Galileo’s telescope

Refractor or Reflector

Refraction• Refraction is the

bending of light when it passes from one substance into another

• Your eye uses refraction to focus light

Refraction makes a pencil appear to be bent when placed in water.

When light passes through a glass slab it first refracts towards the normal then away from the normal.

Example: Refraction at Sunset

• Sun appears distorted at sunset because of how light bends in Earth’s atmosphere

Refraction telescope images

The main lens in a refracting telescope is called the primary lens (or the objective lens). This lens is a part of the telescope and is fixed.

Problems with refractors.

Light of differing frequency travel at different speeds in glass. This results in a spectrum from a prism and leads to chromatic aberration in lenses.

Large achromatic lenses are expensive.

Lenses must be supported at the edges. Large lenses sag in the middle under their own weight.

Long focal length= long tubes. Costly mounts and domes.

Chromatic Aberration

Achromatic Lenses

Yerkes Observatory

Williams Bay Wis.

Worlds largest at 40 inches in diameter

63ft long

ReflectorsMost large telescopes are reflectors.

Isaac Newton presented a reflecting telescope to the Royal Society in 1671.

Mirrors avoid chromatic aberration.

Objective mirror instead of an objective lens.

Early reflectors were made of polished metal alloys. Tarnished rapidly dimming images.

Reflectors 2

1850 method for depositing silver on glass. Most telescopes became reflectors

Still tarnished, normal glass changes shape with temperature changes

1940’s technique for casting Pyrex glass and aluminum coatings (more durable)

Now Pyrex or Fused Quartz

Palomar 200 inch mirror took 11 years to build and required 5 tons of glass.

Reflecting telescope images

Light in a reflecting telescope does not have to pass through glass at all.

The main mirror in a reflecting telescope is called the primary mirror (objective mirror).

Images are inverted

Types of reflectors

Prime or Cassegrain

The Cassegrain focus is at the bottom and requires a secondary mirror.

Prime focus is often used in photography. It places the observer in a cage.

Cassegrain Telescopes

41” Telescope

18” Telescope

Earl of Rosse’s mid –nineteenth century telescope

Observers stood at the prime focus.

Newtonian

A Newtonian focus is inconvenient for large telescopes.

SCT

Telescope Mounts

The choice of mount depends on the telescopes main function and size.

Good optics are useless if you can’t control the telescope.

Good control is useless if the optics are poor.

Alt-Azimuth Mounts

Equatorial Mounts

Equatorial Mounts

200in

Russian 6m

First large telescope to use alt-azimuth mount. A key move to the new generation of telescopes.

For a given aperture this permits a cheaper lighter structure in a more compact dome.

Telescope Rating Characteristics

Powers of a telescopeLight gathering powerResolving powerMagnifying power.

Light Gather PowerHighLow

ResolutionHighLow

Magnification

HighLow

Light Gathering PowerA telescope is a light bucket.

Like a bucket catching rain the diameter is the determining factor in hiw much light gets caught.

Area= π r2

The large the radius, diameter, the greater the area.

Bigger is better. The more light you collect the farther out you see.

Resolving Power

… ability of a telescope to see fine detail.

Defined as the angular distance between two objects that are barely visible as separate.

Due to the wave nature of light magnified images have diffraction fringes.

Diffraction Limits

The edge of a telescope acts as slits causing diffraction patterns. These are only seen at the highest magnification for a telescope.

Overlap

Closely spaced objects begin to overlap, becoming indistinguishable.

Increasing resolving power.

The smaller the number the better the resolution.

)()(

mDm

Angular resolution = 0.25 *

Resolution formula

For visible wavelengths in the middle of the visible spectrum

“ α” ≈ 13.7( )D cm

Note aperture is in the denominator. Large D smaller resolution.Bigger telescope is better.

Magnifying PowerUsually the big sell. Least important. It can be changed.

M =

effo

Magnification is calculated by the focal length of the objective divided by the focal length of the eyepiece.

To Increase magnification just change to a shorter focal length eyepiece.

Maximum Magnification

• As magnification of an instrument is increased the image will be dimmer.

• As a rule of thumb the maximum magnification of a telescope can be found by

• Mmax = 20(X/cm) *D(cm)

F5_4 Focal Length of a Lens

The focal length of a lens or mirror is the distance from the center of the lens to the image formed from an object placed at a great distance.

Observing Problems: Light Pollution

Many of the most interesting objects are dim. Bright skies wash out the images. The darker the sky the better.

Seeing

Seeing

To reduce the effects of the atmosphere observatories are placed at high elevations and in regions where the air is dry.

Paranal Observatory ESO

The best seeing is from remote, high and dry locations.

Telescopes

New Generation telescopes use advances in technology to correct images for bad seeing conditions.

New Generation Telescopes

Large mirrors had to be made thick to avoid sagging.

A mirror can be supported from the back. Traditional telescopes were big, heavy, and expensive. Control devices had to be massive too.

High speed computers have helped to reduce costs and improve performance.

Computer control makes alt-azimuth mounts usable.

Computer control makes it possible to control the shape of thin mirrors rapidly. This reduces the costs of making the mirror, smaller mounts and allows for…

Thin floppy mirrors Mirrors are backed by movable

pistons able to change the shape of the mirror quickly under computer control.

B. One of the Keck hexagonal mirror segments.

Thin mirrors are lighter require less support and they change temperature faster (less trouble with convection currents at surface)

Floppy Mirrors

Nordic 2.6 M

Canary Islands 1989

First large instrument whose dome and primary mirror shape are continually adjusted for the sharpest possible image.

Gemini 8.1 m mirrorsRapid control of mirror shapes makes it possible to correct some distortions caused by poor seeing.

Real time mirror control achieved by 120 actuators under the mirror and 60 around the edge.

Right: The Gemini mirrors have adaptive optics

Adaptive optics

AO allows this 1.5m telescope to reach 0.1” resolution

Adaptive Optics Corrected Image

Adaptive Optics

Rapidly changing the shape of a telescope’s mirror compensates for some of the effects of turbulence

Without adaptive optics With adaptive optics

Pluto and Charon by adaptive optics

Star by AO

The central star is blocked out to show a disk of matter ..

Inside Gemini

Gemini openThe air inside and outside the dome must be the same temperature so the dome is opened shortly before sunset.

Inside looking out

Mauna Kea Observatory Hawaii

At 4 km altitude sky is clear. Keck 10 m mirror

Keck

Keck (I and II) have 9.8 m segmented mirrors[ 36 of them, 90cm on each side]. A major advance in telescope design.

Keck I Segmented Mirror

Segmented Mirrors

Moons by Keck

Segmented Mirror

Texas Hobby-Eberly Telescope, HET:9.1 effective aperture mirror is made from 91 spherically segmented mirrors forming a hexagon 11 m x 10 m.

Plans

Giant Telescopes are proposed. The California Extremely Large Telescope would be a 30 m telescope featuring 1080 segments 40cm on a side.

More plans

XLTExtremely large telescope

The mirror

Enormously large telescope

OWL

Overwhelmingly Large Telescope

Optical InterferometryCombining signals requires control of signal path lengths to a fraction of the wave length of the e-m radiation being combined.Only radio waves could be used until recently.

The resulting image has the resolution of the distance between mirrors.

Very Large Telescope Array

Paranal Observatory at Atacama ChileFour 8.2 m reflecting telescopes .Used together have the effective area of a 16 m .

VLT

Large Binocular Telescope

Binoc mirrorSpinning oven makes mirror closer to end shape, requires less finish grinding.

Giant Magellan Telescope

CCD (charge coupled device) Images

CCDs can detect both dim and bright objects in the same exposure, are more sensitive the a photographic plate, and can be read directly into a computer.

CCDs,2

CCDs can not only image but give relative intensity data that was once required a photometer. This leads to interesting contour plots and the use of false color images.

Imaging• Astronomical

detectors generally record only one color of light at a time

• Several images must be combined to make full-color pictures

Imaging• Astronomical

detectors can record forms of light our eyes can’t see

• Color is sometimes used to represent different energies of nonvisible light

•Early 90s

Spectra

Spectrograph:Most use a grating instead of a prism.Details about the intensity at specific wavelengths gives a wealth of information.

• The Earth’s atmosphere is mostly transparent for visible light and radio waves.

• For that reason, there are two major types of telescopes:

• Common Optical Telescopes• Radio Telescopes

Earth’s Atmosphere

The other window

Radio ?Why don’t we see in the radio region?

Components of Radio telescopebecause you would have to have huge eyes. Remember the resolution formula ?

Radio telescopesStill bigger is better

)()(

mDm

“α” = 0.25

To get the same resolution radio telescopes must be huge.

Green Bank 100 m telescope

Largest steerable radio telescope in the world.

300 m Radio TelescopeArecibo Puerto Rico

Very Large Array

Radio interferometry increases the resolution, by combining signals. VLA consists of 27 dishes has the resolution of a 22 mile diameter radio telescope. Soccoro NM

Radio images

Very Long Baseline Array VLBA

Combines electronically signals from Hawaii and the Virgin Islands. This gives the resolution of an Earth sized telescope

IR Infrared Telescopes Top:Longer wave length light doesn’t see the smog particles.Dust doesn’t block IR as easily in space either.

NASA 3 m Infrared TelescopeWater absorbs IR IR Telescopes must be at high elevations, in the mountains, on balloons or in space. Still only the near infrared is visible under even the thin atmosphere.

NASA’s Great Observatories

Figure 6.22

• Hubble launched April 1990. Visible light and UV ( Shuttle Discovery)

• Compton Gamma-Ray Observatory.(Shuttle Atlantis). April 1991 to June 2000 (lost gyroscope)

• Fermi Gamma-Ray Observatory (formerly GLAST) 2008 replaces Compton

• Swift. 2004. Has gamma ray detectors, as well as x-ray and visible light telescopes.

• Chandra X-ray Observatory July 1999 (Shuttle Columbia)

• Space Infrared Telescope Facility (SIRTF), now called the Spitzer Space Telescope

SIRTF Space Infrared Telescope Facility [Spitzer]

SIRTF, Spitzer Space Telescope

Was launched by Delta rocket first expected in mid April 2003. Saved a lot of money, but required a redesign. Was to have been launched by a shuttle.

IRAS

Infrared Astronomy Satellite surveyed cooler gas and dust.

Dust gets in the way

Resolution

Orbit

Chandra Xray Telescope

Gamma ray telescopes

Design

X ray mirrors

• From Chandra Ed at Harvard

Extreme Ultraviolet Explorer

F5-23a Hubble

F5-23bMars by Hubble

James Webb Space Telescope – 2013… I mean 2018 launch…