Chapter 6 Optics and Telescopes. Optical telescopes Two types –refractors (use lenses)...

$: Chapter 6 Optics and Telescopes. Optical telescopes Two types –refractors (use lenses) –reflectors (use mirrors) Both focus light from a large opening.$
Chapter 6

Optics

and

Telescopes

Optical telescopes

• Two types– refractors (use lenses)– reflectors (use mirrors)

• Both focus light from a large opening to a smaller opening (eyepiece), with magnification.

Refracting telescopes

• The first telescope ( a refractor) was invented in early 17th

century.• Refracting telescopes makes use of a lens to collect light.• All lenses make use of a physical phenomena called

Refraction. – Light travels at a slower speed in a dense substance. – Speed of light in a vacuum is 3.0 x 108 m/s.– Speed of light in glass is less than 2 x 108 m/s.– Therefore, when light travels from a rare medium to a

denser medium, light bends!

The study of Light is Optics

Refraction and Lenses

• Previous figure showed the refraction (or bending) of a beam of parallel light passing from a vacuum to a glass.– The amount of bending depends on the speed of

light in glass.

• If the glass is curved instead of flat…..– When parallel rays of light falls on a convex

(glass) lens, refraction cause all the rays to converge at a point called the focal point.

• The distance from the lens to the focal point is called the focal length.

Refraction and Lenses

When a beam of parallel light rays pass through a convex lens, refraction causes the rays to converge to a point - focal point.

Image from a Lens

•Light rays from a point source radiate in all directions.

•If the lens is at a great distance from the source, the rays arriving are essentially parallel.

•The rays will then converge onto the focal point giving rise to an image at this point.

Image of an extended object

• Consider an extended object - an object with a larger angular size.

•Light rays from each point of the object is brought to focus at its individual point on the focal plane of the lens - a plane that includes the focal point.

• An extended image will form on the focal plane.

Uses of Lenses: Camera

•Light rays from a distant object fall parallel onto the lens.

• Then the rays will converge onto the focal plane.

•If you keep a photographic film at the focal plane, an image of the object will form on the film.

Photographic filmlens Focal length

Focal plane

Uses of Lenses: Refracting Telescope

• Instead of getting the image on a film, if you want to observe the image with your eyes, you would use another lens to magnify the image formed at the focal plane.

•This arrangement of two lenses is called a refracting telescope.


• The larger lens at the front is called the Objective lens - Large Diameter, Longer Focal Length.

• The smaller lens at the back is called the Eyepiece lens - Smaller diameter, shorter Focal length.

Place the eyepiece at a distance from the focal plane of the objective that is equal to the focal length of the eyepiece.

A Refracting Telescope

40-in refractor at Yerkes Observatory near Chicago.

Magnifying power (m)

• Telescopes magnify distant objects.

– Moon’s angular diameter when observed with your naked eyes = 0.50

– When observed by Galileo through his telescope the angular diameter of the Moon = 100 .

– He saw craters, valleys, mountain ranges, etc.

Characteristics of Refracting Telescopes:


• Definition of magnification or magnifying power:

m = angular diameter seen through the telescopeangular diameter seen with your naked eyes

• Example: Galileo’s telescope

m = 100 / 0.50 = 20 times or 20X



• What determines the magnification of a telescope ? the focal lengths of the lenses!

magnification (m) = focal length of the objective lensfocal length of the eyepiece lens

• In order to increase magnification:

– increase the focal length of the objective

– decrease the focal length of the eyepiece



• Larger diameter lenses capture more light; produces brighter images.

•It is important for astronomers that telescopes have large diameter objective lenses.

• light-gathering power area of the objective

(Diameter of objective)2

• If you double the diameter, the light gathering power increase by a factor 22 = 2 x 2 = 4

Light Gathering Power


• Light gathering power is so important that telescopes are described by the diameter of the objective

• Example: 90-cm refractor on Mount Hamilton in California.

This telescope has 900 time the light gathering power as Galileo’s 3-cm refracting telescope!



• For astronomers magnifying power is not the most important factor in a telescope.

•The light gathering power is more important.

• The reason is that there is a limit to how sharp sharp an image can be.

– for Earth base telescopes this is determined by the atmospheric disturbances.

Magnifying a blurred image gives a bigger, but still a blurred image.



Andromeda galaxy as seen from two telescopes. We can see the effect of doubling the diameter of the objective.


Characteristics of a Refracting Telescope

•Passing of light through lenses causes several effects:

–chromatic aberration: lens acts slightly like prism.

The lens bends different color light by different amounts.

–some light absorbed by glass.

–UV absorbed by glass. Chromatic Aberration is corrected by adding a second lens made from a different kind of glass.

Disadvantages


Disadvantages


• It is impossible to produce a large lens that is entirely free of chromatic aberration.

•Since you can only support a lens around it’s edge, the lens tends to sag under it’s own weight and distort the image.

• Nowadays astronomers avoid these problems by building Reflecting telescopes that use mirrors to collect light.

Disadvantages

Characteristics of reflecting telescopes

• Incoming light reflected by several mirrors to eyepiece.

• Light weight construction, since mirrors are lighter than lenses.

• Reflection from mirror has several advantages:– Light not absorbed.– UV not absorbed.– No chromatic aberration.– Can be fully supported.

Characteristics of a Reflecting Telescope

• All modern telescopes form images using the principle of reflection.

• If i and r are the angles that the incident & reflected rays make with the perpendicular

i = r

Principle of reflection

Flat mirror

Characteristics of a Reflecting Telescope

• Parallel rays of light incident on a concave mirror reflect and converge at point - focal point.

•The distance between the reflecting surface and the focus is the focal length.

Reflection by a Concave Mirror


• Because light reflects off the silver coated surface and does not pass through the glass, the defects in the glass does not effect the image.

Advantages

•No chromatic aberration (no refraction). All color light converge to the same focus.

•The mirror can be supported by a bracket on it’s back since light does not pass through.


• Problem: Since the focal point is in front of the mirror how can you view the image? – Your head will block part of the light

Designs for reflecting telescopes

•To get around this problem in 1668 Newton placed a small mirror at a 450 angle in front of the focal point - Newtonian Focus.

•For Reflecting telescopes the magnification is defined as:

Magnification = focal length of the primary mirror

focal length of the eyepiece

Characteristics of reflecting telescopes Designs for reflecting telescopes

• Subsequently different designs of refracting telescopes were introduced.


• Prime Focus: Astronomers place their recording instruments at the prime focus.

Designs for reflecting telescopes

• Newtonian Focus: Reflected light from the primary mirror is deflected by 900 by a secondary mirror, usually to an eyepiece at the side of the telescope.• Cassegrain Focus: An astronomer wanting to place a heavy piece of instrument that is too big to be place at prime focus can use a Cassegrain where the light from the primary is reflected back.

• Coude’ Focus: A more complex cassegrain type that uses two secondary mirrors.

Reflecting Telescopes

• At the moment there are 8 reflecting telescopes with primary mirrors of diameters greater than 26.2 feet( 8meters)

• Photograph shows the 10-m Keck I Telescope on Mauna Kea, Hawaii.

• Hole in the middle of the primary is for the Cassegrain focus.

Characteristics of Reflecting Telescopes:

Spherical Aberration

• One problem with reflecting telescopes is Spherical Aberration.

– Light from different parts of the mirror converge at different focal points due to the shape of the mirror.

• This problem is corrected by two methods:

– Use a parabolic mirror instead of a spherical one.

– Use a correcting lens in front of the mirror.

Characteristics of Reflecting Telescopes:

Spherical Aberration

Characteristics of Telescopes:

Resolving Power

• Another advantage of large telescopes is their finer Angular Resolution.

– The ability of the telescope to form, distinct, separate images of two objects that are close together, or small angular separation.

Characteristics of Telescopes:

Resolving Power• Finer the resolution the more details we can see.

• Angular Resolution

(wavelength of light) / (Diameter of the primary)

• Larger the D, smaller the ang. resolution, and hence better.

• However atmospheric turbulence makes it impossible for a telescope to have the desired angular resolution.

• These effects are corrected by using a technique known as - Adaptive Optics.

– mirror shape is corrected every few seconds.

10’, 1’, 5”, 1”

Image processing in Astronomy

• Instead of using photographic film to record images present day astronomers uses electronic detectors known as charge coupled devices (CCD).

• Data is stored on a silicon wafer that is divided into a 2-dimensional array of elements (pixels).

• CCD’s are much more sensitive than film and therefore can detect much fainter objects.

• These images are then read by a computer and processed

Image Processing

• A CCD

Image processing

Photographic film vs. CCD: a) Photographic film. b) & c) using a CCD with the same telescope.

Limitations of optical telescopes

• Available sky– can only see part of sky at any time on particular

night• Atmospheric distortion

– blurring of image caused by atmospheric “heat waves” which cause image to shimmer

– good “seeing” means less shimmering• Light pollution

– growth of night time lights has distanced us from the night sky

Atmospheric distortion

Observing at other wavelengths

• Light and radio (some infrared) are only ground based observations possible– rest must be space-based due to atmospheric

absorption

Radio Astronomy

• Until the mid 20th century our view of the universe was based on visible light.

• Since then astronomers have used other forms of electromagnetic waves to study the skies, and these observations have revealed startling aspects of the cosmos.

• Radio telescopes were the first telescopes built that used non-visible part of the EM spectrum.

Radio Astronomy

• The curved metal dish, usually made of wire mesh captures cosmic radio waves and reflect them to the focus.

• A receiver at the focus collects the signals and directs them to a computer.

Radio Astronomy

• Optical vs. Radio view of Saturn. • a) shows Saturn seen at 2-cm wavelength radio

waves. This radio emission is caused by tiny charged dust particles moving in Saturn’s strong magnetic field. (Blue for weak and red for strong emission)

Radio Astronomy

•

300-m Arecibo Radio Telescope in Puerto Rico.

Earth orbiting telescopes

• Our atmosphere is mostly transparent to two wavelength regions, the optical window and the radio window.

• In order to observe the universe using other forms of light we have to place telescopes above the atmosphere.

• These invisible astronomies opens up a whole new window on the universe.

Infrared Astronomy

• Water vapor in the atmosphere absorbs most IR.

• Infrared Astronomical Satellite (IRAS) was launched in 1983 on a 9-month mission.

• It mapped most of the sky.• Discovered the presence of dust-disks around

nearby stars, presenting us with the first evidence of planets orbiting other stars.

Infrared Astronomy

• Infrared Space Observatory (ISO) was launched in 1995 by ESA.

• Made ground breaking discoveries of very distant galaxies.

Ultraviolet Astronomy

• Observing at UV wavelengths has given us valuable insight into hot stars, ionized clouds of gas between the stars and the Sun’s corona. All these emit a lot of UV radiation.

• International Ultraviolet Explorer (IUE) launched in 1978 was the first UV orbiting telescope.

• The Hubble (Optical) telescope is also a very good UV telescope.

The Hubble Space Telescope

• The HST was placed in 600-km orbit by the space shuttle Discovery in 1990.– Has a 2.4 m (7.9-ft) primary mirror– Designed to observe from near-infrared

through visible light and into the UV region.

– better resolution in bigger telescopes– Uses a CCD to record images and radio

them to Earth.


• Soon after being placed in orbit astronomers found a big problem with HST– The primary mirror manufacturer had made

a mistake and the mirror was suffering from Spherical Aberration

– Images were blurred. • This problem was corrected by a 1993 space

shuttle mission when the a set of correcting secondary mirrors were installed.



Images taken before and after repair of HST of Galaxy M100

X-ray Astronomy

• A series of X-ray observatories have been launched since 1970’s. – The latest of these being NASA’s Chandra X-

Ray Observatory and ESA’s XMM-Newton

• These telescopes have observed X-ray bursts coming from heated gas around compact massive objects, possibly Black Holes.

Infrared Astronomy

Chandra X-ray Observatory and XMM-Newton

Infrared Astronomy

• Observing with Gamma rays can give us insight into extremely high energy phenomena, such as Supernova Explosions.

The Compton Gamma ray Observatory (CGRO) launched in 1991 by NASA

Observing at other wavelengths

Chapter 6 Optics and Telescopes. Optical telescopes Two types –refractors (use lenses)...

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