the Sun, not the Earth. And the instru- 

 ment was soon to be given such names 

 as perspicillutn (Galileo's own choice of 

 terminology, from the Latin for "seeing 

 through"), perspective-glasse, spyglass, 

 starrie tubus, and of course telescopium 

 (from the Greek for "far-seeing"). 



If modern astronomy is 

 the child of the telescope, 

 modern astrophysics is 

 the child ot astronomy. Its 

 midwives were two nine- 

 teenth-century techno- 

 logical innovations. The 

 first was photography, lit- 

 erally "light drawing," 

 which yielded straight- 

 forward evidence of an 

 object's existence and vis- 

 ible features. The second 

 was spectroscopy, which separates light 

 into its component colors and enables 

 us to glean scads of information about 

 its source. Today we astrophysicists car- 

 ry out observations and analyses with 

 the aid of telescopes that collect tens 

 of thousands of times more light than 

 did Galileo's first attempts at a spyglass. 

 In addition, we're armed with dozens 

 of auxiliary tools — adaptive optics, 

 digital detectors, spectrometers, super- 

 computers. But no matter the innova- 

 tions, no matter the complexity of the 

 technology, the astrophysicist's funda- 

 mental challenge is to collect light from 

 dim and distant objects, and then ex- 

 tract from that light as much informa- 

 tion as possible. 



Telescopes make it possible for you 

 to detect things too faint to see 

 and to resolve detail where your eyes 

 would otherwise fail you. But a tele- 

 scope is little more than a bucket for 

 catching photons of light that happen 

 your way. Whether your goal is detec- 

 tion or resolution, the bigger your 

 bucket, the better off you'll be. For a 

 circular bucket the collection area in- 

 creases as the square of the diameter, 

 and so if you triple the diameter, you 

 increase the buckets capacity for de- 

 tection ninefold. (Same math applies 

 to the famous pizza equation: a ten- 

 inch pie (10X 10 = 100) has more than 



twice the area of a seven-inch pie 

 (7x7 = 49).) And because the capacity 

 to resolve detail comes from the width 

 ot your lens divided by the wavelength 

 ot light you're measuring, you always 

 want the bucket to be much, much 

 wider than the wavelength of light 





than a bucket 

 it's best 

 le as possible. 



A telescope is little more i 

 for catching photons. And 

 to make the bucket as wic 





you're observing — in fact, as wide as 

 you can afford to make it. 



The telescopes built by Galileo were 

 good enough to detect a couple of 

 auxiliary shapes in Saturn's vicinity, 

 but none of his telescopes could re- 

 solve them into a clear image of the 

 planet's ring system. Haifa century lat- 

 er, Christiaan Huygens's telescope re- 

 solved the shapes into one ring — not 

 just because Huygens had a bigger 

 telescope, which could collect more 

 light, but also because he had a better 

 telescope. Its lenses were more clever- 

 ly shaped and more cleverly positioned 

 in the tube than the lenses in Galileo's 

 instruments. But Huygens's discovery 

 was only the beginning. Subsequent 

 telescopes with ever-better resolution 

 showed the single, wide ring to be 

 made up of two, then several, then 

 many rings. Ultimately, countless in- 

 ternal ringlets of the system revealed 

 themselves in images taken by space 

 probes [see "Ringside Seat," by Neil 

 deGrasse Tyson, October 2004]. 



Lens shape and focal length are two 

 other telescopic features to con- 

 sider. When the telescope was an in- 

 fant invention, and visible light was the 

 only kind of light people were trying 

 to snare, the lens closest to the object — 

 the objective lens — was convex. Like 

 all lenses, it refracted, or bent, the light 



rays that passed through it and brought 

 them to a focus somewhere down the 

 tube. The objective lens was the cen- 

 terpiece of every telescope. It was also 

 a big headache. Its glass had to be free 

 of impurities and its surface unmarred; 

 in addition, the shape of the lens had 

 to be true to the curva- 

 ture of a sphere. Glass- 

 makers of the seven- 

 teenth century could 

 meet those specs. But 

 getting all the rays of 

 light to come into focus 

 at the same point was 

 quite another matter. 

 Steeply curved lenses 

 can cause the light that 

 passes through their 

 various parts to focus at 

 different points along the tube, creat- 

 ing blurnness — spherical aberration — 

 in the image. Mildly curved lenses are 

 better, but the problem persists. 



There's an even more pernicious fo- 

 cusing problem, though. Whenever a 

 ray oflight passes at an angle from one 

 medium to another of different densi- 

 ty — from air to glass, from glass back in- 

 to air, even from one kind of lens into 

 another kind of lens — not only does the 

 passage bend the light, but it also splits 

 the light into its component colors, or 

 wavelengths. Trouble is, each color of 

 light bends at a slightly different angle, 

 and so it comes into focus at a slightly 

 different point. When you're looking 

 through a lens, whichever color you fo- 

 cus on — red, green, blue, or anywhere 

 else in the rainbow — you see that col- 

 or at the center of the object, sur- 

 rounded by fuzzy rings of each of the 

 remaining colors. The whole dismal ef- 

 fect is called chromatic aberration. 



By the mid-seventeenth century, a 

 common way to correct for both kinds 

 of aberration was to add more lenses 

 to your telescope. The idea was that 

 the light rays would bend and re-bend, 

 refract and re-refract, and eventually 

 come to focus in the same plane. The 

 lenses had to be positioned in a clever 

 sequence and at just the right position 

 along the length of the tube. Some 

 (Continued on page 29) 



NATURAL HISTORY March 2006 



