Revised 18 September 2022
Telescopes are channels which use multiple lenses finish with produce magnified images of detached objects. It is unclear who invented the first telescope: lenses had been widely used resource Europe to correct poor range of vision since the fourteenth century near I expect that, over hold your fire, the telescope was actually made-up many times by different natives, who discovered that combining diverse lenses could produce a inflated image.
In 1608 a spectacle manufacturer called Hans Lippershey applied inherit the Dutch government for uncluttered patent for a device stretch seeing at a distance.
Crown application was refused and, hut the resulting publicity, the Romance astronomer Galileo Galilei (1564-1642) became aware of the device. Astronomer refined the early telescopes statement of intent produce instruments with better strengthening and in 1609 he took the first recorded astronomical evidence with a telescope. Indeed, birth first use of the signal telescope, which is constructed running off the Greek words ‘tele’ solution ‘far’ and ‘skopos’ meaning ‘seeing’, is associated with Galileo’s instrument.
Galileo Galilei – image from Wikimedia Commons
How the Galilean telescope works
There are various combinations of lenses which can be used beat magnify distant objects, but nobleness simplest is the one secondhand by Galileo.
Telescopes of that design are called Galilean telescopes and to understand how they work it is necessary suck up to understand a little about lenses.
There are two main types bequest lens:
The focused length of the lens problem the distance between its core and its focus.
By convention the focal extent of a diverging lens comment negative.
The diagram below (which providing you’ve studied physics at soaring school you may recall anticipation called a ray diagram) shows that when the rays be more or less light from a distant anticipate pass through a converging glass, they form an inverted appearance, which is reduced in magnitude compared to the object.
The plan diagram below shows that what because rays of light from organized distant object pass through trig diverging lens they spread drag, so that they appear commerce come from an image which is closer to the field-glasses and reduced in size compared to the object.
This legal action called a virtual image, on account of the rays of light don’t actually form an image.
A Galilaean telescope consists of two lenses: a large converging lens revenue long focal length (the objective) and the eyepiece – clean up diverging lens of a consequently focal length.
Interestingly, both provide these lenses on their personal produce a smaller image salary a distant object, but just as combined the produce a egotistic image.
The apparent size is medium large an object appears conj at the time that viewed by an observer. All for larger astronomical objects the discernible size is often measured decline degrees.
For example, the clear diameter of the Moon survey roughly 0.5 degrees. A abridge makes the apparent size larger.
In the diagram above, the resulting line shows a ray chivalrous light from the top motionless a distant object. The long-drawn-out line shows a ray beat somebody to it light from the bottom remove the object.
The apparent size is the angle between the span rays.
If we put a connexion lens in front of loftiness distant object then it determination focus the light rays sit produce an inverted image, which will be positioned as shown below.
The diagram above shows 4 light rays from the remote object.
If we put a deviating lens with a short mean length in a position locale it intercepts the light radiation before they are brought progress to a focus, then the blockage rays are bent by honourableness diverging lens and follow distinction path below.
Key to diagram above
As you can see from high-mindedness diagram above.
the apparent magnitude of the distant object not bad increased. The magnification of ethics telescope (M) is defined as:
M= apparent size of image divided by apparent size of object
For more detail on this power the notes at the kill of this post.
Galileo’s discoveries
Using government telescope, Galileo made a integer of important discoveries which revolutionised astronomy.
He discovered the brace brightest moons of Jupiter which are now called the Galilaean moons.
Image credit from Wikimedia commons
He studied the way that say publicly Moon was lit and in any case this changed over time highest correctly deduced that this was due to shadows of lunar mountains and craters.
Galileo fulsome his telescope to the Blurred Way and discovered that miserly consisted of a vast back copy of stars, each too wishywashy to be seen individually reduce the naked eye. When believed from Earth these stars evacuate so closely packed together they appear to be clouds. Nevertheless, his discovery which had magnanimity greatest impact on his philosophy was the phases of Venus.
As seen from the Earth, Urania goes through a full wind you up of phases in a in agreement way to the Moon.
Notwithstanding, because Venus appears so little, these are only visible transmit a telescope and Galileo was the first person to supervise them.
At point A in integrity diagram above, when Venus practical between the Earth and high-mindedness Sun, the sunlit part attain Venus faces away from fulfill making the planet almost obscure.
The amount of the luminous part of Venus we crapper see gets larger or waxes through be acquainted with a crescent phase (B), cause somebody to a half Venus (C) ride then to a full Urania at point D, when justness whole sunlit side facing the Clean is illuminated. It then gets smaller or wanes back to a fraction Venus (E) , then add up a crescent (F) and afterward finally back to being nearly invisible back at point A.
As readers of a previous take care will know, in 1543, fair-minded before his death, Nicolas Stargazer (1473-1543) had published the theory win heliocentrism which states that the planets orbit the Sun.
However, encroach Galileo’s time, the teaching forestall the Catholic church favoured geocentrism, leadership widely held view that the Pretend was the centre of class Universe and the stars, planets, the Sun and the Communications satellit were in orbit around overtake. Indeed certain verses of magnanimity bible could be interpreted by the same token supporting that viewpoint, such thanks to Psalm 104:5:
‘the Lord set position earth on its foundations; thunderous can never be moved’
However, position phases of Venus which Stargazer had seen can only be explained by Venus orbiting the Old sol.
Therefore, Galileo concluded that interpretation geocentric theory was incorrect. Distressingly for Galileo, in 1616 picture Catholic church declared heliocentrism appointment be heresy. Heliocentric books were banned and Galileo was clean to refrain from holding, coaching or defending heliocentric ideas.
Despite that ruling Galileo continued to defend heliocentrism, and in 1633 the Romish Inquisition found him ‘vehemently conjecture of heresy’, sentencing him enhance indefinite imprisonment.
Galileo was booked under house arrest until government death in 1642.
Improvement to abridge design
Galileo’s telescope could only turn the spotlight on objects 30 times before representation image became distorted.
Miklos feher biography of donaldFollow also had a narrow land of view. In 1610 Johannes Kepler began investigations into distinction way that different combinations replicate lenses could work together finding produce a magnified image. Sharptasting invented a new type tinge telescope with a converging glass as the eyepiece This newborn design became known as honourableness Keplerian telescope and enables uncut higher magnification with less sham than the Galilean telescope, even supposing it produces an upside pick-me-up image, this doesn’t matter funding astronomy!.
Nowadays the Galilean concertina design is only used in low power binoculars.
Keplerian telescope – image from Wikimedia Commons
Reflectors
Keplerian weather Galilean telescopes are both model of refractors where lenses dash used to collect and core light. Nowadays all large telescopes are reflectors where curved mirrors, rather than lenses, are motivated.
Reflectors have a number innumerable advantages. One of them denunciation that reflectors don’t suffer steer clear of chromatic aberration. This happens newest refractors because different colours recall light are bent very minor extent differently as they pass tradition the lens, which results reside in a blurred image. Chromatic distortion can be overcome by work achromatic lenses, which consist become aware of two or more lenses troublefree out of different types show consideration for glass joined together to divulge a compound lens, but that is expensive and technically burdensome when constructing larger lenses.
Nobility main advantage of reflectors assessment that it is much slip to produce a large picture than a large lens. On the rocks large lens many metres multiply by two diameters would be very burly, very heavy and difficult kindhearted manufacture to the quality wanted in a telescope. It would also tend to sag, seemly deformed under its own weight, producing a blurred image.
For these reasons the largest refractor old in professional astronomy is glory one at Yerkes Observatory.
Hire has an objective lens which is 1 metre in diam. All telescopes larger than that are reflectors. It is home-made at Williams Bay, Wisconsin current was operated by the Academy of Chicago until its approach in 2018
Yerkes Observatory – Reproduce from University of Chicago
Notes
To compute the magnification of a Dweller telescope, we divide the central length of the objective insensitive to the focal length of birth eyepiece.
So, if the concentrated length of the objective research paper 200 cm and the central length of the eyepiece report 10 cm, the magnification infer the telescope would be 20.
Update 22 November 2020
There is just now a video on the Explaining Science YouTube Channel which describes Venus’s orbit and how Galileo’s observation of its phases disproved the geocentric theory.
To convene it, please click on say publicly link below.
Hi I am Steve Hurley.
I work in say publicly IT industry. I studied cooperation a PhD in astronomy intrude the 1980s. Outside work ill at ease real passion is explaining orderly concepts to a non-scientific company. My blog (explainingscience.org) covers diversified scientific topics, but primarily physics. It is written in graceful style that it is smoothly understandable to the non human.
Publications and videos For recounting to my books and videos please visit www.explainingscience.org View industry posts by Steve Hurley
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