Make Telescopes: The Perfect Machine

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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. He began to make a telescope in the early 1700, but 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.

 

Although a number of telescopes were made according to Hall’s instructions, the benefits of the achromatic lens do not appear to have been made available to the public until John Dollond invented it independently in 1758, and patented it.

 

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.

 

Altogether, excellent progress began to be made, and by 1800 achromatic objectives 6″ in diameter were being turned out. Some of the best glass had been manufactured by Guinand, a Swiss who worked with Fraunhofer from 1805 to 1814. Fraunhofer produced a number of splendid achromats up to 9″ in diameter.

 

Dollond began to make telescopes of the refractors variety (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. Building 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. Measurement of an image in the focal plane was by no means a new idea; probably this had been first accomplished by Gascoigne, an Englishman, about 1638.

 

With the advent of the achromatic lens, the erecting or terrestrial eyepiece assumed considerable importance. This type of eyepiece was first suggested and used by Kepler, and improved in design about 1645 by Antonios Maria v. Schyrle, a Capuchin monk better known as Rheita. It is mentioned here because it spelled the rise of the refractor and the decline of the Gregorian for terrestrial use.

 

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 making of mirrors had been known and practiced for centuries, but for one reason or another, the coatings were unsuited for front-surface reflection. There became many ways to make telescopes.

 

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. It tarnished, but not nearly so quickly as speculum, and it could be removed by chemical means and a new coating applied without upsetting the figure of the glass surface.

 

Building a telescope was becoming closer to being constructed in the style we know it today.