Word: doublet
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Dates: during 1890-1899
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...eight-inch photographic telescope for the observatory is in process of construction. It is called a doublet, or rectilinear instrument, from the fact that it is to contain two object glasses, by which the linear distortions caused by ordinary lenses are nearly eradicated. The new telescope differs from the present Draper instrument in that it will take successive stellar photographs automatically, changing and exposing the plates according to the particular programme decided upon. The charts produced will be about eight inches square...
...telescope is the largest of its kind in the world. Its peculiar feature is the possession of two independent objectives, a "doublet," forming a rectilinear combination, which does away with distortion, and thus makes possible accurate charting of a large portion of the heavens...