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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 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
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
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
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
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
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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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