Reflections on the Reflecting Telescope

Reflections On The Reflecting Telescope

By

Randy Boswell

It is a given that the reflecting telescope is the workhouse of astronomy. It established itself on the professional level due to the fact that it did not suffer from the limitations of aperture size as is the case with refracting telescopes. And, on the amateur level due to less cost per aperture size. This aside, much of the popular accounts do not reflect (yes, pun intended) accurately on the origin and development of the reflecting telescope. For example, scores of entry-level science and astronomy books claim Isaac Newton as the inventor of the reflecting telescope. However, such is not the case. This is not a knock, nor does it detract from Newton’s genius. It is like the lawyers say: you have to read the fine print.

The credit for the first reflecting telescope goes to the Italian monk, physicist and astronomer Niccolo Zucchi (1858-1670) [1], long before Newton constructed his reflecting telescope in 1668. It is said that Zucchi developed a keen interest in astronomy after a meeting with the famous astronomer Johannes Kepler. Around 1616, Zucchi created a basis reflecting telescope. Zucchi’s telescope involved a bronze primary mirror to focus the image and a concave lens as an eyepiece to view the image. Zucchi’s telescope failed to yield satisfactory images. The factors were the angle the mirror was tilted at to deflect the focused image a measure to the side towards the observer, along with the observer’s head partially obstructing the incoming light from the viewed object, and the possibility that the mirror did not adequately focus the image. Consequently, Zucchi further construction on his reflecting telescope. Nevertheless, in 1652 Zucchi published his book, “An optical philosophy and the experimental nature of the bases determined”, in which he described his experiments with his reflecting telescope. His book later inspired scientists James Gregory and Isaac Newton to pursue improved designs for reflecting telescopes [2].

The earliest design for a practical reflecting telescope goes to the Scottish mathematician and astronomer James Gregory (1638-1675) [3]. Gregory’s design predated Newton’s reflecting telescope by five years. The blueprint for Gregory’s telescope was set forth in his 1663 book, “Promotion of Optics”. Gregory pointed out that a reflecting telescope with a parabolic mirror would correct spherical and chromatic aberrations endemic to refracting telescopes of his day – a finding that preceded Newton’s conclusions.

Gregory’s design placed a secondary concave mirror with an elliptical surface past the focal point of the parabolic mirror, reflecting the image back through a hole in the primary mirror where it could be viewed. Gregory’s proposed design became appropriately known as the Gregorian telescope. However, Gregory had no telescope building skills. Others eventually constructed Gregory’s telescope many years later. However, the Gregorian telescope is seldom used today due to the fact that later telescope designs proving more efficient. Despite this, Gregory’s basic design is used in today’s radio telescopes – the most famous being the Arecibo Radio Telescope in Puerto Rico [3].

Regarding Newton’s 1668 reflecting telescope, it marked a turning point – of sorts. Newton’s design employed a primary mirror to capture light and a smaller secondary mirror to reflect the light out the side of the telescope to a magnifying eyepiece. This, along with the fact that his telescope was easier and less costly to construct than Gregory’s telescope eventually made Newton’s design popular with amateur astronomers and was thenceforth known as the Newtonian Reflector. But there was a problem.

While Newton’s telescope eliminated the problem of chromatic aberration it still suffered from spherical aberration, resulting in a blurry image caused by the spherical shape of the primary mirror. And this was the problem. Newton tried to grind a parabolic mirror in 1666 to eliminate this problem but was unable to do so because the technology was not available at the time. This resulted in Newton’s telescope becoming unusable [4]. The net outcome was that only two were built and given to the Royal Society if only for the purpose of proving the concept of a reflecting telescope [5].

It was not until more than fifty years later that the English inventor and mathematician John Hadley 1682-17440 ground the first parabolic mirror in 1721, thus produced the first practical reflecting telescope [4]. Hadley’s techniques has since been used, notably by the late John Dobson (1915-2014), the inventor of the widely-popular Dobsonsian telescope.

The succeeding years after Hadley’s innovation witnessed new variations on the reflecting telescope such as the Cassegrain and Schmidt-Cassegrain designs that we know today, for example. Burt the salient point is that the conception and prototype of the reflecting telescope occurred before Newton’s telescope. And, moreover, long after Newton built his telescope did the reflecting telescope prove itself and become the mainstay in professional astronomy. And this is, well, something to reflect on.

Notes

  1. Will Kaliff, “How Newton’s Telescope Changed The World”, March 23, 2018, www.Insightobservatory.com
  2. “Niccolo Zucchi: Italian astronomer,” n.d., www.Britannica.com
  3. Jonathan Powell, Rare Astronomical Sights and Sounds, 1st Edition, (Cham, Switzerland: Springer International Publishing AG, 2018).
  4. Mel Bartels, “Think you know who invented the reflecting telescope?”, January 2014, www.bbastrodesigns.com

Image

Wikipedia contributors. (2020, November 19). Niccolò Zucchi. In Wikipedia, The Free Encyclopedia. Retrieved 17:16, December 17, 2020, from https://en.wikipedia.org/w/index.php?title=Niccol%C3%B2_Zucchi&oldid=989594139

Editor’s Trivia Note: Lunar crater Zucchius is named after Niccolo Zucchi

 

 

The Great Conjunction of December 2020

Have you been watching Jupiter and Saturn? If you have, you probably noticed they appear to be getting closer to one another. From our vantage point here on Earth, they are. Both planets are heading towards the Great Conjunction on December 21st, when they will appear closest to each other. Many of our members are genuinely excited about this special and rare event. BAA member Patrick Crants discussed the Great Conjunction during the November 21st “Looking UP!” virtual star gazing event (click here to view the video on our Facebook page: https://fb.watch/22lkkJo-rW/ ). The following was assembled from his presentation.

Live Event Alert: The BAA is planning on sharing this event live on Facebook, weather depending, between December 18 – 23. You can find out more on information about this live event on our Website (https://www.buffaloastronomy.com/events/) as the date approaches. All our videos and Facebook events can be accessed here https://www.facebook.com/BuffaloAstronomy/videos/?ref=page_internal.

The Jovian and Saturnian Systems as they will appear during the Great Conjunction on December 21, 2020. Image created using Sky Safari 6 Pro for Mac.

What is a Conjunction?

  • A conjunction is simply an optical alignment of two celestial bodies (normally solar system objects – planets, moon, comet, asteroids, etc.) as seen from our perspective on Earth.
  • Jupiter/Saturn conjunctions are called “Great Conjunctions” by astronomers.
  • A conjunction is only an optical alignment. It does not imply any proximity between the two bodies. (See Figure 1)
  • During the great conjunction of December 21, Jupiter will be 5.927 AU’s from Earth (555 Million miles) while Saturn will be 10.829 AU’s (1 Billion miles), meaning Saturn will still be in excess of 400 million miles from Jupiter.
Figure 1 – From our view here on Earth, both planets will appear to be very close to one another even though they will be hundreds of millions of miles apart.

How often do Great Conjunctions Occur?

  • A conjunction of the planets Jupiter and Saturn occurs once every 19.6 years on average.
  • Due to the nuances of planetary orbits, not every conjunction reaches the same degree of separation. The conjunction of 12/21/2020 will be the closest great conjunction witnessed in the last 400 years (1623).
  • Oddly enough, this degree of separation will be repeated in just another 60 years (in 2080)

How close will they appear to be?

  • At its closest conjunction (appulse), Jupiter and Saturn will be separated by an angle of just over 6 arc-minutes. An arc-minute is one 60th of a degree.
  • For comparison, the full moon has an angular size of 30 arc-minutes, 5 times greater than the separation we will see between Jupiter and Saturn.
  • This means that both planets and many of their respective moons will be visible (see figure 2) within the eyepiece of a medium length telescope at a medium magnification (1-150x). To the naked eye, the two planets may be difficult to “split”. Note: Magnification may be limited by atmospheric conditions as both planets will be very close to the horizon.
Figure 2 – A simulation of the view through a telescope on December 21st. Image created with Stellarium.

When and how can this be viewed?

  • Jupiter and Saturn will be closely spaced for several days before and after minimum conjunction of 12/21.
  • They will be low in the Southwest at dusk.
  • They will set within 90 minutes of sunset.
  • The conjunction can be viewed naked eye, using binoculars, or with a telescope.
  • Weather permitting, the BAA will be providing a “Looking Up!” live stream of the conjunction on at least one evening between 12/18 and 12/23. For more details, check out out our website and Facebook Page as the date approaches.

Clear Skies!