The development of building a telescope was greatly aided by the construction of the achromatic lens.
In 1733, the achromatic lens was invented by Chester Moore Hall, an English barrister. This was accomplished by combining a convex crown and a concave flint lens in such a way that their focal lengths were inversely proportional to their dispersions.
Dollond’s efforts led to a demand for clearer glasses of more varied densities and of less equal dispersions, needed to improve achromatism, and chemists pursued experiments in learning how to control the refractive indices of melts, and in the pouring of large disks of limpid, homogeneous glass.
Dollond was making refractors (spyglasses) with single-lens objectives as early as 1742, his price for a 2-foot telescope then being 7s 6d. In comparison, in 1762 he sold a 2-foot telescope with a two-lens objective (achromat) for 2 guineas. The lens diameters in each case were just under 2″.
In 1783, with a view to combining the benefits of the wide field of Huygens’ eyepiece with a means of making micrometric measurements of an image in the focal plane, Jesse Ramsden, an English optician, designed the compound eyepiece. Therefore, the process to make a telescope was becoming more like the process undertaken today. It can be seen that a measuring device, such as adjustable parallel wires, set in the focal plane would be magnified along with the image. With the advent of the achromatic lens, the erecting or terrestrial eyepiece assumed considerable importance. In the early part of the 19th century, small achromatic refractors were being manufactured by several concerns. For those not having the means to buy achromats, telescopes with single-lens objectives continued to be made. Enterprising opticians were also offering lens sets that could be assembled into simple refractors.
The Modern Era
The method of chemically depositing silver on glass discovered about 1840 by Justus von Liebig, of Nuremberg, was successfully applied to a small glass telescope mirror in 1856 by Karl Steinheil, a German physicist, and independently in the following year by Jean Foucault, the famous French physicist.
Various processes of plating glass with metal for the make telescopes had been known and practiced for centuries, but for one reason or another, the coatings were unsuited for front-surface reflection.
Then, in 1858, Foucault announced the development of his amazingly delicate and simple test for a concave reflecting surface, using an illuminated pinhole and a straightedge placed in the vicinity of the center of curvature of the mirror. The pinhole and straightedge were the outgrowth of earlier experiments in which simultaneous microscopic comparison was made of a pin point, likewise placed at the center of curvature of a mirror, and its reflected image, which was caused to fall alongside.
The last speculum of note to be constructed was one four feet in diameter, made by Grubb in 1870 for the Melbourne Observatory. Silver-on-glass mirrors replaced the more expensive and difficult-to-work speculum.
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