Chapter 6 Optics and Telescopes Optical telescopes Two

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 17 th 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 Focal plane lens Photographic film Focal length • 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.

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.

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

Uses of Lenses: Refracting Telescope

A Refracting Telescope 40 -in refractor at Yerkes Observatory near Chicago.

Characteristics of Refracting Telescopes: 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: Magnifying power (m) • Definition of magnification or magnifying power: • Example: Galileo’s telescope è m = 100 / 0. 50 = 20 times or 20 X

Characteristics of Refracting Telescopes: Magnifying power (m) • What determines the magnification of a telescope ? Þ the focal lengths of the lenses! • In order to increase magnification: – increase the focal length of the objective – decrease the focal length of the eyepiece

Characteristics of Refracting Telescopes: Light Gathering Power • 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

Characteristics of Refracting Telescopes: 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. è This telescope has 900 time the light gathering power as Galileo’s 3 -cm refracting telescope!

Characteristics of Refracting Telescopes: Light Gathering Power • 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 an image can be. – for Earth base telescopes this is determined by the atmospheric disturbances. è Magnifying a blurred image gives a bigger, but still a blurred image.

Characteristics of Refracting Telescopes: Light Gathering Power Andromeda galaxy as seen from two telescopes. We can see the effect of doubling the diameter of the objective.

Characteristics of a Refracting Telescope Disadvantages • 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. è Chromatic Aberration is corrected by adding a second lens made from a different kind of glass.

Characteristics of a Refracting Telescope Disadvantages

Characteristics of a Refracting Telescope 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.

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 Principle of reflection • 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 Flat mirror Þ i=r

Characteristics of a Reflecting Telescope Reflection by a Concave Mirror • 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.

Characteristics of reflecting telescopes Advantages • 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. • 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.

Characteristics of reflecting telescopes Designs for reflecting telescopes • 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 • 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.

Characteristics of reflecting telescopes Designs for reflecting telescopes • Prime Focus: Astronomers place their recording instruments at the prime focus. • 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( 8 meters) • 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.

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 20 th 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 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.

The Hubble Space Telescope • 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.

The Hubble Space Telescope Images taken before and after repair of HST of Galaxy M 100

X-ray Astronomy • A series of X-ray observatories have been launched since 1970’s. – The latest of these being NASA’s Chandra XRay 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 The Compton Gamma ray Observatory (CGRO) launched in 1991 by NASA • Observing with Gamma rays can give us insight into extremely high energy phenomena, such as Supernova Explosions.

Observing at other wavelengths