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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 can chip be made from sand
Eke Reply
are nano particles real
Missy Reply
yeah
Joseph
Hello, if I study Physics teacher in bachelor, can I study Nanotechnology in master?
Lale Reply
no can't
Lohitha
where we get a research paper on Nano chemistry....?
Maira Reply
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
Ali
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..
learn
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
learn
Google
da
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Bhagvanji
hey
Giriraj
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
revolt
da
Application of nanotechnology in medicine
has a lot of application modern world
Kamaluddeen
yes
narayan
what is variations in raman spectra for nanomaterials
Jyoti Reply
ya I also want to know the raman spectra
Bhagvanji
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.
Damian
yes that's correct
Professor
I think
Professor
Nasa has use it in the 60's, copper as water purification in the moon travel.
Alexandre
nanocopper obvius
Alexandre
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
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
Rafiq
if virus is killing to make ARTIFICIAL DNA OF GRAPHENE FOR KILLED THE VIRUS .THIS IS OUR ASSUMPTION
Anam
analytical skills graphene is prepared to kill any type viruses .
Anam
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
Hafiz
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
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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