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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

how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
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Maciej
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s. Reply
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Devang Reply
are you nano engineer ?
s.
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Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
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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
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s.
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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
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what's the easiest and fastest way to the synthesize AgNP?
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Cied
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abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
Porter
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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
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what is system testing?
AMJAD
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
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AMJAD
what is system testing
AMJAD
what is the application of nanotechnology?
Stotaw
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
Azam
anybody can imagine what will be happen after 100 years from now in nano tech world
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after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
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Damian
silver nanoparticles could handle the job?
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not now but maybe in future only AgNP maybe any other nanomaterials
Azam
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I'm interested in Nanotube
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this technology will not going on for the long time , so I'm thinking about femtotechnology 10^-15
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how did you get the value of 2000N.What calculations are needed to arrive at it
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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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