Next Generation Extremely Large Telescopes

Outline

• What are we looking for?

• Telescope types

• Telescope style

• Mirror types

• Size comparison – Existing vs New

• New Ground Based Telescopes • Electromagnetic Spectrum

• New Space Based Telescopes

What are we looking for?

• More distant and fainter objects:

• discover new stars, galaxies, black holes, exoplanets

• first objects to emit light in the Universe

• Investigate dark energy and dark matter

• Determine if there is life elsewhere in the galaxy

• identify potentially habitable planets

• Measure quasars, pulsars, gravitational waves

• Mapping small objects in the solar system

• near-Earth asteroids, Kuiper belt objects

• Detecting transient optical events

• novae and supernovae

What are the objects? • Stars, starlike objects

– Pulsars – Pulsing stars

– Quasars – Quasi Stellar radio sources

– novae and supernovae – exploding stars

• Galaxies

• Exoplanets – planets around other stars

• Invisible items – don’t emit light

– Black holes – location of intense gravity

– Dark Energy

– Dark Matter

How do we find them?

• Images of objects

• Object motion

• Effect on motion or image of nearby matter

(other objects)

– Exoplanets – transit and movement of star

– Dark energy – expansion of universe

– dark matter – attraction of objects

Extremely Large Telescopes • Why is “bigger”/larger is better?

• Greater resolving power •see more detail

• More light gathering power •See dimmer objects

• Space based vs Ground based • No atmospheric interference

• Adaptive Optics • Removes effects of atmospheric distortion

High resolution Low resolution

Resolution

Telescope Styles

• Refracting – Lenses

• Reflecting – Mirrors

• Multiple Mirror

– Aberration Correction

Refracting telescope:

Objective lens

Cassegrain

Reflecting telescope

Parabolic Hyperbolic

Hyperbolic

Objective

Mirror

Hyperbolic

Secondary Mirror

Ritchey-Cretien

Multiple Mirror Telescopes • Aberration

– any disturbance of the rays of a pencil of light such that they

can no longer be brought to a sharp focus or form a

clear image.

• A telescope with only 1 curved mirror will always have

aberrations

• Spherical aberration

• Coma

• Astigmatism

• Aberration correction

– Multiple mirrors

– Mirror Shapes

• Parabolic

• hyperbolic

Spherical Aberration

How a spherical mirror creates spherical aberration

A parabolic mirror focus all light to a single point

Spot diagram of a star

at the edge of the field

affected by coma

Stars in the center of the field are

not affected by coma, but the effect

grows stronger toward the edge of

the field.

Stars affected by pure coma are

shaped like little comets (hence the

name) pointed toward the center of

the field.

Coma Aberration

Spot diagram of a star at the

edge of the field affected by

astigmatism

Astigmatism

Two distinct focal surfaces exist

in the presence of astigmatism

• a Tangential focal surface

•Sagittal focal surface

• Two-mirror 3-reflection system.

• Concave secondary mirror (S) reflects light back to the

primary (P), which then forms the final focus through an

opening on the secondary.

• Correction of all three aberrations, spherical, coma and

astigmatism

• The only remaining aberration is relatively strong field

curvature.

Gregorian Telescope

Final Focus

ellipsoidal primary hyperbolical secondary

Multiple Mirror Telescopes

• parabolic mirrors

– eliminates spherical aberration from spherical mirror

• Two curved mirrors -

– Ritchey–Chrétien telescope eliminates coma

– Gregorian Eliminates spherical, coma, astigmatism

• 3 curved mirrors

– Anastigmat telescope also cancels astigmatism.

– Korsch Corrects astigmatism and field curvature

– Larger field of view than one or two mirrors

Telescope Styles • Gregorian

– Giant Magellan Telescope

• Three-mirror anastigmat

– European Extremely Large Telescope (EELT)

– Large Synoptic Survey Telescope (LSST)

• Ritchey-Chretien

– Thirty Meter Telescope

• Korsch

– James Webb Space Telescope (JWST)

• Radio Telescope

– Deformable Fixed primary - FAST

Segmented Mirror Telescopes

• Segmented mirror

– an array of smaller mirrors designed to act as

segments of a single large curved mirror.

– used as objectives for large reflecting telescopes.

• mirror segments

– have to be polished to a precise shape

– actively aligned by a computer-controlled active

optics

• future large optical telescopes

essentially all plan to use segmented mirrors.

Existing Large Telescopes

Ground Based

Ground Based Telescope

Size Comparsion

EXISTING NEW

2.5 m 6.5 m 10 m 24.5 m 30 m 39.3 m Human

i

Existing

Ground Based

Telescopes

tennis

court

Basketball

court

New Ground

Based

Telescopes

New Large Ground Based Telescopes

• 500 M Aperture Spherical Telescope (FAST)

• 30 Meter Telescope

• Giant Magellan Telescope

• European Extremely Large Telescope (EELT)

• Large Synoptic Survey Telescope (LSST)

Electromagnetic Spectrum

300 300

GHz MHz

microwave

Interstellar Communication

10 GHz 1 GHz

. Radio astronomy Study of celestial objects at radio frequencies

• FAST telescope - 70 MHz – 3 GHz

• Interstellar communication (1 GHz to 10 GHz)

• radiation coming from the Milky Way

• stars and galaxies

• radio galaxies, quasars, pulsars

• Cosmic microwave background radiation

FAST

telescope

Electromagnetic Spectrum

Larger diameter Telescope diameter

for

same resolving power

smaller diameter

wavelength

Five Hundred Meter Aperture Spherical Telescope

FAST China – Sept 25, 2016

Five Hundred Meter Aperture

Spherical Telescope

FAST

• Features

• Single dish - Can shift its 4450 reflectors for pointing

and focusing

• Collecting area – almost 450 basketball courts –

world’s largest

• Type – Deformable Fixed primary

• Radio telescope - 70 MHz to 3.0 GHz

• Potential to search for more strange objects

• to better understand the origin of the universe

• boost the global hunt for extraterrestrial life

• 169% larger than next largest – Arecibo

• Twice as sensitive

Ground Based Telescopes Key Characteristics

Telescope Operational Location Aperture (ft/m)

Signal type (visible, IR, etc

Primary objective

FAST

500 M

Aperture

Spherical

Telescope

Sept 25,

2016

China 500

meters

(30 soccer

fields)

radio

Objectives

• quasars

• pulsars

• gravitational waves

• extra-terrestrial life

See below

Electromagnetic Spectrum

James Webb

Visible (orange) – Mid IR

Hubble Near UV, visible, Near IR

Infrared astronomy

• detect objects such as planets

• view highly red-shifted objects from the early days of the

universe caused by expansion of the universe

• penetrates dusty regions of space such as molecular clouds

No shift

Red shift

Blue shift

Stationary galaxy

Receding galaxy

Approaching galaxy

Galaxy motion vs Electromagnetic Spectrum

Planet rotation around star effect on Electromagnetic Spectrum

Blue Shift

Red Shift

Locates exoplanets that cannot be seen visually

Thirty Meter Telescope TMT International Observatory

Hawaii ?? - 2022

Consortium: United States, Canada, Japan, China, and India.

Alternate Locations:

Baja California in Mexico, Canary Islands, Chile, India and China

Thirty Meter Telescope

Hawaii -?? - 2022

Thirty Meter Telescope

• Features

– Telescope style: Ritchey-Chretien

– 9 x light gathering power of Keck telescope

– Highest altitude of all proposed Extremely Large Telescopes (ELT’s)

– Adaptive optics to correct image blur due to earth’s atmosphere

• Delayed due to islanders

Ground Based Telescopes Key Characteristics

Telescope Operational Location Aperture (ft/m)

Signal type (visible, IR, etc

Primary objective

30 Meter Telescope

2022? Hawaii??

Altitude 13,287 ft

Blue Canyon 4695 ft

30 meters 492 segmented mirrors

Near UltraViolet

Visible

Mid Infrared

Objectives

• More distant and fainter objects

• exoplanets

• black holes

• clues left from early universe

See

below

Giant Magellan Telescope GMT Consortium

Chile 2021 1st light

US-led in partnership with Australia, Brazil, and Korea,

with Chile as the host country

Giant Magellan Telescope

• Features

• Telescope style - Gregorian

• Resolving power 10 x Hubble Space Telescope

• Advanced adaptive optics to correct atmosphere

distortion

• Largest optical observatory in world at first light

Ground Based Telescopes Key Characteristics

Telescope Operational Location Aperture (ft/m)

Signal type (visible, IR, etc

Primary objective

Notes

Giant Magellan Telescope

1st light 2021 Completion 2025

Chile

Altitude 8225 ft

Palomar San Diego 5617 ft

7 8.4m Mirrors

Resolving power of 25 meter primary

Visible light Near Infrared

• Objectives

• It should be able to probe the first objects to emit light in the Universe

•To investigate dark energy and dark matter

•To identify potentially habitable planets

See

below

European Extremely Large Telescope (EELT)

European Southern Observatory (ESO) Chile – first light 2024

• Features

European Extremely Large Telescope

(EELT)

• Telescope type

•Three mirror anastigmat with two flat folding

mirrors providing the adaptive optics.

• Adaptive optics

• to correct image blur due to earth’s atmosphere

• Images 16 times sharper than Hubble

• Will be largest “light” telescope in the world

Ground Based Telescopes Key Characteristics

Telescope Operational Location Aperture (ft/m)

Signal type (visible, IR, etc

Primary objective

Notes

European Extremely Large Telescope

2024 Atacama desert Chile Altitude 10,039 ft

39.3 meters

798 hexagonal segments

Visible light Near IR

• Objectives

•Detailed studies of:

• planets around other stars,

• first galaxies in the Universe,

• super-massive black holes,

• nature of the Universe’s dark sector,

• water and organic molecules in protoplanetary disks around

other stars

See

below

Large Synoptic Survey Telescope (LSST)

LSST Corporation Chile – first light 2019

• Features

Large Synoptic Survey Telescope

(LSST)

• Telescope type

• three-mirror anastigmat

• Advance in speed – not size

• Photograph entire sky every 3 nights

• movies

Ground Based Telescopes Key Characteristics

Telescope Operational Location Aperture (ft/m)

Signal type (visible, IR, etc

Primary objective

Large Synoptic Survey Telescope (LSST)

2019-2022 Chile

Altitude 8739 ft

James Lick 4209 ft

8.4 m Visible light

Objectives • Measuring weak gravitational lensing in the deep sky to detect

signatures of dark energy and dark matter

• Mapping small objects in the Solar System, particularly near-

Earth asteroids and Kuiper belt objects

• Detecting transient optical events such as novae and

supernovae

• Mapping the Milky Way

See

below

FAST

China

30 Meter Telescope

Hawaii ??

EELT

GIANT MAGELLAN

LSST

Chile

Existing

Space Based

Telescopes

tennis court basketball court

New

Space Based

Telescopes Hubble

Space Telescope Orbit Location

Earth

Lagrange points, L-points, or libration points)

• positions in an orbital configuration of two large bodies

where a small object affected only by gravity can maintain a

stable position relative to the two large bodies.

New Large Space Telescopes

1. James Webb Space Telescope

2. Advanced Technology Larger Aperture Space Telescope (ATLAST)

3. Wide Field Infrared Survey Telescope (WFIRST)

James Webb Space Telescope (JWST)

NASA Launch – Oct 2018

James Webb Space Telescope

• Features

• Type Korsch - Corrects astigmatism and field curvature

• Unprecedented resolution and sensitivity from :

• long-wavelength (orange – red) visible light through near-infrared to the mid-infrared

• 3 X Hubble IR spectrum

• Aperture 6.5 m vs Hubble 2.4 m

• Can View

• high-redshift objects have their visible emissions shifted into the infrared,

• cold objects such as debris disks and planets emit most strongly in the infrared

• this band is difficult to study from the ground or by existing space telescopes such as Hubble.

Space Based Telescopes Key Characteristics

Telescope Operational Location Aperture (ft/m)

Signal type (visible, IR, etc

Primary objective

James Webb Space Telescope

Oct 2018 Orbit L2

Lagrange point

6.5m Visible (orange) to Infrared

• Objectives

•Observing some of the most distant objects in the universe

• Very first stars

• Epoch of reionization, formation of the first galaxies

• Understanding the formation of stars and planets

• Imaging molecular clouds and star-forming clusters

• Studying the debris disks around stars

• Direct imaging of exoplanets

• Spectroscopic examination of planetary transits

See

below

Advanced Technology Larger Aperture Space Telescope

ATLAST Space Telescope Science Institute

Launch – Oct 2018 8m Telescope

The architectures are: • a telescope with a monolithic primary mirror

• two variations of a telescope with a large segmented primary mirror

Advanced Technology Larger Aperture

Space Telescope ATLAST

• Features

• 5 – 10 times better resolution than

Hubble or James Webb Space Telescope

• Sensitivity limit 2000 times better than Hubble

• Hubble Replacement (2.4 m)

Space Based Telescopes Key Characteristics

Telescope Operational Location Aperture (ft/m)

Signal type (visible, IR, etc

Primary objective

Notes

Advanced Technology Larger Aperture Space Telescope (ATLAST)

Concept 2030 - 2035

Orbit L2

Lagrange point

3 concepts: 8 m 9.2 m 16.8

UV, Visible Near IR

Objectives

• Determine if there is life elsewhere in the galaxy

• By searching the spectra of terrestrial exoplanets for

“biosignatures”

• molecular oxygen, ozone, water. methane

See

below

Wide-Field Infrared Survey Telescope

WFIRST NASA

Launch Mid 2020’s

Wide-Field Infrared Survey Telescope WFIRST

• Features

• Telescope type -Three-mirror anastigmat

• Wider FOV than Hubble due to shorter focal length

• Hubble-quality imaging

• over an area of sky 100x larger than Hubble

• Hubble Space Telescope/WFC3/IR PHAT Survey

• required 432 pointings to cover M31

• while only 2 WFIRST pointings are required.

• NASA repurposed spy telescope

Space Based Telescopes Key Characteristics

Telescope Operational Location Aperture (ft/m)

Signal type (visible, IR, etc

Primary objective

Notes

WFIRST Wide Field Infrared Survey

Mid 2020’s Orbit Sun-Earth

L2 Lagrange

point

2.4m Infrared

Objectives

• Settle fundamental questions about the nature of dark

energy

• Could reveal all kinds of new information about the

evolution of the universe

• Discover new exoplanets and entire galaxies

See

below

Telescope “First Light” Schedule

Ground Based Space Based

Sept Oct Mid 2019- 2030-

2016 2018 2020 2021 2022 2022 2024 2035

FAST James WFIRST Giant Large 30 Meter European ATLAST

500 m Webb Magellan Synoptic Extremely

Survey Large

LSST EELT

FAST – 500 Meter Aperture Spherical Telescope

WFIRST – Wide Field Infrared Survey Telescope

ATLAST – Advanced Technology Larger Aperture Space Telescope

END

• Three mirror anastigmat system.

• Uses elliptical secondary to achieve a flat focal plane

Paul-Baker telescope

Final Focus

Elliptical secondary

Parabolic primary

Spherical

tertiary

• Distance Measurement Techniques

• Hubble’s Law V = Ho D • Ho = Hubble’s Constant

• D = distance

• V measured by doppler (spectral line) shift

• Parallax • difference in apparent position of an object viewed along two

different lines of sight measured by the angle between the two lines

• Earth on opposite sides of the sun in it’s orbit

• Cepheid variable • Star that pulsates in diameter and temperature –

• Luminosity relates directly to pulsation period

• Type Ia supernova • Occurs in binary systems when white dwarf explodes

• Consistent peak luminosity – Standard Candle

• Visual magnitude to measure distance to host galaxies

• Hubble – four postage stamp-sized pieces of high-tech circuitry

called Charge-Coupled Devices (CCDs

• In celestial mechanics, the Lagrangian points (/ləˈɡrɑːndʒiən/; also Lagrange points, L-points, or libration points) are positions in an orbital configuration of two large bodies where a small object affected only by gravity can maintain a stable position relative to the two large bodies. The Lagrange points mark positions where the combined gravitational pull of the two large masses provides precisely the centripetal force required to orbit with them. There are five such points, labeled L1 to L5, all in the orbital plane of the two large bodies. The first three are on the line connecting the two large bodies and the last two, L4 and L5, each form an equilateral triangle with the two large bodies. The two latter points are stable, which implies that objects can orbit around them in a rotating coordinate system tied to the two large bodies.

• The L2 point lies on the line through the two large masses, beyond the smaller of the two. Here, the gravitational forces of the two large masses balance the centrifugal effect on a body at L2.

• Explanation: On the opposite side of Earth from the Sun, the orbital period of an object would normally be greater than that of Earth. The extra pull of Earth's gravity decreases the orbital period of the object, and at the L2 point that orbital period becomes equal to Earth's. Like L1, L2 is about 1.5 million kilometers from

Earth

Classical two-mirror arrangements are Gregorian - the very first reflective telescope design, conceived by James Gregory in 1663 –

In the former, concave secondary mirror is placed outside the focal point of the primary mirror,

and Cassegrain (FIG. 121), probably introduced in 1672 by Laurent Cassegrain.

latter the secondary is convex, placed inside the primary mirror focus.

In either case, the final focus is made accessible either by focusing through an opening on the primary, or by inserting small diagonal flat in front of it, to reflect converging cone out to the side (Nasmyth arrangement).

• FIGURE 121: TOP: Left: Gregorian reflector consists from a pair of concave mirrors. The secondary, placed axially outside of the primary's focus F1, forms properly oriented final focus F by re-focusing diverging light cone coming from the primary. In the classical arrangement, primary is paraboloid, hence for the secondary to maintain zero spherical aberration it has to be an ellipsoid with its near focus coinciding with the primary's focus. Right: Cassegrain reflector also consists from a pair of mirrors, but the secondary is convex, and placed inside the primary's focus. In the classical arrangement with paraboloidal primary, secondary has to be hyperboloidal, with its near (virtual) focus coinciding with the primary's. The image is reversed, left-to-right and up-side-down. Effective focal length is determined by extending marginal ray converging from the secondary to the final focus backwards to the point of intersection with the incident marginal ray, or ƒ=s/k, k being the relative (in units of aperture radius) height of marginal ray on the secondary. Since the primary is paraboloid in the classical arrangement, its focus in either Gregorian or Cassegrain has to coincide with the near (conic) focus of the secondary (not to confuse with secondary's Gaussian focus F2, for object at infinity), in order for the latter to form an aberration-free axial image. BOTTOM: Projected image of the primary is, effectively, the object for the secondary. It is located at a distance ℓ from secondary's surface, which re-images it to the final focus. The final image is constructed by a reverse projection of the reflections from the concave side of the secondary of two imaginary rays, one parallel with the axis (blue) and the other extended to secondary's vertex (red). The final top point image forms at the point of their intersection. The primary is the exit pupil for the secondary (i.e. chief, of central ray CR for the off-axis image point is coming from the primary's center), and it is also the aperture stop. The stop-to-secondary separation σ, in units of the secondary radius of curvature R2 (secondary-to-primary separation s=σR2) is, according to the sign convention, negative; it is a factor in calculating off-axis aberrations at the secondary (due to the chief ray - and with it the entire converging wavefront - being shifted off the center of the secondary, as a result of displaced stop). Optical image of the primary formed by the secondary is the system exit pupil (ExP2); it is a factor - aperture stop - in calculating aberrations of the tertiary mirror in three-mirror systems, due to the chief ray of off-axis points appearing as if coming (onto the tertiary) from its center, and the marginal rays (1 and 2) appearing as if coming from its boundary.

WFIRST-FTA • Wide-Field Infrared Survey Telescope –

Astrophysics Focused Telescope Assets – Repurposed spy telescopes

– 2.4 m primary

– Telescope type -Three-mirror anastigmat

– Could be more powerful that Hubble

– Study dark energy from orbit

– Hubble-quality imaging over an area of sky 100x larger than Hubble

– Settle fundamental questions abut the nature of dark energy

– Could reveal all kinds of new information about the evolution of the universe

– Discover new exoplanets and entire galaxies

• About WFIRST

• WFIRST, the Wide Field InfraRed Survey Telescope, is a NASA observatory designed to settle essential questions in the areas of dark energy, exoplanets, and infrared astrophysics. The telescope has a primary mirror that is 2.4 meters in diameter (7.9 feet), and is the same size as the Hubble Space Telescope's primary mirror. WFIRST will have two instruments, the Wide Field Instrument, and the Coronagraph Instrument.

• The Wide Field Instrument will have a field of view that is 100 times greater than the Hubble infrared instrument, capturing more of the sky with less observing time. As the primary instrument, the Wide Field Instrument will measure light from a billion galaxies over the course of the mission lifetime. It will perform a microlensing survey of the inner Milky Way to find ~2,600 exoplanets. The Coronagraph Instrument will perform high contrast imaging and spectroscopy of dozens of individual nearby exoplanets.

• WFIRST is designed for a 6 year mission, and will launch on a EELV out of Cape Canaveral.

• WFIRST Observatory Concept

• The wide field imaging capability of WFIRST enables large area surveys at a much faster rate but at the same resolution as HST/WFC3/IR. The HST/WFC3/IR PHAT Survey required 432 pointings to cover M31 while only 2 WFIRST pointings are required.

• A three-mirror anastigmat is a telescope built with three curved mirrors, enabling it to minimize all three main optical aberrations - spherical aberration, coma, and astigmatism. This is primarily used to enable wide fields of view, much larger than possible with telescopes with just one or two curved surfaces.

• A telescope with only one curved mirror, such as a Newtonian telescope, will always have aberrations. If the mirror is spherical, it will suffer from spherical aberration. If the mirror is made parabolic, to correct the spherical aberration, then it must necessarily suffer from coma and astigmatism. With two curved mirrors, such as the Ritchey–Chrétien telescope, coma can be eliminated as well. This allows a larger useful field of view. However, such designs still suffer from astigmatism. This too can be cancelled by including a third curved optical element. When this element is a mirror, the result is a three-mirror anastigmat. In practice, the design may also include any number of flat fold mirrors, used to bend the optical path into more convenient configurations.

• Diagram of the lightpath through a Gregorian telescope.

The Gregorian telescope consists of two concave mirrors; the primary mirror (a concave paraboloid) collects the light and

brings it to a focus before the secondary mirror (a concave ellipsoid) where it is reflected back through a hole in the

centre of the primary, and thence out the bottom end of the instrument where it can be viewed with the aid of the

eyepiece.

The Gregorian design solved the problem of viewing the image in a reflector by allowing the observer to stand behind the

primary mirror. This design of telescope renders an upright image, making it useful for terrestrial observations. It also

works as a telephoto in that the tube is much shorter than the system's actual focal length.

The design was largely superseded by the Cassegrain telescope. It is still used for some spotting scopes because this

design creates an erect image without the need for prisms. The Steward Observatory Mirror Lab has been making mirrors

for large Gregorian telescopes at least since 1985.

In the Gregorian design, the primary mirror creates an actual image before the secondary mirror. This allows for a field

stop to be placed at this location, so that the light from outside the field of view does not reach the secondary mirror. This

is a major advantage for solar telescopes, where a field stop (Gregorian stop) can reduce the amount of heat reaching the

secondary mirror and subsequent optical components. The Solar Optical Telescope on the Hinode satellite is one

example of this design.

For amateur telescope makers the Gregorian can be less difficult to fabricate than a Cassegrain because the concave

secondary is Foucault testable like the primary, which is not the case with the Cassegrain's convex seconda

Telescope Style • Refracting/Reflecting

• Cassegrain Convex secondary – vs Concave (Gregorian) – The classic Cassegrain configuration uses a parabolic reflector as the primary while the

secondary mirror is hyperbolic.[

• Gregorian – superseded by Cassegrain – Giant Magellen Telescope – consists of two concave mirrors;

– the primary mirror (a concave paraboloid) collects the light and brings it to a focus before the secondary mirror (a concave ellipsoid) where it is reflected back through a hole in the centre of the primary, and thence out the bottom end of the instrument where it can be viewed with the aid of the eyepiece.

• Three-mirror anastigmat, - European Extremely Large Telescope (EELT), Large Synoptic Telexcope

• Ritchey-Chretien - Thirty Meter Telescope

• Paul-Baker

• Korsch – James Webb Space Telescope

• Segmented Mirror

• Radio Telescope - Deformable Fixed primary - FAST

73

Naked Eye to Eyepiece

Basic telescope types

Refractors

Reflectors: Newtonian

Hybrids: Schmidt-Cassegrain

Basic Parameters

Diameter of objective lens or mirror (Do)

Focal Length (Fo)

• Diagram of the lightpath through a Gregorian telescope.

two concave mirrors;

the primary mirror (a concave paraboloid) collects the light and brings it to a focus before the

secondary mirror (a concave ellipsoid) where it is reflected back through a hole in the centre of the primary, and thence

out the bottom end of the instrument where it can be viewed with the aid of the eyepiece.

The design was largely superseded by the Cassegrain telescope.

Telescope style

• The classic Cassegrain configuration uses a parabolic reflector as the primary while the secondary mirror is hyperbolic.

• Modern variants often have a hyperbolic primary for increased performance (for example, the Ritchey–Chrétien design), or the primary and/or secondary are spherical or elliptical for ease of manufacturing

• A reflecting telescope (also called a reflector) uses a single or combination of curved mirrors that reflect light and form an image.

Cassegrain telescope: a mirror/lens combo

Multiple Mirror Telescopes • A segmented mirror is an array of smaller mirrors designed to act as segments of a single

large curved mirror.

• The segments can be either spherical or asymmetric (if they are part of a larger parabolic reflector[1]).

• They are used as objectives for large reflecting telescopes.

• To function, all the mirror segments have to be polished to a precise shape and actively aligned by a computer-controlled active optics system using actuators built into the mirror support cell. essentially all future large optical telescopes plan to use segmented mirrors.

There is a technological limit for primary mirrors made of a single rigid piece of glass. Such non-segmented, or

monolithic mirrors can not be constructed larger than about eight meters in diameter. The largest monolithic

mirror in use are currently the two primary mirrors of the Large Binocular Telescope, each with a diameter of 8.4

meters. The use of segmented mirrors is therefore a key component for large-aperture telescopes.[2][3] Using a

monolithic mirror much larger than 5 meters is prohibitively expensive due to the cost of both the mirror, and the

massive structure needed to support it. A mirror beyond that size would also sag slightly under its own weight

as the telescope was rotated to different positions,[4][5] changing the precision shape of the surface. Segments

are also easier to fabricate, transport, install, and maintain over very large monolithic mirrors.

Segmented mirrors do have the drawback that each segment may require some precise asymmetrical shape,

and rely on a complicated computer-controlled mounting system. All of the segments also cause diffraction

effects in the final image.

objective is the optical element that gathers light from the object being observed

and focuses the light rays to produce a real image.

Multiple Mirror Telescopes • A segmented mirror is an array of smaller mirrors designed to act as segments of a single

large curved mirror.

• The segments can be either spherical or asymmetric (if they are part of a larger parabolic reflector[1]).

• They are used as objectives for large reflecting telescopes.

• To function, all the mirror segments have to be polished to a precise shape and actively aligned by a computer-controlled active optics system using actuators built into the mirror support cell. essentially all future large optical telescopes plan to use segmented mirrors.

There is a technological limit for primary mirrors made of a single rigid piece of glass. Such non-segmented, or

monolithic mirrors can not be constructed larger than about eight meters in diameter. The largest monolithic

mirror in use are currently the two primary mirrors of the Large Binocular Telescope, each with a diameter of 8.4

meters. The use of segmented mirrors is therefore a key component for large-aperture telescopes.[2][3] Using a

monolithic mirror much larger than 5 meters is prohibitively expensive due to the cost of both the mirror, and the

massive structure needed to support it. A mirror beyond that size would also sag slightly under its own weight

as the telescope was rotated to different positions,[4][5] changing the precision shape of the surface. Segments

are also easier to fabricate, transport, install, and maintain over very large monolithic mirrors.

Segmented mirrors do have the drawback that each segment may require some precise asymmetrical shape,

and rely on a complicated computer-controlled mounting system. All of the segments also cause diffraction

effects in the final image.

objective is the optical element that gathers light from the object being observed

and focuses the light rays to produce a real image.

New Ground

Based

Telescopes

tennis court basketball court

New Space

Based

Telescopes

tennis court basketball court

Ground Based Telescopes Key Characteristics

Telescope Operational Location Aperture (ft/m)

Signal type (visible, IR, etc

Primary objective

Notes

500 M Aperture Spherical Telescope

July 2016 China 500 meters (30 soccer fields)

radio quasars, pulsars, gravitational waves, extraterrestrial life

169% larger than Arecibo

30 Meter Telescope

?? Planned 2016

Hawaii 30 meters (now 10 m)

Visible light Delayed due to islanders

Large Synoptic Survey Telescope

2019-2022 Chile 8.4 m Visible light

Advance speed – not size Scan entire sky every 4 days, movies

Ground Based Telescopes Key Characteristics

Telescope Operational Location Aperture (ft/m)

Signal type (visible, IR, etc

Primary objective

Notes

500 M Aperture Spherical Telescope

July 2016 China 500 meters (30 soccer fields)

radio quasars, pulsars, gravitational waves, extraterrestrial life

169% larger than Arecibo

30 Meter Telescope

?? Planned 2016

Hawaii 30 meters (now 10 m)

Visible light Delayed due to islanders

Large Synoptic Survey Telescope

2019-2022 Chile 8.4 m Visible light

Advance speed – not size Scan entire sky every 4 days, movies

Space Based Telescopes Key Characteristics

Telescope Operational Location Aperture (ft/m)

Signal type (visible, IR, etc

Primary objective

Notes

WIFIRST Wide Field Infrared Survey

NASA Infrared NASA repurposed spy telescope

James Webb Space Telescope

Oct 2018 Orbit? ) IR 3 X Hubble IR spectrum

Advanced Technology Larger Aperture Space Telescope (ATLAST)

Concept 2030 - 2035

8.4 m Visible light

Hubble Replacement Space Telescope Science Institute

Three-mirror anastigmat of Paul or Paul-Baker form. A Paul design

has a parabolic primary with spherical secondary and tertiary

mirrors; A Paul-Baker design modifies the secondary slightly to

achieve a flat focal plane.

FAST Five Hundred Meter Aperture Spherical Telescope

China - July 2016

Ground Based Telescopes Key Characteristics

Telescope Operational Location Aperture (ft/m)

Signal type (visible, IR, etc

Primary objective

Notes

500 M Aperture Spherical Telescope

July 2016 China 500 meters (30 soccer fields)

radio quasars, pulsars, gravitational waves, extraterrestrial life

169% larger than Arecibo

30 Meter Telescope

?? Planned 2016

Hawaii 30 meters (now 10 m)

Visible light Delayed due to islanders

Giant Magellan Telescope

2021 Chile 7 round mirrors 25 m diameter

Visible light

4 yrs to polish mirror

Ground Based Telescopes Key Characteristics

Telescope Operational Location Aperture (ft/m)

Signal type (visible, IR, etc

Primary objective

Notes

European Extremely Large Telescope

2022 Visible light Will be largest “light” telescope n the world

Large Synoptic Survey Telescope

2019-2022 Chile 8.4 m Visible light Advance speed – not size Scan entire sky every 4 days, movies

Spherical Aberration

Spherical aberration is an axial aberration, affecting the entire field equally, including stars at the center. All telescope designs

strive to eliminate or minimize spherical aberration. Normally, spherical aberration should not be visible in an optical

system. But it is important to understand how it arises to see how it is eliminated in certain designs. The elimination of

spherical aberration is critical to how certain telescopes such as Schmidt-Cassegrains and Newtonians are designed.

Above: How a spherical mirror creates spherical aberration

A simple spherical mirror cannot focus light to a single point. As the diagram above shows, light from the edge of the field is focused

closer to the mirror along the optical axis than is light from the center of the field. This means it is not possible to find a single point of

best focus, only a point where the image is smallest but still not sharp. The simplest way to eliminate coma with a single mirror is to

change the shape from spherical to parabolic. A parabolic mirror does not suffer from spherical aberration and can focus all light to a

single point.

Above: A parabolic mirror focus all light to a single point

Note that this is the same principle used in radar and satellite dishes. Radio waves are simply electromagnetic radiation, just like

visible light only with much longer wavelengths. Satellite dishes are parabolic in shape. Even the sound-collecting dishes you see

along the sidelines of NFL games are parabolic to focus the incoming sound waves onto the microphone located at the focal point of

the dish.

Above: Spot diagram of a star at the edge of the field affected

by coma

Coma is an off-axis aberration. Stars in the center of the field

are not affected by coma, but the effect grows stronger toward

the edge of the field. Stars affected by pure coma are shaped

like little comets (hence the name) pointed toward the center of

the field. In the diagram above the center of the field is down. In

essence, what occurs to cause coma is that light passing through

the center of the field (but at an angle so it focuses off-axis) does

not focus at the same distance along the optical path as light

from farther off axis

Above: Diagram of how coma arises in an optical system

Since coma affects the edges of a field, and grows larger with

increasing distance from the optical axis, it is a significant

aberration with regards to wide-field viewing and imaging. For

professional astronomers and advanced amateurs who are

interested in scientific study, coma can be very problematic

because it is an asymmetrical aberration. This is problem

because it makes it impossible to accurately measure the

position of stars (astrometry). For this reason, most professional

instruments are designed specifically to eliminate coma

(although sometimes by introducing another less problematic

aberration). For most amateur astronomers, a small amount of

coma is tolerable. For wide-field applications, some telescope

designs eliminate or minimize coma, while coma-correcting

lenses are available for other designs to minimize the effect if

desired.

Spherical aberration

•Occurs due to the increased refraction of light rays when they strike a

lens or a reflection of light rays when they

• strike a mirror near its edge, in comparison with those that strike

nearer the centre.

Perfect lens

Real lens with

spherical surface

Spherical aberration

Telescope Styles

• Refracting – Lenses

• Reflecting – Mirrors

• Mirror Shapes – Spherical

– Parabolic

– hyperbolic

Above: Spot diagram of a star at the edge of the field affected by astigmatism

Like coma, astigmatism is an off-axis aberration. Unlike coma, it is a symmetrical aberration. For professional astronomers this

can be an important distinction, since asymmetrical aberrations do not allow accurate astrometric (positional) measurements to be

made. For amateur astronomers viewing the sky and taking pretty pictures, it is hard to choose one aberration over the other

(and preferably neither would be present).

Understanding how astigmatism arises can be somewhat difficult. An optical system has two planes, the tangential and the

sagittal planes, which are perpendicular to each other. If you imagine an optical diagram (below) that shows the optical path from

the side, the tangential plane corresponds to the flat plane of the diagram (the computer screen) while the sagittal plane

corresponds to the plane along the optical axis at a 90-degree angle--in other words the sagittal plane sticks out of the screen at

90 degrees to the screen itself.

Above: Optical diagram viewed along the tangential plane

of the system. The sagittal plane sticks straight out of the

screen at 90 degrees to the tangential plane.

Imagine then that the telescope has a different power (focal

length) in each plane. For example, the telescope may

focus closer along the tangential plane and farther along the

sagittal plane. This is shown in the diagram below

Above: Astigmatism arises from a

difference of optical power in the two

optical planes, tangential and sagittal

This effectively creates two focal surfaces,

one corresponding to each plane. Seen

from the side, as in a normal optical

diagram, the two focal surfaces are

normally curved (see the Field Curvature

section on the next page) and coincide

only at the optical axis.

Three-mirror anastigmat of Paul or Paul-Baker form

Parabolic mirror

• Parallel rays coming in to a parabolic mirror are focused at a point F. The vertex is V, and the axis of symmetry passes through V and F. For off-axis reflectors (with just the part of the paraboloid between the points P1 and P3), the receiver is still placed at the focus of the paraboloid, but it does

not cast a shadow onto the reflector.

Parabolic reflectors collect star light) and bring it to a common focal point,

correcting spherical aberration found in simpler spherical reflectors

Above: Two distinct focal

surfaces exist in the

presence of astigmatism, a

tangential focal surface and

sagittal focal surface

If astigmatism is eliminated,

the focal surfaces will

coincide across the entire

field of view

Telescope Designs with Astigmatism

Very few telescope designs have significant

astigmatism. The most popular telescope

with this aberration is the Ritchey-Chrétien

(RC). RCs trade coma for astigmatism for

the purpose of positional

measurements. Hence the popularity of the

design among professional astronomers. The

Hubble Space Telescope, the Keck

Telescope, and many other large professional

telescopes are RCs. These designs have

become popular with advanced amateur

astronomers for CCD imaging, but for the

purposes of pretty pictures there is little

advantage to astigmatism over coma. In fact,

the preference would be to not have either

aberration. For this reason most RCs employ

a correcting lens to eliminate the residual

astigmatism.

Telescope Designs Free from Astigmatism

Most telescope designs are free from noticeable

astigmatism. Parabolic mirrors such as those used in

Newtonians do not suffer from astigmatism. Most two-mirror

Cassegrain telescopes are designed specifically to eliminate

astigmatism (again the RC being the exception). However, these

designs suffer instead from coma.

Note that while few telescopes have astigmatism, many eyepiece

designs, especially inexpensive wide-field designs, suffer from

fair amounts of astigmatism. Often the poor edge quality seen in

many wide-field eyepieces is attributed to coma in the telescope

design while it is actually the result of eyepiece

astigmatism. Fancier wide-field designs such as the TeleVue

Nagler eyepieces are designed to minimize eyepiece

astigmatism and hence sharpen stars at the edge of the field.

• Many combinations of three mirror figures can be used to cancel all third-order aberrations. In general these involve solving a relatively complex set of equations. A few configurations are simple enough, however, that they could be designed starting from a few intuitive concepts.

• Paul[edit]

• The first were proposed in 1935 by Maurice Paul.[1] The basic idea behind Paul's solution is that spherical mirrors, with an aperture stop at the centre of curvature, have only spherical aberration - no coma or astigmatism (but they do produce an image on a curved surface of half the radius of the spherical mirror, for a mirror in air). So if the spherical aberration can be corrected, a very wide field of view can be obtained. This is similar to the conventional Schmidt design, but the Schmidt does this with a refractive corrector plate instead of a third mirror.

• Paul's idea was to start with a Mersenne beam compressor, which looks like a Cassegrain made from two (confocal) paraboloids, with both the input and output beams collimated. The compressed input beam is then directed to a spherical tertiary mirror, which results in traditional spherical aberration. Paul's key insight is that the secondary can then be converted back to a spherical mirror. One way to look at this is to imagine the tertiary mirror, which suffers from spherical aberration, is replaced by a Schmidt telescope, with a correcting plate at its centre of curvature. If the radii of the secondary and tertiary are of the same magnitude, but opposite sign, and if the centre of curvature of the tertiary is placed directly at the vertex of the secondary mirror, then the Schmidt plate would lie on top of the paraboloid secondary mirror. Therefore the Schmidt plate required to make the tertiary mirror a Schmidt telescope is eliminated by the paraboloid figuring on the convex secondary of the Mersenne system, as each corrects the same magnitude of spherical aberration, but the opposite sign. Also, as the system of Mersenne + Schmidt is the sum of two anastigmats: the Mersenne system is an anastigmat, and so is the Schmidt system, the resultant system is also an anastigmat, as third-order aberrations are purely additive.[2] In addition the secondary is now easier to fabricate. This design is also called a Mersenne-Schmidt, since it uses a Mersenne configuration as the corrector for a Schmidt telescope.

• Paul-Baker[edit]

• Paul's solution had a curved focal plane, but this was corrected in the Paul-Baker design, introduced in 1969 by James Gilbert Baker.[3] The Paul-Baker design adds extra spacing and reshapes the secondary to elliptical, which corrects field curvature to obtain a flat focal plane.[4]

• Korsch[edit]

• A more general set of solutions was developed by Dietrich Korsch in 1972.[5] A Korsch telescope is corrected for spherical aberration, coma, astigmatism, and field curvature, meaning that images on a flat detector will be the same size at the center as at the edges, and can have a wide field of view while ensuring that there is little stray light in the focal plane.

• Korsch Telescope

Five Hundred Meter Aperture Spherical Telescope (FAST)

• 500 meters - 30 Soccer fields wide

– 169% bigger than next largest - Arecibo 305 meters -1963

– Can shift its 4450 reflectors to different directions

– Type – Deformable Fixed primary

– 70 MHz to 3.0 GHz

• Research projects involving: quasars, pulsars, gravitational waves, extraterrestrial life

– “Potential to search for more strange objects to better understand the origin of the universe and boost the global hunt for extraterrestrial life”

Space Based Telescopes Key Characteristics

Telescope Operational Location Aperture (ft/m)

Signal type (visible, IR, etc

Primary objective

Notes

James Webb Space Telescope

Oct 2018 Orbit? ) IR 3 X Hubble IR spectrum

Advanced Technology Larger Aperture Space Telescope (ATLAST)

Concept 2030 - 2035

8.4 m Visible light Hubble Replacement Space Telescope Science Institute

WIFIRST Wide Field Infrared Survey

NASA Infrared NASA repurposed spy telescope

Space Based Telescopes Key Characteristics

Telescope Operational Location Aperture (ft/m)

Signal type (visible, IR, etc

Primary objective

Notes

James Webb Space Telescope

Oct 2018 Orbit? ) IR 3 X Hubble IR spectrum

Advanced Technology Larger Aperture Space Telescope (ATLAST)

Concept 2030 - 2035

8.4 m Visible light Hubble Replacement Space Telescope Science Institute

WIFIRST Wide Field Infrared Survey

NASA Infrared NASA repurposed spy telescope

Ground Based Telescope Size Comparsion

EXISTING NEW

2.5 m 6.5 m 10 m 24.5 m 30 m 39.3 m Human

i

• Enabled by NASA's proposed space launch system (SLS) vehicle.

– The 8m ATLAST offers the inherent advantages of a monolithic aperture telescope in terms of high-contrast imaging and superb wavefront control.

– The 16m ATLAST represents a pathway to truly large apertures in space and uses the largest extrapolation of a JWST-like chord-fold primary mirror packaging.

• Evolved Expendable Launch Vehicle (EELV) – 9.2m segmented is compatible with and also adopts JWST design heritage

Advanced Technology Larger Aperture Space

Telescope 2025 - 2035

• Space Telescope Science Institute

• Hubble Replacement • UV, Optical, Near IR Telescope • 8 to 16.8 meter (26 to 55 ft)

• Features – Spectroscopic and imaging observation in the ultraviolet, optical, and infrared wavelengths – Substantially better resolution than Hubble or James Webb Space Telescope (5 – 10 times

better) – Sensitivity limit 2000 times better that Hubble

– Goals – Determine if there is life elsewhere in the galaxy – By searching for “biosignatures” (molecular oxygen, ozone, water, and methane) in the spectra

of terrestrial exoplanets

– The architectures are a telescope with a monolithic primary mirror and two variations of a telescope with a large segmented primary mirror.

• Two of the concepts, the 8m monolithic mirror telescope and the 16.8m segmented mirror telescope, span the range of UVOIR observatories that are enabled by NASA's proposed space launch system (SLS) vehicle. The 8m ATLAST offers the inherent advantages of a monolithic aperture telescope in terms of high-contrast imaging and superb wavefront control. The 16m ATLAST represents a pathway to truly large apertures in space and uses the largest extrapolation of a JWST-like chord-fold primary mirror packaging. However, the ATLAST mission is not solely dependent on Ares V. Our third concept, a 9.2m segmented telescope, is compatible with an Evolved Expendable Launch Vehicle (EELV) and also adopts JWST design heritage

Advanced Technology Larger Aperture Space Telescope

• Visible Telescope

• Features

– Unprecedented resolution and sensitivity from :

– long-wavelength (orange –red) visible light through near-infrared to the mid-infrared

– Goals

New Large Ground Based Telescopes

• 1. 500 M Aperture Spherical Telescope (FAST) • China Sept 2016 • Radio Telescope

• 2. 30 Meter Telescope • Hawaii ?? May relocate due to local resident protests • New locations that are being considered include:

• Baja California in Mexico, the Canary Islands and Chile, as well as locations in India and China.

• construction planned to begin in April 2018 with completion in 2022.

• 4. Giant Magellan Telescope • Chile 2021 1st light • 7 round mirrors 25 m (80') diameter • 4 years to polish mirror

• 3. European Extremely Large Telescope (EELT) • Chile 39 m 2022 • Will be largest "light" telescope in the world

• 5. Large Synoptic Survey Telescope • Chile 2019-2022 • 8.4 m • Advance in speed - not size • scan entire sky every four days, movies

Thirty Meter Telescope

• Near UltraViolet, Visible, Mid Infrared Telescope • Mauna Kea Hawaii – planned Oct 2016 – delayed by

residents • 30 meters

– Largest existing light telescopes are about 10 meters – 492 segmented mirrors – Telescope style: Ritchey-Chretien – 9 x light gathering power of Keck telescope – Highest altitude of all proposed Extremely Large Telescopes (ELT’s)

• More distant and fainter objects – exoplanets, black holes, clues left from early universe – Adaptive optics to correct image blur due to earth’s atmosphere

Giant Magellan Telescope (GMT) • Optical, Near Infrared Telescope • Las Campanas Observatory, Chile– first light 2021, completion 2025 • Seven 8.4 meter (27.6 ft) primary mirrors

• Worlds largest primary mirrors • Resolving power of 24.5 m (80.4 ft) primary • Collection area equal to 22.0 m (72.2 ft) • Telescope style - Gregorian

• Resolving power 10 x Hubble Space Telescope • Image sharpness should exceed Hubble Space Telescope • Largest optical observatory in world at first light • Advanced adaptive optics to correct atmosphere distortion • it should be able to probe the first objects to emit light in the

Universe, to investigate dark energy and dark matter, and to identify potentially habitable planets

European Extremely Large Telescope • Visible, Near Infrared Telescope

• Cerro Armazones, Atacama Desert Chile – 2024

• 39.3 meters

– 798 hexagonal segments

– Telescope type – Reflector folded three-mirror anastigmat

– Gather 13 times more light that largest existing optical telescopes

– Adaptive optics to correct image blur due to earth’s atmosphere

– Images 16 times sharper than Hubble Space Telescope

• Detailed studies of – planets around other stars,

– first galaxies in the Universe,

– super-massive black holes,

– nature of the Universe’s dark sector,

– water and organic molecules in protoplanetary disks around other stars

Large Synoptic Survey Telescope • Visible Telescope

• El Penon peak of Cerro Pachon (2682 meter high mountain) – northern Chile – 2019

• 8.4 meters - advanced in speed – not size

• scan entire sky every four days, movies

– 3 mirror design

– Sharp images over very wide 3.5o – diameter undistorted field of view

– 3.2 gigapixel CCD imaging camera, largest digital camera ever constructed

– Telescope style – 3 Mirror anastigmat, Paul Baker/Mersenne-Schmidt wideangle

• Particular scientific goals

– Measuring weak gravitational lensing in the deep sky to detect signatures of dark energy and dark matter

– Mapping small objects in the Solar System, particularly near-Earth asteroids and Kuiper belt objects

– Detecting transient optical events such as novae and supernovae

– Mapping the Milky Way

Telescope “First Light” Schedule

Ground Based

FAST Magellan European Large Synoptic 30 Mtr

500 m Extremely Large Survey

9/2016 2021 2024 2019-2022 2022

Space Based

James Webb WFIRST ATLAST

Oct 2018 Mid 2020 2030-2035

James Webb Space Telescope • Visible (orange) to mid-infrared Telescope

– NASA - Northrop Grumman, Ball Aerospace

• Korsch type - 6.5 meters (21 ft) mirror diameter

• Successor to Hubble space Telescope (3 x larger mirror) and Spitzer Space Telescope

• Located near the Earth-Sun L2 point

• Large sunshield keeps mirror and science instruments below -370o F

• Features

– Unprecedented resolution and sensitivity from :

– long-wavelength (orange –red) visible light through near-infrared to the mid-infrared

– Can View

– high-redshift objects have their visible emissions shifted into the infrared,

– cold objects such as debris disks and planets emit most strongly in the infrared

– this band is difficult to study from the ground or by existing space telescopes such as Hubble.

• Goals

– Observing some of the most distant objects in the universe

– Very first stars

– Epoch of reionization, formation of the first galaxies

– Understanding the formation of stars and planets

– Imaging molecular clouds and star-forming clusters

– Studying the debris disks around stars

– Direct imaging of exoplanets

– Spectroscopic examination of planetary transits

Telescope “First Light” Schedule

Ground Based Space Based FAST James WFIRST Magellan Large 30 Mtr European ATLAST

500 m Webb Synoptic Extremely

Survey Large

Sept Oct Mid

2016 2018 2020 2021 2019- 2022 2024 2030- 2022 2035

Telescope Designs with Coma

The classic example of coma comes from the Newtonian telescope. Coma is the primary aberration inherent in

the Newtonian design and is the limiting factor in this design. Most Newtonian designs show coma at the edge

of the field. Coma is a function of both off-axis distance and focal ratio, meaning faster-focal-ratio (smaller f-

number) telescopes will have more coma than a similar size but slower telescope. Therefore, an 8" f/4

Newtonian has more coma than an 8" f/6 Newtonian. f/4 is usually considered the fastest a Newtonian can be

made without having excessive coma. Coma correctors are available that can minimize the amount of coma in

a Newtonian design. These lenses fit into the focuser ahead of the eyepiece of camera. They are typically used

on f/5 and faster telescopes.

Most commercial Schmidt-Cassegrain and Maksutov-Cassegrain designs also suffer from coma. Since they

typically have long focal ratios, in the range of f/10 to f/15, the coma is less than in a similar-sized

Newtonian. The amount of coma is not normally problematic when observing, but can appear at the edges of a

large photographic field. Note that coma is not necessarily inherent in the Schmidt- and Maksutov-Cassegrain

design, but exists because of the choice of optical parameters chosen to minimize the cost of manufacturing

these commercial scopes. See the Optical Design section on Schmidt-Cassegrains for more details.

Telescope Designs Free from Coma

Some telescope designs are free from coma. Classical Cassegrain telescopes, for example, have coma

inherent in the design. But by slightly changing the configuration of the mirrors, the very similar Ritchey-

Chrétien design eliminates coma but instead suffers from astigmatism. This compromise is made because

astigmatism is a symmetrical aberration and allows astronomers to make accurate positional

measurements. For amateur astronomers interested in viewing or taking pretty pictures, there is little advantage

to one aberration over the other.

Most refractors have little or no coma, contributing to their being well-suited to wide-field viewing and imaging.

Other imaging systems such as hyperbolic astrographs, Schmidt cameras, and other uncommon designs are

Figure 1. Nasmyth configuration

Figure 2. Coudé configuration

• E-ELT OPTICAL DESIGN

•

The optical design for the E-ELT is that of a folded three-mirror anastigmat, folding being provided by two flat mirrors sending the beam to either Nasmyth foci along the elevation axis of the telescope (see Figure 1). The optics are mounted on an altitude azimuth telescope main structure.

• The present concept features as a baseline for the primary mirror (M1) an elliptical f/0.93 segmented mirror of 39-m diameter and a 11.1-m central obstruction. The 4.2-m secondary mirror (M2) is convex and returns the beam, through a hole in the quaternary mirror (M4), to the 3.8-m mildly aspheric concave tertiary mirror (M3) located at the vertex of the primary. The beam is reflected by M3 to the adaptive optics system, the 2380x2340mm quaternary flat adaptive mirror M4, supported by up to 8000 actuators, and the fifth mirror in the train (M5) that allows for the final image correction. M4 is inclined at 7.75 degrees to the beam direction. M5 is a flat mirror, elliptical in contour, defines the altitude axis of the telescope and steers the beam towards the Nasmyth focus. The output beam at f/17.48 is very nearly diffraction limited over the entire 10-arcminute field of view. The total Nasmyth field of view is limited to 10 arcmin by the dimensions of the way-through hole in M4. The rather large Nasmyth focal ratio is constrained by the backfocal distance, and the location and size of mirrors M4 to M5.

• Additionally, the design allows for the f/17.48 beam to be redirected through relay optics to a Coudé focus (see Figure 2) envisaged within telescope foundations at the ground level. The need for a Coudé focus is driven by the narrow field high-resolution ultra stable spectrograph top level requirement. While this is not an absolute requirement and alternate solutions may be feasible for such an instrument, all necessary provisions have been included in the telescope design for implementing a Coudé train.

What are we looking for? • FAST

– quasars, pulsars, gravitational waves, extra-terrestrial life

• better understand the origin of the universe and boost the global hunt for extraterrestrial life

• Thirty Meter More distant and fainter objects:

• exoplanets

• black holes

• clues left from early universe what?

• Giant Magellen

– the first objects to emit light in the Universe,

– to investigate dark energy and dark matter

– identify potentially habitable planets

• Quasars are believed to be powered by accretion of material into supermassive black holes in the nuclei of distant galaxies, making these luminous versions of the general class of objects known as active galaxies

• A pulsar (short for pulsating radio star) is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation.

What are we looking for? • European Extra Large Telescope (EELT)

– planets around other stars,

– first galaxies in the Universe,

– super-massive black holes,

– nature of the Universe’s dark sector,

– water and organic molecules in protoplanetary disks around other stars

• Large Synoptic Survey Telescope (LSST)

– Measuring weak gravitational lensing in the deep sky to detect signatures of dark energy and dark matter

– Mapping small objects in the Solar System, particularly near-Earth asteroids and Kuiper belt objects

– Detecting transient optical events such as novae and supernovae

– Mapping the Milky Way

What are we looking for? • James Webb Space Telescope (JWST)

• Observing some of the most distant objects in the universe

• Very first stars

• Epoch of reionization, formation of the first galaxies

• Understanding the formation of stars and planets

• Imaging molecular clouds and star-forming clusters

• Studying the debris disks around stars

• Direct imaging of exoplanets

• Spectroscopic examination of planetary transits

• Advanced Technology Space Telescope (ATLAST) • Determine if there is life elsewhere in the galaxy

• By searching for “biosignatures” (molecular oxygen, ozone, water, and methane) in the spectra of terrestrial exoplanets

• Wide Field Infrared Survey Telescope • Settle fundamental questions abut the nature of dark energy

• Could reveal all kinds of new information about the evolution of the universe

• Discover new exoplanets and entire galaxies

European

Extremely Large Telescope

• primary mirror (M1) an elliptical segmented mirror of 39-m diameter

• secondary mirror (M2) is convex and returns the beam, through a hole in the

quaternary mirror (M4), to the 3.8-m mildly aspheric concave tertiary mirror (M3)

•The beam is reflected by M3 to the adaptive optics system, the quaternary flat

adaptive mirror M4, supported by up to 8000 actuators

•fifth mirror in the train (M5) that allows for the final image correction.

M5

M4

M3

M2

M1

Reflector folded three-mirror anastigmat

Extraterrestrial Communication

• Television Broadcast Frequencies

– Band RF Channels Frequency

– VHF-Lo 2 – 6 54 - 88 MHz

– VHF-Hi 7 – 13 174 - 216 MHz

– UHF 14 – 69 470 - 806 MHz

• FAST 70 MHz to 3.0 GHz

• SETI (Water Hole) 1.42 GHz – 1.72 GHz

• Interstellar Communication 1 .0 GHz - 10 GHz

• Microwave 300 MHz – 300 GHz

• Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object was made in the 1930s, when Karl Jansky observed radiation coming from the Milky Way. Subsequent observations have identified a number of different sources of radio emission. These include stars and galaxies, as well as entirely new classes of objects, such as radio galaxies, quasars, pulsars, and masers. The discovery of the cosmic microwave background radiation, regarded as evidence for the Big Bang theory, was made through radio astronomy.

• Radio astronomy is conducted using large radio antennas referred to as radio telescopes, that are either used singularly, or with multiple linked telescopes utilizing the techniques of radio interferometry and aperture synthesis. The use of interferometry allows radio astronomy to achieve high angular resolution, as the resolving power of an interferometer is set by the distance between its components, rather than the size of its components.

• Radio telescopes[edit]

• Main article: Radio telescope

• An optical image of the galaxy M87 (HST), a radio image of same galaxy using Interferometry (Very Large Array-VLA), and an image of the center section (VLBA) using a Very Long Baseline Array (Global VLBI) consisting of antennas in the US, Germany, Italy, Finland, Sweden and Spain. The jet of particles is suspected to be powered by a black hole in the center of the galaxy.

• Radio telescopes may need to be extremely large in order to receive signals with high signal-to-noise ratio. Also since angular resolution is a function of the diameter of the "objective" in proportion to the wavelength of the electromagnetic radiation being observed, radio telescopes have to be much larger in comparison to their optical counterparts. For example, a 1-meter diameter optical telescope is two million times bigger than the wavelength of light observed giving it a resolution of roughly

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Electromagnetic Spectrum

300 300

GHz MHz

800 TV 50

MHz MHz

microwave

Interstellar

Communication

10 GHz 1 GHz

. Radio astronomy Study of celestial objects at radio frequencies

• radiation coming from the Milky Way

• stars and galaxies

• radio galaxies, quasars, pulsars, and masers

• Cosmic microwave background radiation

• FAST telescope - 70 MHz – 3 GHz

Extraterrestrial Communication

• Television Broadcast Frequencies

– Band RF Channels Frequency

– VHF-Lo 2 – 6 54 - 88 MHz

– VHF-Hi 7 – 13 174 - 216 MHz

– UHF 14 – 69 470 - 806 MHz

• FAST 70 MHz to 3.0 GHz

• SETI (Water Hole) 1.42 GHz – 1.72 GHz

• Interstellar Communication 1 .0 GHz - 10 GHz

• Microwave 300 MHz – 300 GHz

• As the most sensitive single dish radio telescope, FAST would be able to discover more mega-masers and measure the radial velocities of masers with higher precision. This may yield more delicate dynamics of their maser spots. FAST will increase the precision of time of arrival (ToA) measurements for pulsars. This will help in detecting the stochastic gravitational wave background and in establishing an independent timing standard based on the long-term stability of the rotations of a group of millisecond pulsars. FAST might also work as a very powerful ground station for the future space missions. In a three-way communication mode, FAST should be able to provide precise ranging and Doppler measurements. Moreover, by joining the international VLBI network, FAST would help to improve the precision of the VLBI astrometry measurements. An astrophysical maser is a naturally occurring source of stimulated spectral line emission, typically in the microwave portion of the electromagnetic spectrum. This emission may arise in molecular clouds, comets, planetary atmospheres, stellar atmospheres, or various other conditions in interstellar space.

Large scale neutral hydrogen survey

Pulsar observations

Leading the international very long baseline interferometry (VLBI) network

Detection of interstellar molecules

Detecting interstellar communication signals

Pulsar timing arrary

• Calculations reveal that if we use an omni directional antenna with a transmitter power of 1000 MW (for comparison, the EIRP of a typical television station is about 1 MW, and the radiated power of the most powerful transmitter on earth is about 10 million MW), then:

• The Parkes 65 meter telescope in Australia could detect the signal to 4.5 light years, and it would reach only one star — α Cen. The Arecibo 305 meter telescope detection distance is 18 light years, and it could reach 12 stars. FAST could search out to 28 light years, and would be able to reach 1400 stars. If we increase the transmitter’s radiated power to 1000,000 MW, Parkes could reach 5000 targets, while FAST would be able to reach a million stars

• Black holes - a place in space where gravity pulls so much that even light can not get out. • Can’t observe with light or other EM radiation Can infer the presence of black holes and study them

by detecting their effecf on other matter nearby e.g. cloud of matter – accretion tears star apart emits

x-rays

• Exoplanets

• first objects to emit light in the Universe

• dark energy – measurements of supernovae movement published observations of Type Ia supernovae. suggest that the expansion of the universe is accelerating

• dark matter its existence and properties are inferred from its gravitational effects such as the motions of visible matter

• Determine if there is life elsewhere in the galaxy

• identify potentially habitable planets

• Quasars a massive and extremely remote celestial object, emitting exceptionally large amounts of energy, and typically having a starlike image in a telescope. It has been suggested that quasars contain massive black holes and may represent a stage in the evolution of some galaxies. Astronomers called them "quasi-stellar radio sources," or "quasars," because the signals came from one place, like a star.

• Pulsars(short for pulsating radio star) is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation

• gravitational waves

• near-Earth asteroids, Kuiper belt objects

• novae and supernovae

• Quasars a massive and extremely remote celestial object, emitting exceptionally large amounts of energy, and typically having a starlike image in a telescope. It has been suggested that quasars contain massive black holes and may represent a stage in the evolution of some galaxies. Astronomers called them "quasi-stellar radio sources," or "quasars," because the signals came from one place, like a star.

• Pulsars(short for pulsating radio star) is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation

• pulsars to search for gravitational waves

• The pulses from Millisecond Pulsars (MSPs) are used as a system of Galactic clocks. Disturbances in the clocks will be measurable at Earth. A disturbance from a passing gravitational wave will have a particular signature across the ensemble of pulsars, and will be thus detected. This radiation can be observed only when the beam of emission is pointing toward Earth (much the way a lighthouse can be seen only when the light is pointed in the direction of an observer), and is responsible for the pulsed appearance of emission. Neutron stars are very dense, and have short, regular rotational periods. This produces a very precise interval between pulses that range roughly from milliseconds to seconds for an individual pulsar

• gravitational waves

• near-Earth asteroids, Kuiper belt objects

• novae and supernovae

• Supernovae are useful for cosmology because they are excellent standard candles across cosmological distances. They allow the expansion history of the universe to be measured by looking at the relationship between the distance to an object and its redshift, which gives how fast it is receding from us. The relationship is roughly linear, according to Hubble's law. It is relatively easy to measure redshift, but finding the distance to an object is more difficult. Usually, astronomers use standard candles: objects for which the intrinsic brightness, the absolute magnitude, is known. This allows the object's distance to be measured from its actual observed brightness, or apparent magnitude. Type Ia supernovae are the best-known standard candles across cosmological distances because of their extreme and consistent luminosity

• And now there’s a planet of 1.3 Earth masses right next door, zipping around its star in 11.2 days. Its distance of 4,349,598 miles (7 million kilometers) from its star may seem tiny, at just one-fifth the distance between Mercury and the Sun, but Proxima Centauri is the runt of the litter in the Alpha Centauri system. At a diameter of 124,274 miles (200,000km), it’s only 1.43 times the diameter of Jupiter,.

• So how was there a planet hiding around the closest star to us, just waiting to be discovered? The simple answer: Finding a planet is really hard. Kepler found thousands of planets by staring at 145,000 stars in a minute region of the sky at the tail end of Cygnus, waiting for the 1 percent chance a planet would directly pass in front of a star and cause a dip in its light, in a method known as transiting.

• But the problem with the Proxima Centauri planet is that it doesn’t transit — at least not from our vantage point. In order to witness a transit, the orbital plane of the planets must be at or near our line of vision, but not all solar systems have the same orientation. A star might have all of its planets aligned at a 90-degree angle from us, with the planets orbiting in such a way that they never pass in front of their star for our telescopes to see. While some planets have been found by direct imaging (that is, appearing in a photo along with its star) it’s not possible of yet with Proxima, a 5 billion year old planet. Unless the planets are very young and very large, no instruments are currently capable of directly imaging these p

• That’s why the Pale Red Dot project, tasked with finding a planet around our nearest neighbor, had to turn to indirect — but reliable — methods of detection. The researchers chose radial velocity, a process that looks for shifts in a star’s light due to the tug of a planet, sometimes called the Doppler shift method. Subtle movements of gravity cause the light of a star to move toward the blue end of the light spectrum, which means it’s moving toward us, or the red end of the spectrum, which means it’s moving away. Based on those changes, researchers can give a mass estimate, and the frequency gives an idea of the orbit.

• The planet itself was found over a series of nights from January 19 to March 31, 2016, during which Proxima was monitored closely for subtle variations on the European Southern Observatory’s HARPS instrument.

How do we find them

• Exoplanets • Transit - passes in front of a star and cause a dip in its light • Proxima Centauri planet – discovered in 2016 – doesn’t transit • radial velocity

• process that look for shifts in a star’s light due to the tug of a planet, sometimes called the Doppler shift method.

• Subtle movements of gravity cause the light of a star to • move toward the blue end of the light spectrum, which

means it’s moving toward us, or move toward the red end of the spectrum, which means it’s moving away.

• Based on those changes, researchers can give a mass estimate, and the frequency gives an idea of the orbit.

James Webb Space Telescope (JWST)

NASA Launch – Oct 2018