Reflecting Telescope

Invented by : Issac Newton

Invented in year : 1668

A reflecting telescope is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. It is also called a reflector and catoptric telescope as reflecting telescopes use mirrors. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from severe chromatic aberration. Although reflecting telescopes produce other types of optical aberrations, it is a design that allows for very large diameter objectives. In reflector, there is no lens to collect the light. The light passes down a tube and falls upon a concave mirror. The mirror sends the light back up the tube onto a smaller, flat mirror placed at an angle of 45 degree. The light is then directed into the side of the tube and is brought to focus. The image is formed outside the incoming light beam and is enlarged by an eyepiece lens - the observer’s head does not obstruct the incoming beam. Almost all of the major telescopes used in astronomy research are reflectors.

History of the invention

There are many inventors who are known for inventing telescope but the most widely known inventor was an Italian scientist, Galileo Galilei. He introduced telescope to astronomy in 1609 and also became the first man to see the craters of the moon, and who went on to discover sunspots, the four large moons of Jupiter, and the rings of Saturn. Galielo's telescope however had limited magnification and suffered from other anomalies.  

Sir Isaac Newton, an English physicist, mathematician and astronomer invented the reflecting telescope in 1668. This was in response to the chromatic aberration (rainbow halo) problem that plagued refractors during his time. Instead of using a lens to gather light, Newton used a curved, metal mirror (primary mirror) to collect the light and reflect it to a focus. Mirrors do not have the chromatic aberration problems that lenses do. Newton placed the primary mirror in the back of the tube. Because the mirror reflected light back into the tube, he had to use a small, flat mirror (secondary mirror) in the focal path of the primary mirror to deflect the image out through the side of the tube, to the eyepiece; otherwise, his head would get in the way of incoming light. The secondary mirror does not block the image because it is so small compared to the primary mirror, which is gathering a great deal of light.

Development in the invention of reflecting telescope

In 1722, John Hadley developed a design that used parabolic mirrors, and there were various other improvements in mirror-making. The major change that took place was the growth in the size of the reflecting mirror, from the 6-inch mirror used by Newton to the 6-meter (236 inches in diameter) mirror of the Special Astrophysical Observatory in Russia, which opened in 1974.

Despite the theoretical advantages of the reflector design, the difficulty of construction and the poor performance of the speculum metal mirrors being used at the time meant it took over 100 years for them to become popular. Many of the advances in reflecting telescopes included the perfection of parabolic mirror fabrication in the 18th century, silver coated glass mirrors in the 19th century, long-lasting aluminium coatings in the 20th century, segmented mirrors to allow larger diameters, and active optics to compensate for gravitational deformation. A mid-20th century innovation was catadioptric telescopes such as the Schmidt camera, which use both a lens (corrector plate) and mirror as primary optical elements, mainly used for wide-field imaging without spherical aberration.

Role of the invention of reflecting telescope in the improvement of human life

• The Newtonian reflector was a highly successful design, and remains one of the most popular telescope designs in use today. One of the biggest example is Hubble telescope.

• The reflector telescope that Newton designed opened the door to magnifying objects millions of times- far beyond what could ever be obtained with a lens.

• It helped in making accurate astronomical observations.