Evolution of the Telescope

By Rachel Alexander

Today, telescopes are used to study far off planets and galaxies, and are even in space themselves. But, they had much more humble beginnings.
The knowledge that lenses can be used to magnify objects has been around for a very long time. In fact, magnifying lenses have been found that date as far back as 400 BCE. But, the first telescope wasn't made or turned to the stars until the 1600's.

Conflicting Patents

In 1608, Dutch lensmaker Hans Lippershey tried to patent his design for a convex objective lens and a concave ocular lens in a metal tube with 3x magnification. However, a week later, another lensmaker, Jacob Metius tried to patent a very similar design. Both were denied the patent, but Lippershey gets the credit for making the first crude telescope.

Galileo - The First Refracting Telescope

In 1609, Galileo Galilei created an improvised version of the Dutch telescopes. His design implemented a plano convex objective lens and a plano concave eyepiece. This original design only had 8x magnification. However later refinements produced both a 14x magnification and a 20x magnification model. These were the first refracting telescopes, and were about the strength of a pair of modern day binoculars.

However, they did have a few flaws, such as a narrow field of view, blurry and distorted images due to the spherical shape of the lens, called spherical aberration, and rings of color surrounding objects, caused by the way light travels through glass, called chromatic aberration.

Johannas Kepler

In 1610, Johannas Kepler turned his attention to telescopes, more specifically, Galileo's telescopes. He improved the design by using a convex lens for both the objective lens and the eyepiece. This not only widened the field of view, but fixed the spherical aberration, and allowed for the use of measuring devices.

The use of two convex lenses had some problems though. It didn't fix the chromatic aberration, and it made the resulting image appear upside down.

*Fun fact: Kepler never actually built his telescope. It was first built a few years later by Christoph Scheiner.

Christoph Schiener

In 1617, Christoph Schiener builds the first Keplerian telescope. After presenting it to several church officials who complain about the inverted image, he adds a third lens. This new addition would flip the image back just before it reached the eyepiece.

Equatorial Mount

In 1620, Christoph Schiener and Father Grienburger designed a mount for telescopes that made it possible to follow stars across the sky using only one axis. Other current mounts required the adjustment of two axis, but the equatorial mount made tracking much easier. This mount is still considered superior by astronomers today.

Christiaan Huygens

1654 - Developed an improved method of grinding and polishing lenses that would provide much better clarity.

- Later designed the compound eyepiece. An eyepiece that uses two lenses instead of one. After it was built in 1703, several years after his death, it became standard equipment for large telescopes for some time.

-Designed and built several massive refracting telescopes: a 125 ft, a 180 ft, and a 210 ft telescope. These were more powerful, but were also too large to put in a metal tube, they were unwieldy, and would lose the image in even the slightest breeze.

Isaac Newton - First Reflecting Telescope

In 1668, Isaac Newton replaced the primary lens with a parabaloid, round, polished metal mirror that was 6 inches in diameter, and made of six parts copper to two parts tin. The mirror was almost as bright as the commonly used silver, it wouldn't corrode as quickly as silver would, and it removed chromatic aberration. It also reached a magnification of 40x. Refracting telescopes had to be between 3 to 6 feet long in order to reach such magnification. However, it couldn't eliminate spherical aberration because the technology at the time could only make spherical mirrors. In addition, refractors still had brighter images, Newton's reflecting telescope only reflected 16% of the light it encountered.

Later, he added a flat, diagonal secondary mirror that would bounce the light out to the side of the telescope into an eyepiece. Is made it so that his head wouldn't get in the way of the reflected light.

James Gregory - "Gregorian Design"

In 1663, James Gregory describes a reflecting telescope with a concave perforated main mirror, and a convex secondary mirror. However, he never actually built his model.

Laurent Cassegrain - Cassegrain Reflector

In 1672, Laurent Cassegrain created a telescope using a parabaloid primary mirror and a hyperboloid secondary mirror. This caused parallel rays of light entering the telescope to be reflected from a large concave mirror, the prime focus, to a small convex mirror. This smaller mirror brings the light rays to a focus near a small hole in the center of the main mirror before they reach the main focus. This design can correct the blurring of the image entirely. The design was not fully appreciated until about a century later. From the twentieth century onwards, Cassegrain designs would dominate astronomical research telescopes.

John Hadley

Built the first working telescope of the Gregorian design in 1721. The finished product was six feet long with a mirror six inches in diameter, which worked as about as well as Huygen's 125 foot long refractor.

Achromatic Lens

  • Chester Moore Hall

In 1733, Hall combined two different types of glass in a lens that reduced the optical distortions it created.

  • John Dollond

In 1758, Dollond patents the Achromatic lens, although, there is some controversy as to if he developed it independently or borrowed the design from Hall.

Eponymous Eyepiece - Ramsden Eyepiece

Created by Jesse Ramsden in 1783, this eyepiece utilizes two plano convex crown-glass lenses of equal focal length, placed with the convex sides facing each other, with a separation between the lenses of two thirds to equal the focal length of the lenses. This helped reduce both chromatic and spherical aberration.

Orthoscopic Eyepiece

In 1880, Ernst Abbe made an eyepiece that had four elements:

  • A plano convex singlet eye lens
  • A cemented convex-convex triplet lens (3 pieces put together)

This eyepiece gave near perfect image quality, but had a narrow field of view.

Ritchey-Chretien Reflector Telescope

In 1908, George Ritchey and Henri Chretien co-invented the Ritchey-Chretien telescope. The mirror was made in a dust free, temperature controlled environment, where only a select few were allowed to enter and carefully work on the lens. This was an example of a very early version of the modern day clean room. The mirror was silver coated glass and 60 inches in diameter, and was for a time, the largest telescope in the world.

The telescope introduced the Coude System, which allowed scientists to deflect the light from the telescope to many other instruments that were too heavy to attach to the telescope itself and analyze it.

Hooker 100 inch Reflector Telescope

George Hale, after being sponsored by the telescope's namesake, John D. Hooker, began work on his telescope in 1908, but it wasn't finished until 1917.

The delay was partially due to the fact that the 100 inch glass disk made to become the mirror had hundreds of tiny air bubbles inside it, and some of the glass crystallized, this meant that the disk would not be able to stand up well to the grinding it would need to become a mirror. After several other failed attempts at making a new disk and being delayed further by World War One, they finally decided to use the original mirror and begin the five year processes of grinding it and coating it in silver to turn it into a mirror.

The finished mirror had a diameter of 100 inches, and the entire telescope weighed 100 tons. The motions of the dome, shutters, and telescope were controlled by a system of 30 motors.

Hale 200 Inch Reflector

George Hale began this project in 1928, but due to delays and a second world war, it was not completed until 1948. The mirror was a thin pyrex glass disk with a ribbed back that would help strengthen it. It was coated in aluminum and was 200 inches in diameter and could gather much more light than the 100 inch Hale had previously built.

COBE - Cosmic Background Explorer

This telescope was launched in 1989 and had three instruments:
  • one to observe infrared radiation
  • one to map microwave radiation
  • one to measure the cosmic background radiation's spectrum
The telescope was equipped with solar panels to provide power and a funnel shaped sunshade to keep it's instruments cold. It also had a liquid nitrogen tank to assist in keeping it cold so than delicate science equipment wouldn't overheat from the infrared light, which we observe as heat.

The COBE orbited at 547 miles above earth, but was not very sensitive, and was eventually decommissioned in 1993.

HST - Hubble Space Telescope

This reflector telescope was launched in 1990, and it had a basic 94.5 inch (diameter) aluminum coated glass mirror, and was equipped with other instruments to give a clear view of the universe in visible, infrared, and ultraviolet light. Unfortunately, the first mirror's curve had been ground slightly too thin, the difference was about 50 times thinner than a human hair, but it made Hubble unable to focus correctly and caused the images to have spherical aberration.

A series of coin sized mirrors were placed in front of the original instruments and corrected the image.

Has been upgraded several more times since then to gives us even better images.

CGRO - Compton Gamma Ray Observatory

Launched in 1991, this telescope was not equipped with a mirror, and did not collect light. It was an entirely new type of telescope that tracked gamma radiation. This enabled us to study black holes, quasars, neutron stars, and supernovae. It's instruments measured the flashes of light that occurred when gamma rays struck the liquid crystal detectors built into them. It was powered by batteries and solar panels and orbited 280 miles above Earth.

Unfortunately, it had to be brought back to Earth in 2000 because one of it's gyroscopes failed.

Chandra X-ray Observatory

Launched in 1999, this reflector telescope has 8 iridium coated glass mirrors - each 32.8 inches in diameter, which it uses to observe x-rays. It follows an oval shaped orbit around Earth at between 9,942 - 82,646 miles above Earth's atmosphere.

Spitzer Space Telescope

Launched in 2003, this reflector telescope studies infrared light using a beryllium metal mirror - 35.5 inches in diameter. It is well insulated and shielded from the sun and heat and has a liquid helium tank to keep itself as cold as 1.4 Kelvin, almost at absolute zero, if needed. It is powered by solar panels and batteries and follows along behind Earth in it's orbit.

Photo Citations

Kepler Telescope Diagram. Digital image. Amazing Space. Web. 09 May 2016. <http://amazingspace.org/resources/explorations/groundup/lesson/basics/g9a/graphics/g9a_kepler.gif>.

Plano Convex. Digital image. Molecular Expressions. Web. 9 May 2016. <https://micro.magnet.fsu.edu/optics/lightandcolor/lenses.html>.

Plano Concave. Digital image. Molecular Expressions. Web. 9 May 2016. <https://micro.magnet.fsu.edu/optics/lightandcolor/lenses.html>.

Hubble Spherical Aberration. Digital image. Hubble Space Telescope. Web. 9 May 2016. <http://www.spacetelescope.org/about/history/aberration_problem/>.

Chromatic Aberration. Digital image. Nikon. Web. 9 May 2016. <http://www.nikon.com/products/sportoptics/how_to/guide/binoculars/technologies/img/technologies_08/pic_011.jpg>.

COBE Telescope. Digital image. NASA. Web. 10 May 2016. <http://science.nasa.gov/missions/cobe/>.

Hubble Restored. Digital image. Hubble Space Telescope. Web. 10 May 2016. <http://hubblesite.org/gallery/spacecraft/04/>.

Compton Gamma Ray Observatory. Digital image. NASA. Web. 10 May 2016. <http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1991-027B>.

Chandra X-ray Observatory. Digital image. Chandra X-ray Observatory. Web. 10 May 2016. <http://chandra.si.edu/about/spacecraft.html>.

Assembly of Telescope. Digital image. NASA. Web. 10 May 2016. <http://www.nasa.gov/centers/jpl/missions/spitzer.html>.

Gregorian Design. Digital image. Wordpress. Web. 10 May 2016. <https://thonyc.wordpress.com/2012/06/21/reflecting-the-heavens/>.

200 Inch Telescope, East. Digital image. Palomar Observatory. Web. 10 May 2016. <http://www.astro.caltech.edu/palomar/media/halemedia.html>.

Hooker 100 Inch. Digital image. Joe's Big Idea. Web. 10 May 2016. <http://www.npr.org/2015/04/25/401843663/hubbles-other-telescope-and-the-day-it-rocked-our-world>.

RCT. Digital image. Wikiwand. Web. 10 May 2016. <http://www.wikiwand.com/en/Ritchey–Chrétien_telescope>.