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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 can chip be made from sand
Eke Reply
is this allso about nanoscale material
are nano particles real
Missy Reply
Hello, if I study Physics teacher in bachelor, can I study Nanotechnology in master?
Lale Reply
no can't
where is the latest information on a no technology how can I find it
where we get a research paper on Nano chemistry....?
Maira Reply
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
Application of nanotechnology in medicine
has a lot of application modern world
what is variations in raman spectra for nanomaterials
Jyoti Reply
ya I also want to know the raman spectra
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
Nasa has use it in the 60's, copper as water purification in the moon travel.
nanocopper obvius
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
analytical skills graphene is prepared to kill any type viruses .
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
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
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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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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