Telescopes

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• Basic function of a telescope: extend human vision– Collect light from celestial object

– Focus light to create image or spectrum of the object

– Use larger aperture than the human eye

– Expose for longer than the human eye

– Achieve better resolution than human eye

– Observe at wavelengths the eye is not sensitive to (i.e. beyond 400 – 700 nm)

– Examine spectral information in detail

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Light Hitting a Telescope Mirror

huge mirror near a star

small mirror far from 2 stars

In the second case (reality), light rays from any single point of light are essentially parallel. But the parallel rays from the second star come in at a different angle. 3

If the mirror is a particular shape, a paraboloid, light rays from a distant source, parallel to the "mirror axis" all meet at one point, the focus.

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Image Formation

"focal plane"

Light rays from a distant, extended source are all focused in the same plane, the "focal plane" creating an image of the source.

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Optical telescopes Kinds of optical telescopes:

1) Refractor – uses a lens that light passes through, to concentrate light. Galileo’s telescope was a refractor.

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<-- object (point of light) image at focus

• Lens can only be supported around edge

• Some light absorbed in glass (especially UV, infrared)

• Air bubbles and imperfections affect image quality

• "Chromatic aberration"7

Problems with Refracting Telescopes

Chromatic Aberration

Lens - different colors focus at different places.

white light

Mirror - reflection angle doesn't depend on color.

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blue focus red focus

Largest Refracting Telescope Built

Yerkes 40-inch (about 1 m).

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Solution:

2) Reflecting telescope

use concave mirror (shape is ideally parabolic), not lens, to focus light. Newton built first one. Big, modern research telescopes are reflectors.

10Gemini South 8-m reflector.

Reflector advantages• Mirrors can be large, because they can be supported from

behind.

• No chromatic aberration

• Less light lost and fewer image quality problems

Largest single mirror built: 8.4 m diameter for the Large Binocular Telescope

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• There are 10 m telescopes, but in segments

Keck 10-m telescope12

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focus options

or Nasmythfocus

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Nasmyth focus platforms

Image of Andromeda galaxy with optical telescope.

Image with telescope of twice the diameter, same exposure time.

Characteristics of telescopes• Light gathering power: area, or D2 Main reason for building

large telescopes!

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Characteristics of telescopes, cont.

• Magnification: angular diameter as seen through telescope/angular diameter on sky– Typical magnifications 10 to 100

• Field of View: how much of sky can you see at once? Typically many arcminutes – few degrees.

• Resolution: The ability to distinguish two objects very close together. Angular resolution:

θ = 2.5 x 105 /D where θ is angular resolution of telescope in arcsec, is wavelength

of light, D is diameter of telescope objective, in same distance units.

• Example, for D=2.5 m, λ=500 nm, θ = 0.05”17

Two light sources with angular separation larger thanangular resolution vs. equal to angular resolution

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But, “seeing” limits resolution for ground-based optical telescopes

* Air density varies => bends light. No longer parallel

Parallel rays enter atmosphere

CCD

No blurring case. Rays brought to same focus.

* Sharp image on CCD.

Blurring. Rays not parallel. Can't be brought into focus.

Blurred image.

resolution limited to about 1”19

fuzziness you would see with your eye.

detail you can see with a telescopeon ground.

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Example: the Moon observed with a 2.5 m telescope

1" => 2 km

0.05" => 100 m

Ground-based telescope image, 1” resolution

Hubble SpaceTelescope image,0.05” resolution 21

DetectorsQuantum Efficiency = how much light they respond to:

– Eye 2%

– Photographic emulsions 1-4%

– CCD (Charge coupled device) 80%• Can be used to obtain images or spectra

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CCDs also provide data directly in digital form – easier to process.

Photographic film CCD

Same telescope, same exposure time!23

Spectrographs: light spread out by wavelength, using prism or “diffraction grating”

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Some future optical telescopes

Large Synoptic Survey Telescope (LSST): 8-m telescopewith large field of view (3.5°). Will survey entire skyrepeatedly. Site in Chile. First light 2019.

Thirty Meter Telescope (TMT):

segmented design, like Keck.First light 2022. 25

Radio Telescopes

Large metal dish acts as a mirror for radio waves. Radio receiver at prime focus.

Surface accuracy not so important, so easy to make large one (surface shouldn’t have irregularities that are larger than 1/16 ).

But angular resolution is poor. Remember: θ = 2.5 x 105 /D

D larger than optical case, but much larger (cm's to m's), e.g. for = 1 cm, diameter = 100 m, resolution = 20".

Andromeda radiomap with Effelsberg telescope

Effelsberg 100-m (Germany)

Andromeda galaxy –optical

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Parkes 64-m (Australia)

Green Bank 100-m telescope (WV) Arecibo 300-m telescope (Puerto Rico)

Jodrell Bank 76-m (England)

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• So how can we get better resolution?

• Interferometers – e.g., VLA

Use interference of radio waves to mimic the resolution of a telescope whose diameter is equal to the separation of the dishes

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InterferometryA technique to get improved angular resolution using an array of telescopes. Most common in radio, but also limited optical interferometry.

D

Consider two dishes with separation D vs. one dish of diameter D. By interfering the radio waves from the two dishes, the achieved angular resolution is the same as the large dish. 29

Example: wavelength = 1 cm, separation = 2 km, resolution = 1"

Very Large Array (NM). Maximum separation 30 km (only about 1km in thisconfiguration).

Very Long Baseline Array. Maximum separation 1000's of km

VLA and optical image of Centaurus A

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Atacama Large Millimeter Array

• 18,000 ft elevation plateau in Chile• USA/Europe/Japan collaboration• Started observing in 2011 with a few dishes• 66 dishes eventually

• UNM is building its own array for =3-10m: the Long Wavelength Array (LWA)

• Far larger than the VLA, to give same resolution. “Stations” of 256 antennas, to be spread across NM

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• Square Kilometer Array, currently being designed, will be 50 times collecting area of VLA, with baselines to 1000’s of km 33

Optical-to-mm-wave Telescope Sites

• Site requirements– Dark skies (avoid light pollution)

– Clear, dry skies

– Good “seeing”, stable atmosphere

• High, dry mountain peaks are ideal observatory sites, for optical to mm waves.

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USA at night

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Mauna KeaObservatory,Hawaii

Kitt PeakNationalObservatory,Arizona 36

Radio Telescope Sites

• Away from radio interference is most important. Radio astronomy can be done in cloudy weather, day or night.

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Telescopes in spacePros – above the atmospheric opacity so can work at

impossible from ground, above turbulence, weather, lights on Earth

Cons – expensive! Repairs difficult or impossible.

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Spitzer Space Telescope - infrared

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Shorter infrared wavelengths allow you to see through dust. Dust is good at blocking visible light but infrared gets through better.

Trifid nebula in visible light Trifid nebula with Spitzer

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Longer infrared wavelengths allow you to see radiation from warm dust in interstellar gas

FERMI – Gamma Ray Telescope

Gamma rays are the most energeticphotons, tracing high-energy events inUniverse such as “Gamma-ray Bursters”.

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Hubble Space Telescope and its successor-to-be: the James WebbSpace Telescope

Advantage of space for optical astronomy: get above blurring atmosphere – much sharper images.

Center of M51: HST (left; 0.05” resolution) vs.ground-based (right; 1” resolution)

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The JWST

Diameter 6.5 meters (vs. HST 2.5 meters) – much higher resolution and sensitivity. Will also observe infrared, whereas Hubble is best at visible light. Expected launch 2018.

Mock-up of JWST

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