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

 

 

Got a New Telescope. So What Now?

Did you or someone in your family get a new telescope? Great welcome to a fantastic hobby! You’re all set, just open the box, point up and you’re good to go. Well, not really. Using a telescope is a learned skill. Sometimes it can be quite challenging, even frustrating. We’ve all been there. No one is born knowing all this stuff. Talk to most amateur astronomers and you’ll see that that had some help along the way. That’s why we are so eager to help out newcomers. That’s the way it is with this hobby. So here’s some friendly advice to help you get started with that new telescope.

1.) Find a local Astronomy Club

That would be us. If you’re reading this, you’ve come to the right place. Most clubs have programs accessible to the public, so you don’t have to join. Given the current circumstances with the Pandemic, we’re not certain when we will hold in-person public events again. In the meantime, we are holding virtual events live-streamed to our Facebook page. Check out our events page on this website to keep track of our events. You can ask questions, we try to answer all the questions in the comments. Like what you see/hear, consider joining. We offer more opportunities to interact and learn from other members.

2.) Set it up and Learn to Use it in the Daytime

Put your telescope together while it’s still light or inside in a well-lit space. Once your telescope is assembled, point it at any distant object(s) (a telephone pole, mailbox, distant tree, etc..). NOTE: NEVER POINT YOUR TELESCOPE AT THE SUN!!! Learn how to move the telescope from object to object, align the finder (more on that later), work the focuser, and switch eyepieces. Try practicing during the day, where it is easy to see instead of fumbling around in the dark.

3.) Learn how to Align the Finder

Most telescopes come with a finder. They can take a few different forms. Some may be Red Dot finders and others may look like a mini-telescope. Either way, their purpose is the same, to assist in pointing telescopes. In order for them to work, they must be aligned with the main telescope. This is best done in daylight. If you plan on observing for the night, set up your scope before dark and check/align your finder. Using your eyepiece with the widest field of view, this will be the eyepiece with the longest focal length (biggest number). Center a terrestrial object (mailbox, top of a telephone pole, tower, streetlight, etc.) in the field of view of the telescope. Look through the finder and see where it is pointing.

A typical Red Dot finder.
A typical Finder Scope. This is a 6 x 30 Straight Through Finder Scope.

Most finders have some sort of a reticule (Red Dot, Circles, Cross-hairs, etc.). Is this reticule pointing at the same thing centered in the eyepiece? Most likely it isn’t. There should be some adjustment screws/knobs on the finder to move it (up/down and left/right). Make the necessary adjustments so that the finder is pointed on the same thing centered in the main telescope when you look through the eyepiece. Here’s the important part, keep checking the eyepiece of the main telescope to make sure the telescope has not been moved when making adjustments. Re-center the object and continue with adjustments if the telescope was moved. Now, wherever your finder is pointing the telescope should be pointing. Optional. If you want to refine the alignment, replace the long focal length eyepiece with a smaller focal length eyepiece, increasing the magnification and providing a narrower field of view. Repeat the alignment process with this eyepiece to really dial in the alignment.

4.) Use Long Focal Length Eyepieces to Find Objects

Your telescope may have come with more than one eyepiece. The eyepiece with the longer focal length (bigger number) will provide less magnification and therefore a wider field of view.

This 25 mm focal length eyepiece is an example of a long focal length eyepiece, providing a generous field of view for finding objects and viewing objects with large angular size.

When looking for objects, use the longest focal length eyepiece you have. This will help increase the chances of the object you’re looking for will be in the field of view once you point the telescope using your finder. Once in the field of view, center the object and if desired, switch to a smaller focal length eyepiece to increase the magnification. Note, if the atmospheric conditions are poor, increasing the magnification may not always provide a favorable result.

This 10 mm eyepiece is an example of n eyepiece that will increase magnification once an object is found and centered in the field of view with a longer focal length eyepiece.

Hope these tips help you get started in a wonderful hobby that can inspire you and challenge you for many years to come. If you find yourself having difficulty and getting frustrated. Cut yourself some slack. As I said in the beginning, this is a learned skill. No one is born knowing how to do all this stuff. Seek out help. Most of us had a lot of help along the way and we’re glad to help others.

Clear Skies