Lens making for telescopes in the 17th century
Venetian glass makers polished their lenses on a rotating felt or deer skin. It was the only known polishing method at the time and we will discuss it in more detail below. With this method, during the polishing process, the spherical lens initially becomes more and more aspherical, starting first from the edge and then gradually towards the central part. Now the big problem for glass manufacturers was that they had no test method and were therefore unable to control the growing aspherical deformation. Finally, they had many lenses with a huge variation of different qualities. Therefore, Galileo had to examine hundreds of lenses to find some suitable for astronomical use. With the best lenses, the useful central part has a diameter of 20 - 25 mm and, with mediocre lenses, only 10 - 20 mm. All other lenses were good to throw away.
Now, how did Galileo examine his lenses? He had no choice but to put each lens in a tube of the right length and then look at the bright fixed stars on clear nights. Prior to this test, he had to reduce the aperture of the lens to such an extent that the chromatic aberration remained below the resolving power of the human eye. In other words, he had to reduce the aperture of the lens until the different colors surrounding the bright celestial bodies disappeared. This aperture was now the best he could achieve with his unique lenses. Now if the image of the stars had been without bright stripes and rays and the stars were round, the objective glass would have been perfect. However, the more Galileo had to reduce the aperture below this optimal diameter to reduce the remaining defects, the worse it was because the image became darker and lost much of its resolution and detail.
It is now important to realize that Galileo with this test could not get any information about the quality of the lenses on the part covered by the aperture. It was impossible for him to find any difference in quality between a lens with a fine image inside with a free aperture of 25 mm, but with an increasing asphericity to the edge and on the other hand, a lens like "single lens" from Florence which was perfect from the center to the outer edge did not exist. Therefore, our question is: how could Galileo successfully select the lenses, from which it could not determine the quality differences in its test method? From his correspondence, especially with Sagredo, we know that, even in 1618, Galileo had difficulty obtaining adequate lenses for his observations. From the example of a letter from Sagredo, dated April 23, 1616, that, out of a set of 300 lenses, only three were suitable for terrestrial telescopic use and not a single lens for astronomical observations.
As far as I know, it is impossible to polish spherical lenses to their full diameter using the rotating felt method. Therefore, the belief that the "single lens" belonged to Galileo leads us in particular to the strange assumption that an unknown manufacturer had a secret method, which allows him to polish lenses of the highest quality, which he then mixed with many weak lenses for you kept Galileo busy with a puzzle and that Galileo was in possession of the glass as good as those of the famous Giuseppe Campani 60 years later. We must recognize that the "Galileo telescopes" in Florence remain a mystery and that their age will remain a controversial issue.
We can find a description of this early polishing technique in Telescopium Sive Ars Perficiendi Novum. Sirturus described the grinding methods of Venetian lens and mirror manufacturers. However, he found the finishing methods of the lenses very rough and primitive. Grinding molds are formed only by hammer wallpaper without any spherical precision. Sirturus now did his own experiments. He corrected the mold as much as he could with a curved profile and filled the concave mold with molten lead to obtain a convex copy. Now his next action was of the utmost importance. He fixed the concave mold in a lathe and tied the mold to the convex copy of the lead together with grinding material until all the irregularities disappeared. Therefore, we can say that Sirturus was the inventor of the extremely important spherical grinding of molds. It is the only method for obtaining an exactly spherical shape. This improvement in lens manufacturing techniques is a clear step, but it has remained the only one for the next 25 years. Then, Sirturus described how to polish the lens. To do this, he cut a mold into a cylindrical piece of wood and glued a piece of felt or deer skin into it. He then polished the lens on a rotating lathe, using tripolite (diatomaceous mineral) or tin ash. This was also the method of polishing Venetian eyewear manufacturers, and Sirturus did not know how to improve it.
Now everything is clear, with such a primitive polishing method, the perfect spherical shape of the lens after the grinding process would gradually deteriorate from the outside to the inside, as the polishing process continued. The reason was that the felt never kept a spherical shape during the polishing process.
Due to the rotational speed of the felt and therefore the polishing force are at maximum at the edge and at minimum in the center, the spherical shape of the lens would deteriorate the fastest on the outside of the lens, during polishing the asphericity increases more and more to the center.
Towards the middle of the 17th century, it seems that the best telescopes were made by Evangelista Torricelli in Rome and Francisco Fontana in Naples. Despite this reputation, none of them grew in knowledge of the celestial bodies. Therefore, we can conclude that these two artists did not know how to improve the manufacture of telescopic lenses. Now we have found the reason why the telescopes of that period could not be upgraded to a higher resolving power. Rather than the poor condition of the glass, it was a completely inappropriate method of polishing the lenses.
Telescope examined from the beginning of the 17th century
Manufactured in 1617 or earlier by an unknown manufacturer in Augsburg, Germany, part of the Pommerscher Kunstschrank.
Main tube and five sections made of cardboard. Each tube is covered with marbled paper; the end tube and the ring stop in the silk velvet with gold thread. The collars, rings and the final tube form a cylinder with a common outer diameter (48 mm), characteristic of other early telescopes.
The box containing this telescope included the documentation of the various artisans involved in the assembly. This documentation makes this instrument the oldest surviving telescope in the world (see Hainhofer's cabinet inventory).
The lens is flat-convex, full aperture 38.4-41.4 mm. Free aperture between 16.3-17.4 mm. Lens velvet diaphragm: 41.4 (full) and 16.9 (free). When the telescope tubes are extended, the lens separation measures 900 mm.
The focal length of the lens is about 960 mm the eyepiece plano concave with a focal length of 60 mm and a magnification of 16x. Lots of bubbles in the lens. The color of the lens is slightly yellow, while the eyepiece showed a bluish or gray-gray hue.
The observed field of view is about 4 m at a distance of 1000 m around 15 minutes of arc, for a 6 mm eye pupil opening, but it offers a surprisingly clear, sharp and vertical image.
The eyepiece lens is plano-concave, full aperture: 22.9-23.8 mm, central aperture: 19.6 mm, thickness 2.2 mm.
1 The telescope
2 Objective and eyepiece
3 Ronchi test of the objective lenss full fdiameter
4 Ronchi test 2 with the center ring indicating the useful diameter.
5 Objective lens
6 Eyepiece lens
7 Objective lens closeup
8 The target statue to be seen through the telescope
9 Image through the telescope at 16x
10 Image through the telescope