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With the acceptance of the astronomical telescope, the limit on magnification caused by the small field of view of the Galileantelescope was temporarily lifted, and a "telescope race" developed. Because of optical defects, the curvature of lenseshad to be minimized, and therefore (since the magnification of a simple telescope is given roughly by the ratio of the focallengths of the objective and ocular) increased magnification had to be achieved by increasing the focal length of theobjective. Beginning in the 1640s, the length of telescopes began to increase. From the typical Galilean telescope of 5 or 6feet in length, astronomical telescopes rose to lengths of 15 or 20 feet by the middle of the century. A typical astronomicaltelescope is the one made by Christiaan Huygens, in 1656. It was 23 feet long; its objective had an aperture of several inches,it magnified about 100 times, and its field of view was 17 arc-minutes.

Aerial telescope (Christiaan Huygensm AstroscopiumCompendiaria,1684)

Telescopes had now again reached the point where further increases in magnification would restrict the field of view ofthe instrument too much. This time another optical device, the field lens came to the rescue. Adding a third convex lens--of appropriate focal length, and in the right place--increased thefield significantly, thus allowing higher magnifications. The telescope race therefore continued unabated and lengthsincreased exponentially. By the early 1670s, Johannes Hevelius had built a 140-foot telescope.

But such long telescopes were useless for observation: it was almost impossible to keep the lenses aligned and any wind wouldmake the instrument flutter. After about 1675, therefore, astronomers did away with the telescope tube. The objective wasmounted on a building or pole by means of a ball-joint and aimed by means of a string; the image was found by trial and error;and the compound eyepiece (field lens and ocular), on a little stand, was then positioned to receive the image cast by theobjective. Such instruments were called aerial telescopes .

Although some discoveries were made with these very long instruments, this form of telescope had reached its limits. Bythe beginning of the eighteenth century very long telescopes were rarely mounted any more, and further increases of powercame, beginning in the 1730s, from a new form of telescope, the reflecting telescope.

Since it was known that the telescopic effect could be achieved using a variety of combinations of lenses and mirrors, a numberof scientists speculated on combinations involving mirrors. Much of this speculation was fueled by the increasingly refinedtheoretical study of the telescope. In his Dioptrique , appended to his Discourse on Method of 1637, RenèDescartes addressed the problem of spherical aberration, already pointed out byothers. In a thin spherical lens, not all rays from infinity--incident parallel to the optical axis--are united atone point. Those farther from the optical axis come to a focus closer to the back of the lens than those nearer the opticalaxis. Descartes had either learned the sine law of refraction from Willebrord Snell (Snell's Law)

The ratio of the sines of the angles of incidence and refraction is constant.
or had discovered it independently, and this allowed him to quantify spherical aberration. In order to eliminate it, heshowed, lens curvature had to be either plano-hyperboloidal or spherico-ellipsoidal. His demonstration led many to attempt tomake plano-hyperboloidal objectives,
The effect is most apparent for the objective; spherical aberration in the ocular affects the image much less.
an effort which was doomed to failure by the state of the art of lens-grinding. Others began considering the virtues of a concaveparaboloidal mirror as primary receptor: it had been known since Antiquity that such a mirror would bring parallel incident raysto a focus at one point.
Newton's reflecting telescope (1671)

A second theoretical development came in 1672, when Isaac Newton published his celebrated paper on light and colors. Newtonshowed that white light is a mixture of colored light of different refrangibility: every color had its own degree ofrefraction. The result was that any curved lens would decompose white light into the colors of the spectrum, each of which comesto a focus at a different point on the optical axis. This effect, which became known as chromatic aberration, resulted ina central image of, e.g., a planet, being surrounded by circles of different colors. Newton had developed his theory of lightseveral years before publishing his paper, when he had turned his mind to the improvement of the telescope, and he haddespaired of ever ridding the objective of this defect. He therefore decided to try a mirror, but unlike his predecessorshe was able to put his idea into practice. He cast a two-inch mirror blank of speculum metal (basically copper with some tin)and ground it into spherical curvature. He placed it in the bottom of a tube and caught the reflected rays on a 45°secondary mirror which reflected the image into a convex ocular lens outside the tube (see ). He sent this little instrument to the Royal Society, where it caused asensation; it was the first working reflecting telescope. But the effort ended there. Others were unable to grind mirrors ofregular curvature, and to add to the problem, the mirror tarnished and had to be repolished every few months, with theattending danger of damage to the curvature.

Hevelius's rooftop observatory, (Machina Coelestis, 1673)

The reflecting telescope therefore remained a curiosity for decades. In second and third decades of the eighteenth century,however, the reflecting telescope became a reality in the hands of first James Hadley and then others. By the middle of thecentury, reflecting telescopes with primary mirrors up to six inches in diameter had been made. It was found that for largeaperture ratios (the ratio of focal length of the primary to its aperture, as the f-ratio in modern cameras for instance), f/10or more, the difference between spherical and paraboloidal mirrors was negligible in the performance of the telescope. Inthe second half of the eighteenth century, in the hands of James Short and then William Herschel, the reflecting telescope withparabolically ground mirrors came into its own.

Questions & Answers

How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
How can I make nanorobot?
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
how can I make nanorobot?
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
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Source:  OpenStax, Galileo project. OpenStax CNX. Jul 07, 2004 Download for free at http://cnx.org/content/col10234/1.1
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