4. Telescopes

Light gathering power and resolution Optical and radio telescopes Limitations of Earths 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.

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 reflectionThis 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

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 telescopes 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

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 Earths 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 airGround level4.4 Limitations of Ground-based observing - Effect of the Atmosphere

Solution 1 to limitation 1 - go to space!Solution 2 - develop adaptive optics to correct for effect of Earths atmosphere

Limitation 2 - The Earths 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

Ground-based observatories tend to be located on tops of mountains with low rainfall to limit atmospheric affectsKitt Peak National Observatory - near Tucson, Arizona

Ground-based Observing

La Campanas Observatory in Chile.

Telescopes

The twin 10 meter Keck reflecting telescopes.

Telescopes

The 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 Observing

Astronomers 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 Up until the early-1980s, 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.

Most CCDs are over 90% efficient at detecting photons.

CCDs are made up of light sensitive diodes laid out in a square array. These diodes are known as pixels (picture elements).Photometry - by calculating the area under this curve, we can measure the apparent brightness of this star.

Photometry - measuring the apparent brightness of this 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 telescopesThe 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:

Radio Observatories