Big Telescopes on a Roll: The World Has a Number of Projects for Telescopes in a Size Range Thought to Be Impossible Not Many Years Ago

By Thomsen, Dietrick E. | Science News, September 12, 1987 | Go to article overview

Big Telescopes on a Roll: The World Has a Number of Projects for Telescopes in a Size Range Thought to Be Impossible Not Many Years Ago


Thomsen, Dietrick E., Science News


Big Telescopes on a Roll

The first astronomical telescope was two lenses in a tube; Galileo could hold it in his hand. Today's telescopes are so big that mountaintops sometimes have to be sheared off to make room for them, and they are getting even bigger.

Until about a decade ago, astronomers thought they had reached the practical limit of size with telescope mirrors of 5 or 6 meters diameter. Now, thanks to technological developments in the construction and management of large mirrors, these limits are being surpassed, even doubled. The world now has approximately 10 projects in different stages of construction, planning or discussion that intend to use mirrors larger than 6 meters.

Glass was the greatest hindrance to building large mirrors. To image the sky properly, a telescope mirror must keep the shape of its curved surface precise. The necessary stiffness seemed to require a thick backing of glass behind the reflecting surface. As diameters got up to 6 meters or more, a catch-22 came in: In large telescope mirrors the amount of glass seemingly required for stiffness would slump under its own weight and distort the reflecting surface it was supposed to maintain.

Two solutions are actively being tried. One stops relying on the glass for stiffness. Such plans envision a segmented mirror with what telescope designers call "active support,' an arrangement of levers and thrusters that holds the overall shape of the mirror, compensating for gravity, wind stress and other distorting factors.

The second solution is still "passive.' It relies on the stiffness of the glass but aims to prevent slumping by leaving out most of the glass. It turns out that this is possible if the back of the mirror is in the form of a proper kind of honeycomb shape rather than a solid slab.

Such honeycombing seems first to have been tried with the 5-meter mirror on Palomar Mountain. J. Roger Angel and his associates at the University of Arizona in Tucson have developed it into a production-line technique, in which plugs of water-soluble material set in the bottom of the mold make the voids in the honeycomb and are then washed out of the finished mirror with a high-pressure stream of water.

A major innovation of Angel's group is to use a rotating mold. Rotation gives the upper surface of the telescope blank a paraboloidal surface instead of the flat surface of ordinary casting. A paraboloid is the surface shape most telescope designers want, and so starting with it greatly simplifies the grinding and polishing of the surface. For a large conventional mirror, shaping and grinding take years, and the time and expense of this step posed another hindrance to planning mirrors bigger than 5 meters.

The recent developments have meant, among other things, that two people are having significant influences on an entire generation of new telescopes. Nearly all the current large projects have had Harland Epps of the University of California at Los Angeles involved in the design of their optics, and most of them will want mirrors of the type Angel casts.

Angel's group started out with a 2-meter rotating furnace. Having successfully cast 2-meter mirrors, they began about three years ago to build a new casting shop under the stands of the football stadium on the Tucson campus with the intention of ultimately casting 8-meter mirrors (SN: 2/16/85, p.106; 1/17/87, p.40). A session on the status of the large-telescope projects at the recent Workshop on Instrumentation for Groundbased Optical Astronomy, held at the University of California at Santa Cruz, heard from John Hill of the University of Arizona that the 8-meter turntable has just been completed. It now carries a furnace of 3.5 meters diameter for its first project, a mirror of that size for the projected ARC telescope, expected to be cast in November.

Later Angel and his colleagues expect to cast another 3. …

The rest of this article is only available to active members of Questia

Sign up now for a free, 1-day trial and receive full access to:

  • Questia's entire collection
  • Automatic bibliography creation
  • More helpful research tools like notes, citations, and highlights
  • Ad-free environment

Already a member? Log in now.

Notes for this article

Add a new note
If you are trying to select text to create highlights or citations, remember that you must now click or tap on the first word, and then click or tap on the last word.
One moment ...
Default project is now your active project.
Project items

Items saved from this article

This article has been saved
Highlights (0)
Some of your highlights are legacy items.

Highlights saved before July 30, 2012 will not be displayed on their respective source pages.

You can easily re-create the highlights by opening the book page or article, selecting the text, and clicking “Highlight.”

Citations (0)
Some of your citations are legacy items.

Any citation created before July 30, 2012 will labeled as a “Cited page.” New citations will be saved as cited passages, pages or articles.

We also added the ability to view new citations from your projects or the book or article where you created them.

Notes (0)
Bookmarks (0)

You have no saved items from this article

Project items include:
  • Saved book/article
  • Highlights
  • Quotes/citations
  • Notes
  • Bookmarks
Notes
Cite this article

Cited article

Style
Citations are available only to our active members.
Sign up now to cite pages or passages in MLA, APA and Chicago citation styles.

(Einhorn, 1992, p. 25)

(Einhorn 25)

1

1. Lois J. Einhorn, Abraham Lincoln, the Orator: Penetrating the Lincoln Legend (Westport, CT: Greenwood Press, 1992), 25, http://www.questia.com/read/27419298.

Cited article

Big Telescopes on a Roll: The World Has a Number of Projects for Telescopes in a Size Range Thought to Be Impossible Not Many Years Ago
Settings

Settings

Typeface
Text size Smaller Larger Reset View mode
Search within

Search within this article

Look up

Look up a word

  • Dictionary
  • Thesaurus
Please submit a word or phrase above.
Print this page

Print this page

Why can't I print more than one page at a time?

Full screen

matching results for page

Cited passage

Style
Citations are available only to our active members.
Sign up now to cite pages or passages in MLA, APA and Chicago citation styles.

"Portraying himself as an honest, ordinary person helped Lincoln identify with his audiences." (Einhorn, 1992, p. 25).

"Portraying himself as an honest, ordinary person helped Lincoln identify with his audiences." (Einhorn 25)

"Portraying himself as an honest, ordinary person helped Lincoln identify with his audiences."1

1. Lois J. Einhorn, Abraham Lincoln, the Orator: Penetrating the Lincoln Legend (Westport, CT: Greenwood Press, 1992), 25, http://www.questia.com/read/27419298.

Cited passage

Welcome to the new Questia Reader

The Questia Reader has been updated to provide you with an even better online reading experience.  It is now 100% Responsive, which means you can read our books and articles on any sized device you wish.  All of your favorite tools like notes, highlights, and citations are still here, but the way you select text has been updated to be easier to use, especially on touchscreen devices.  Here's how:

1. Click or tap the first word you want to select.
2. Click or tap the last word you want to select.

OK, got it!

Thanks for trying Questia!

Please continue trying out our research tools, but please note, full functionality is available only to our active members.

Your work will be lost once you leave this Web page.

For full access in an ad-free environment, sign up now for a FREE, 1-day trial.

Already a member? Log in now.