Jim's Corner Blog

Musings of a Backyard Astronomer

A Historical View of Telescopes

From a historical viewpoint, today’s commercially available telescopes and eyepieces are technological marvels that the great astronomers of history would have loved using. Who knows what further discoveries that William Herschel or Charles Messier would have made with today’s high quality and sophisticated telescopes and wide field eyepieces.

Charles Messier was a French astronomer of the middle to late 1700’s to early 1800’s, who during his lifetime was more noted as a comet hunter than a deep sky observer. As a result of stumbling upon diffuse “fuzzy” objects that did not move in the sky, Messier, the comet hunter, began compiling a list of fixed diffuse objects in the night sky which could be mistaken for comets. Ironically, in today’s world, Charles Messier is best remembered for his Messier catalog of over 110 deep sky objects rather than for his 13 comets that were discovered during his lifetime.

Messier’s favorite instrument was a 7.5” aperture Gregorian reflector with a 3 foot focal length, with a fixed magnification of 104x. With its speculum metal mirrors, it has been calculated that the effective aperture of this instrument was equivalent to a 3.5-inch refractor. Even worse was the situation for the old 8-inch Newtonian reflector he occasionally used, which again was equipped with speculum mirrors and could only achieve the performance of a modern 2.5” refractor. Later he preferred to use several 3.5” achromatic refractors, with focal lengths of about 3.5 feet, and magnifying 120 times. He selected to use these scopes because they were the most easily accessible instruments for him. All of Messier’s telescopes appeared to have fixed magnifications. Apparently, telescopes of his time did not have interchangeable eyepieces.
All of Messier’s instruments are not as capable as a modern 4” refractor or 8” Schmidt-Cassegrain telescope. Backyard astronomers of today can observe all the objects of the Messier catalog using a 3” or 4” refractor in dark skies.

The story of William Herschel, his sister Caroline, and his son John, and their contributions to the science of astronomy is well documented. Sir William Herschel, born in Germany as Frederich Wilhelm Herschel in 1738, escaped the French occupation of Hanover, Germany and immigrated to England in 1757. A music teacher by trade, he became obsessed with astronomy. Telescopes were not a common place item in England of the 1700’s, so Herschel resolved to learn how to build his own. With his sister Caroline helping, William Herschel began experimenting with grinding mirrors and building reflecting telescopes.

Using innovations, such as the parabolic mirror and a new reflective coating alloy that used an increased mixture of copper in its formulation, Herschel built his first telescope, a 6” aperture with a 7-foot focal length. This initial foray into telescope making was easily capable of seeing Saturn’s rings.

But William knew he could build a bigger mirror, and followed that with a telescope with a mirror diameter of 9 inches. As the mirror size increased, so did the focal length of the telescope, with a focal length growing to 10 feet in length.

His next instrument proved to be his life-long favorite telescope. An 18” aperture with a focal length of 30′, an extremely long instrument by today’s standards. It took him three tries before successfully completing the fabrication of this 18” mirror, with the first attempt ending with a cracked mirror and the second with a molten metal mess.
He went on to complete the construction of a massive 48” instrument, with a focal length of 40 feet! Although certainly the greatest light gathering could be achieved with this instrument, it proved to be very cumbersome to operate, requiring at least two assistants to operate the massive telescope. This telescope and the 18” telescope were not of the commonly accepted Newtonian reflector design. Herschel tilted the primary mirror so that the focus would occur slightly to the side of the telescope. The observer had to be on a platform and lean over the telescope to see the image. This meant that the difficult-to-make-flat diagonal normally used in a Newtonian design could be eliminated. In the case of the 48”, it meant that Herschel was hanging precariously over the end of the telescope tube as much as 40 feet in the air! Few “Herschelian” telescopes exist today, and their design has been left to amateur telescope makers to attempt the challenge of building.
With these telescopes, William Herschel was able to discover the planet Uranus. He and his sister Caroline went onto discover over 2,500 deep sky objects and 848 double stars.
Telescope technology has advanced far beyond the equipment that Messier and Herschel used. Modern telescopes, with advanced telescope optics, advanced coatings, modern eyepiece designs, and the mechanical and electronic innovations of telescope mounting systems, can out-perform any of the classic telescopes of Messier’s or Herschel’s era. The average backyard astronomer with today’s high quality 4” refractor, 8” Schmidt-Cassegrain, or 10” Dobsonian can, with dark skies and due diligence, observe all of Messier’s and most of Herschel’s discoveries.

Leave a Reply

Your email address will not be published. Required fields are marked *