource provided by nature.

Binoculars, infrared viewers and other compact optical instruments often have rigidly attached ocular lenses, which cannot be easily exchanged.

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As noted above, the rare earths are fairly abundant, but their availability is somewhat limited, primarily because their concentration levels in many ores are quite low (less than 5 percent by weight). An economically viable source should contain more than 5 percent rare earths, unless they are mined with another product—e.g., zirconium, uranium, or iron—which allows economic recovery of ore bodies with concentrations of as little as 0.5 percent by weight.

Officially, 130,000 metric tons of REO equivalent was mined in 2017, but a black market in rare earths was said to produce an additional 25 percent of that amount. Most black-market rare-earth materials are smuggled out of China.

Besides the eye lens, which does the actual imaging, an ocular can contain an additional field lens for expanding the field of view. If that field lens is placed in the intermediate image plane, it does not affect the image magnification. However, the field lens is sometimes slightly moved away from that location.

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Some optical instruments such as telescopes, infrared viewers and microscopes contain certain groups of optical elements, mostly lenses, which are called objectives and ocular lenses. While an objective is on the side of the observed object, the ocular lens (also called ocular or eyepiece, sometimes loupe) is on the side of the observing eye. It may contain a single optical lens or some combination of lenses and is normally placed in a cylindrical housing (barrel).

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Note that the assumed pupil diameter is assumed to be significantly larger for an astronomical telescope than for a microscope because the viewing conditions are usually relatively dark, leading to a wide open eye.

The magnification of an telescope or microscope can usually be understood as the product of the magnification factors of objective and ocular lens. The magnification of an eyepiece depends on its focal length. Generally, short focal lengths lead to higher magnification, but also to a smaller field of view and possibly stronger image distortions.

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Of the 83 naturally occurring elements, the 16 naturally occurring rare-earth elements fall into the 50th percentile of the elemental abundances. Promethium, which is radioactive, with the most stable isotope having a half-life of 17.7 years, is not considered to be naturally occurring, although trace amounts have been found in some radioactive ores. Cerium, which is the most abundant, ranks 28th, and thulium, the least abundant, ranks 63rd. Collectively, the rare earths rank as the 22nd most abundant “element” (at the 68th percentile mark). The non-lanthanide rare-earth elements, yttrium and scandium, are 29th and 44th, respectively, in their abundances.

Chinese deposits accounted for about 80 percent of the rare earths mined in the world in 2017 (105,000 tons of rare-earth oxide). About 94 percent of the rare earths mined in China are from bastnasite deposits. The major deposit is located at Bayan Obo, Inner Mongolia (83 percent), while smaller deposits are mined in Shandong (8 percent) and Sichuan (3 percent) provinces. About 3 percent comes from laterite (ion absorption) clays located in Jiangxi and Guangdong provinces in southern China, while the remaining 3 percent is produced at a variety of locations.

Lanthanum and the light lanthanides (cerium through europium) are more abundant than the heavy lanthanides (gadolinium through lutetium). Thus, the individual light lanthanide elements are generally less expensive than the heavy lanthanide elements. Furthermore, the metals with even atomic numbers (cerium, neodymium, samarium, gadolinium, dysprosium, erbium, and ytterbium) are more abundant than their neighbours with odd atomic numbers (lanthanum, praseodymium, promethium, europium, terbium, holmium, thulium, and lutetium).

Professional optical instruments usually allow the easy exchange of oculars and the use of oculars with different parameters. One can often modify the magnification of an instrument that way, and at the same time other parameters such as the field of view and the image brightness can change. Instruments normally have a standardized kind of barrel (sleeve) for insertion of an ocular. There is also a mounting thread with which the eyepiece can be fixed after insertion.

The optical parameters of an ocular and not only the barrel diameter should fit to the optical instrument. It is sometimes possible to use oculars from different manufacturers with an instrument, but the performance may be degraded to some extent if the optical parameters do not fully fit.

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