U N I VERSE (Continued from page 20) 



might have one flat side, and the 

 amount of curvature on the other side 

 might vary. Others might be made of 

 glass of a different density. But despite 

 all the challenges, some astronomers 

 and opticians built some impressive re- 

 fracting telescopes. 



Lenses refract; mirrors reflect. Both 

 relay light from one place to anoth- 

 er. The tenth-century 

 Iraqi mathematician 

 Abu All al-Hasan ibn 

 al-Haytham (Alhazen) 

 understood this. Gali- 

 leo understood this. 

 Descartes understood 

 this. Seventeenth-cen- 

 tury astronomers quick- 

 ly realized that using a 

 mirror rather than a lens 

 could eliminate chro- 

 matic aberration. Instead 

 of being bent out of shape and refracted 

 into its component colors, light would 

 just bounce off the mirror — intact. 



Unlike the silver- or aluminum- 

 backed transparent glass mirrors with 

 which people mercilessly examine 

 their personal flaws, astronomical mir- 

 rors can be made of an opaque, pol- 

 ished material. In fact, they have to be: 

 the light rays must be reflected off a 

 front surface, not refracted through in- 

 tervening glass. So the entire back, not 

 merely the edges, of an astronomical 

 mirror can be designed to strengthen 

 and support the mirror. The size of 

 any mirror designed on this principle 

 can far surpass the size of a clear, re- 

 fracting lens. But how do you look at 

 the focused light without having your 

 head get in the way? 



Enter Isaac Newton and the re- 

 flecting telescope, closely preceded 

 and followed by a couple of less fa- 

 mous contemporaries. Farewell, chro- 

 matic aberration. 



Newton made his mirrors of cop- 

 per-tin alloy, with a pinch of arsenic to 

 increase the alloy's reflectivity. And in- 

 stead of applying his powdery polish- 

 ing abrasive with leather or cloth, he 

 used pitch — a method still popular to- 

 day. As for the best shape and arrange- 



ment for his telescope's components, 

 Newton's solution was simplicity itself: 

 a large, concave primary mirror col- 

 lects and reflects the light onto a small- 

 er, flat, tilted secondary mirror. The 

 secondary mirror redirects the reflect- 

 ed light out the side of the tube; since 

 it is flat, the secondary doesn't alter the 

 image except to swap left with right. 

 The viewer looks at the image in the 



The world's most famous mirror, 

 however, is the primary on the beloved 

 Hubble Space Telescope, fashioned 

 from an eight-toot-wide, flat block of 

 glass. It was to be a perfectly polished 

 hyperboloid. Perfectly polished it was: if 

 the mirror were the size of Texas, its 

 biggest bump would be half an inch 

 high. A few hours after the Hubbies 

 launch in April 1990, though, test ob- 

 servations showed that the 

 mirror suffered from seri- 



Technicians recently cast 40,000 pounds 

 of glass for a telescope mirror 

 twenty-seven feet in diameter. 



secondary mirror through an eyepiece 

 inserted in the side of the tube, happi- 

 ly removed from the path of the light 

 entering at the front. Although the pri- 

 mary mirror of Newton's telescope was 

 less than an inch and a half across, and 

 its metal less than brilliantly reflective, 

 on January 1 1, 1672, Newton demon- 

 strated its workings for members of the 

 scientific organization known (both 

 then and now) as the Royal Society. 

 Duly impressed, they awarded him the 

 rank of Fellow. 



Improvements and enlargements fol- 

 lowed fast and furious. Nowadays 

 the most massive, most modern, and 

 most famous telescopes, regardless of 

 the wavelengths they gather, all rely on 

 mirrors rather than lenses as their pri- 

 mary light bucket. Just this past No- 

 vember, astronomers and technicians 

 at the University of Arizona in Tucson 

 finished casting 40,000 pounds of glass 

 for a mirror twenty-seven feet in di- 

 ameter. That mirror and five more like 

 it will surround a seventh giant mirror 

 on what will be (at least for a while) 

 the world's largest optical telescope: 

 the Giant Magellan Telescope, sched- 

 uled to be up and running in northern 

 Chile in about a decade. 



ous spherical aberration. 

 • Its outer edge turned out 

 to be 0.0001 inch too flat, 

 yet that deviation was 

 enough to make it practi- 

 cally useless for image- 

 taking. Happily for us all, 

 astronauts fixed the prob- 

 lem during the first ser- 

 vicing mission to the Hub- 

 ble, in December 1993. 

 They gave the telescope a set of correc- 

 tive lenses — eyeglasses, if you will — that 

 perfectly compensated for the error. 



Astrophysicists these days aren't con- 

 tent with just a bucket o' photons. We 

 analyze the properties of detected 

 light — its spectra — because from its 

 spectra we can often extract the source's 

 distance, temperature, chemical com- 

 position, motion through space, rota- 

 tion, polarization, and surrounding 

 magnetic fields. Those are our data. 

 And, just as the wine lover wants a 

 wineglass to be so thin that it is nearly 

 absent as a boundary between lips and 

 wine, the astrophysicist wants extrane- 

 ous influences to be as absent as pos- 

 sible from the data. Sleepy observers in- 

 troduce too many extraneous things in- 

 to the data stream, particularly if their 

 skill at drawing what they've seen is 

 variable. Almost as bad as a sleepy as- 

 tronomer is the atmosphere's habit of 

 altering a photon's path to a ground- 

 based telescope. 



Twentieth-century photography got 

 faster and faster as the decades rolled 

 by, minimizing the problem of record- 

 ing data accurately. And launching 

 telescopes into space eliminated the 

 problem of atmospheric turbulence. 

 (Continued on page 66) 



March 2006 NATUK.AI HISTORY 



