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A typical Galilean telescope with which Jupiter's moons could be observed was configured as follows. It had a plano-convexobjective (the lens toward the object) with a focal length of about 30-40 inches., and a plano-concave ocular with a focallength of about 2 inches. The ocular was in a little tube that could be adjusted for focusing. The objective lens was stoppeddown to an aperture of 0.5 to 1 inch. , and the field of view was about 15 arc-minutes (about 15 inches in 100 yards). Theinstrument's magnification was 15-20. The glass was full of little bubbles and had a greenish tinge (caused by the ironcontent of the glass); the shape of the lenses was reasonable good near their centers but poor near the periphery (hence therestricted aperture); the polish was rather poor. The limiting factor of this type of instrument was its small field ofview--about 15 arc-minutes--which meant that only a quarter of the full Moon could be accommodated in the field. Over the nextseveral decades, lens-grinding and polishing techniques improved gradually, as a specialized craft of telescope makers slowlydeveloped. But although Galilean telescopes of higher magnifications were certainly made, they were almost uselessbecause of the concomitant shrinking of the field.

As mentioned above, the telescopic effect can be achieved with different combinations of lenses and mirrors. As early as 1611,in his Dioptrice , Johannes Kepler had shown that a telescope could also be made by combining a convex objective and a convex ocular. He pointed outthat such a combination would produce an inverted image but showed that the addition of yet a third convex lens would makethe image erect again. This suggestion was not immediately taken up by astronomers, however, and it was not until Christoph Scheiner published his Rosa Ursina in 1630 that this form of telescope began to spread. In his study of sunspots, Scheiner had experimented withtelescopes with convex oculars in order to make the image of the Sun projected through the telescope erect.

The Galilean telescope produces an erect image of an object viewed directly but an inverted image of a projected object;by substituting a convex for the concave ocular, this situation is reversed.
But when he happened to view an object directly through such an instrument, he found that, although the image was inverted, itwas much brighter and the field of view much larger than in a Galilean telescope. Since for astronomical observations aninverted image is no problem, the advantages of what became known as the astronomical telescope led to its generalacceptance in the astronomical community by the middle of the century.

The Galilean telescope could be used for terrestrial and celestial purposes interchangeably. This was not true for theastronomical telescope with its inverted image. Astronomers eschewed the third convex lens (the erector lens) necessary forre-inverting the image because the more lenses the more optical defects multiplied. In the second half of the seventeenthcentury, therefore, the Galilean telescope was replaced for terrestrial purposes by the "terrestrial telescope," which hadfour convex lenses: objective, ocular, erector lens, and a field lens (which enlarged the field of view even further).

Hevelius's 60 foot telescope
Hevelius's 140 foot telescope
(Machina Coelestis, 1673)

Questions & Answers

what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
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
Daniel
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
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
Do somebody tell me a best nano engineering book for beginners?
s. Reply
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
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.
Tarell
what is the actual application of fullerenes nowadays?
Damian
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.
Tarell
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.
Virgil
is Bucky paper clear?
CYNTHIA
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
China
Cied
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
Porter
what is the k.e before it land
Yasmin
what is the function of carbon nanotubes?
Cesar
I'm interested in nanotube
Uday
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
AMJAD
preparation of nanomaterial
Victor 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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