4. Telescopes

• Light gathering power and resolution

• Optical and radio telescopes

• Limitations of Earth’s atmosphere and satellite missions.

• Instruments (prism spectrographs and diffraction gratings), detection devices (CCDs) and methods (photometry and spectroscopy)

4.1 What is a telescope?

A telescope gathers and focuses electromagnetic radiation.

Optical telescope designs - we use reflecting telescopes today

Telescopes with lenses work because of refraction: at the interface between two transparent media the light path changes because the speed of light depends on the medium that it is passing through.

2211 sinsin :Law sSnell' nn

Problems for refracting telescopes: the degree of refraction depends on wavelength - can lead to chromatic aberration - wavelength dependent focal point.

Telescopes with mirrors work because of reflection:

angle of incidence = angle of reflection

This effect is wavelength independent and can be exploited to bring light of all wavelengths to the same focus with concave or convex mirrors.

Think of a telescope as a bucket sitting in a rain of photons: increasing the aperture (diameter) of a telescope its light gathering power (it can capture more photons in the same amount of time because it is bigger). I.e.

4.2 Why a BIG Telescope? - Light Gathering Power

LGP D2

E.g. Two pictures of the Andromeda galaxy taken with the same exposure time…...

Light from the same object incident on different portions of the telescope’s mirror can interfere constructively and destructively to form a pattern of light and dark Airy rings (similar to the double slit experiment).

4.3 Why a BIG telescope? - Resolution and the Diffraction Limit

Airy rings around a single star. The interference pattern produced as the Airy rings of two stars overlap each other.

The larger the aperture size of the telescope, the smaller or more compact the Airy rings.

The smaller the aperture size of the telescope, the larger or more extended the Airy rings.

No image can be smaller than the innermost Airy ring, and this sets the diffraction limit of a telescope to be

min 1.22(206,265)D

where D is the telescope's aperture

diameter of the telescope's mirror

(in same units used to measure the wavelength).

min (the above expression gives the answer

in arcseconds) sets a limit to the resolution of

the image.

E.g. The Andromeda galaxy seen by a small telescope………

……vs the Andromeda galaxy seen by a telescope with a larger aperture.

Limitation 1: turbulence in the Earth’s atmosphere causes starlight to be blurred - an effect known as seeing.

Seeing typically limits the resolution of ground-based telescopes to no better than 1 arcsecond

Cells of air

Ground level

4.4 Limitations of Ground-based observing - Effect of the Atmosphere

Ground-based Image Ground-based Image HSTHST

Because the Hubble Space Telescope (HST) is above the atmosphere, it is not affected by atmospheric turbulence that blurs ground-based observations.

Solution 1 to limitation 1 - go to space!

Solution 2 - develop adaptive optics to correct for effect of Earth’s atmosphere

Limitation 2 - The Earth’s atmosphere is not transparent to all wavelengths, so IR, UV, X-ray and gamma-ray observations require satellite missions (the only solution).

Space-Based Observing

The Chandra X-ray satellite…….

Ground-based observatories tend to be located on tops of mountains with low rainfall to limit atmospheric affects

Cerro Tololo Inter-American Observatory - in the Chilean Andes near La Serena, Chile

Ground-based observatories tend to be located on tops of mountains with low rainfall to limit atmospheric affects

Kitt Peak National Observatory - near Tucson, Arizona

Ground-based Observing

La Campanas Observatory in Chile.

Telescopes

The twin 10 meter Keck reflecting telescopes.

TelescopesThe 10-meter Keck primary mirror consists of many smaller mirror segments which fit together precisely to create the reflecting surface.

Just one of the Keck mirror segments…..

Ground-based ObservingAstronomers no longer look through the eyepieces of telescopes and draw pictures - rather they collect data using instruments and analyze them on computers

Going observing can be quite a magical experience….

4.5 Detecting the Light

We no longer use our eyes to look through telescopes for the purpose of astronomical study.

Up until the early-1980’s, the principal light detector that optical astronomers used was the photographic plate. This consisted of a flat piece of rectangular glass about 1/4 of an inch thick which was coated on one side with a photographic emulsion.

Photographic plates were extremely inefficient! For every 100 photons that fell on the plate, only one was detected - 1% efficiency.

Nowadays, we use sensitive cryogenically cooled solid state light detectors known as Charge Coupled Devices (CCDs).

Most CCDs are over 90% efficient at detecting photons.

(a)(a) (b)(b) (c)(c)

(a) A negative print (black stars on white sky) of a photographic image. (b) A CCD image. Notice the many faint objects that are revealed. (c) This color picture was created by combining a series of CCD images taken through different filters.

CCD’s are made up of light sensitive diodes laid out in a square array.CCD’s are made up of light sensitive diodes laid out in a square array.

These diodes are known as These diodes are known as pixelspixels ( (picpicture ture elelements).ements).

xx

yy

Star on a CCDStar on a CCD

00

# of

pho

tons

# of

pho

tons

xx

PhotometryPhotometry - by calculating the - by calculating the area under this curve, we can area under this curve, we can measure the apparent brightness of measure the apparent brightness of this star.this star.

x

dxcurve)(

PhotometryPhotometry - measuring the apparent brightness of - measuring the apparent brightness of this starthis star. . Astronomers also use filters to measure brightness in a specific range of wavelengths (e.g. red light vs blue light).

Spectroscopy - using a diffraction grating or prism astronomers split the light into a spectrum and measure the intensity of light at each wavelength.

The wavelength dependence of refraction allows prisms of glass to to split light into spectra:

At the first interface (air/glass) the light rays are bent towards the normal perpendicular to the surface.

At the second interface (glass/air) the light rays are bent away from the normal.

Used in prism spectrographs to split light into spectra.

4.5 Radio telescopes

The diffraction limit means that telescopes to detect longer wavelengths have to be larger to maintain a reasonable angular resolution. Radio telescopes achieve this either with huge single dishes, or by synthesizing signals from more than one dish:

Very Large Array (VLA) - near Socorro, New Mexico

Radio Observatories

Arecibo Observatory - Puerto Rico