Bob Doyle, Columnist
A few weeks ago, I purchased a new book on space, of which I have a number. Even in the book store, I was struck by the excellent illustrations and accompanying text (on nearly every page).
“Space: A Visual Encyclopedia” is another DK science book (DK = Dorling Kindersley) listing a consultant (Peter Bond), a veteran space writer and a number of designers and editors.
This large format book has ISBN 978-0-7566-6277-6, was published in 2010 and costs about $25. “Space” has about 250 pages, including a glossary and index.
For students in middle school and novices to astronomy, this book offers an excellent introduction to the universe. There are nine sections and in this column, I will consider the telescope material in the first section, Observing the Heavens.
The history of the development of telescope starts with Hans Lippershey in 1608. Then Galileo in 1610 published his telescopic discoveries (the large moons of Jupiter, the craters on the moon, the lighted shapes of Venus).
Lippershey and Galileo’s telescopes used only simple lenses, which made the majority of early celestial discoveries.
In 1668, Isaac Newton invented a small telescope that used a concave metallic mirror to form an image.
In the late 1700s, William Hershel began to make much larger mirror telescopes, allowing fine views of gas clouds and star clusters. During the 1800s, there were fine telescopes of both kinds built.
The two largest lens telescopes were both in America: the Yerkes 40-inch telescope in Wisconsin and the Lick 36-inch telescope in California.
The dimensions stated are the width of the front lens or objective. Both telescopes had tubes about a dozen times as long as their lens width.
Around 1900, large high quality reflecting telescopes began to be built on California mountain tops.
Combined with precise motorized mounts that could accurately track the stars, the photographs of these telescopes revealed the delicate tendrils of gas clouds in the Milky Way.
Also captured were the highly concentrated globular clusters that are similar to swarms of bees flying through the halo of our galaxy.
These large reflecting telescopes extend about three times as far (in length) as their mirrors are wide.
The 100-inch telescope at Mount Wilson made photographic observations resolving the individual stars in a nearby spiral galaxy (in the star group of Andromeda).
A few years later, this same telescope found that all distant galaxies were moving away from us. In fact, the farther away, the faster they moved. The only reasonable explanation is that the universe is expanding, with space stretching between galaxies.
More sensitive digital detectors (larger versions of the image receptors in our digital cameras) replaced the photographic plates several decades ago, capturing 50 times as many photons (units of light) as film.
Today, some talented amateurs using backyard telescopes can produce images of gas clouds and galaxies that rival the mountain top observatory photographs of the 1950’s and 1960’s.
Improvements in mirror making has allowed a laboratory in Arizona to turn out 27.5-foot wide mirrors (331 inches) which have been used in telescopes in Hawaii, Chile and Arizona. Another technique is to produce smaller hexagonal mirrors that can be honeycombed to produce even larger mirror widths.
Before the end of this decade, there will be a 138-foot wide mirror assembly operating in Chile. This telescope is called the E-ELT or European Extremely Large Telescope.
On Hawaii’s Mauna Kea mountain top will stand the Thirty Meter Telescope, whose primary compound mirror will be 98 feet across.
Bob Doyle invites any readers comments and questions. E-mail him at firstname.lastname@example.org . He is available as a speaker on his column topics.