Astronomy 101Astronomy 101Section 020Section 020

Lecture 6Lecture 6

Optics andOptics andTelescopesTelescopes

John T. McGraw, ProfessorJohn T. McGraw, Professor

Laurel Ladwig, Planetarium ManagerLaurel Ladwig, Planetarium Manager

TelescopesTelescopes

The fundamental purpose of The fundamental purpose of any telescope is to gather any telescope is to gather more light than the eye. more light than the eye.

The second purpose of a The second purpose of a telescope is to see finer telescope is to see finer detail.detail.

The third purpose is to form The third purpose is to form an image.an image.

Magnification is NOT a Magnification is NOT a primary purpose of a primary purpose of a telescope.telescope.

Refraction of Light

Magnification by a TelescopeMagnification by a Telescope

The magnification of a telescope is equal to the focal length of The magnification of a telescope is equal to the focal length of the objective divided by the focal length of the eyepiece the objective divided by the focal length of the eyepiece

Light Gathering PowerLight Gathering Power

The The light-gathering power light-gathering power of a telescope is directly proportional to of a telescope is directly proportional to the area of the objective lens, which in turn is proportional to the the area of the objective lens, which in turn is proportional to the square of the lens diameter.square of the lens diameter.

Why astronomers don’t use refracting telescopes - Why astronomers don’t use refracting telescopes - chromatic aberrationchromatic aberration

Lenses bend different colors of light through different angles, just as a prism doesLenses bend different colors of light through different angles, just as a prism does As a result, different colors do not focus at the same point, and stars viewed through a telescope As a result, different colors do not focus at the same point, and stars viewed through a telescope

that uses a simple lens are surrounded by fuzzy, rainbow-colored halosthat uses a simple lens are surrounded by fuzzy, rainbow-colored halos

If the telescope designer carefully chooses two different kinds of glass for two lenses that make If the telescope designer carefully chooses two different kinds of glass for two lenses that make up one objective, different colors of light can be brought to a focus at the same pointup one objective, different colors of light can be brought to a focus at the same point

The theorist and the proto-observer

Glass Glass impurities, impurities, chromatic chromatic aberration, aberration, opacity to opacity to certain certain wavelengths, wavelengths, and structural and structural difficulties make difficulties make it inadvisable to it inadvisable to build extremely build extremely large refractors.large refractors.

(And they make (And they make astronomers astronomers look old!)look old!)

A reflecting telescope uses a mirror to A reflecting telescope uses a mirror to concentrate incoming light at a focusconcentrate incoming light at a focus

Reflecting telescopes, or Reflecting telescopes, or reflectors, produce images reflectors, produce images by reflecting light rays to a by reflecting light rays to a focus point from curved focus point from curved mirrors.mirrors.

Reflectors are not subject Reflectors are not subject to most of the problems to most of the problems that limit the useful size of that limit the useful size of refractors.refractors.

The basic reflecting telescope

Reflecting TelescopesReflecting Telescopes

Gemini North TelescopeGemini North Telescope

1.1. The 8.1-meter The 8.1-meter primary mirrorprimary mirror

2.2. The 1.0-meter The 1.0-meter secondary mirror secondary mirror

3.3. The Cassegrain holeThe Cassegrain hole

Telescope images are degraded by the blurring Telescope images are degraded by the blurring effects of the atmosphere and by light pollutioneffects of the atmosphere and by light pollution

Angular Resolution: Angular Resolution: A telescope’s angular resolution, which indicates A telescope’s angular resolution, which indicates ability to see fine details, is limited by two key factorsability to see fine details, is limited by two key factors

Diffraction is an intrinsic property of light waves.Diffraction is an intrinsic property of light waves.

The blurring effects of atmospheric turbulence can be minimized by placing The blurring effects of atmospheric turbulence can be minimized by placing the telescope atop a tall mountain with very smooth air.the telescope atop a tall mountain with very smooth air.

Blurring (astronomical “seeing”) can be dramatically reduced by the use of Blurring (astronomical “seeing”) can be dramatically reduced by the use of adaptive optics and can be eliminated entirely by placing the telescope in adaptive optics and can be eliminated entirely by placing the telescope in orbit orbit

The Very Large Telescope (VLT)

An electronic device is commonly used to An electronic device is commonly used to record the image at a telescope’s focusrecord the image at a telescope’s focus

Sensitive light Sensitive light detectors called detectors called charge-coupled charge-coupled devices (CCDs) are devices (CCDs) are often used at a often used at a telescope’s focus telescope’s focus to record faint to record faint images.images.

Why use a CCD?

Spectrographs record the spectra of Spectrographs record the spectra of astronomical objectsastronomical objects

A spectrograph uses a diffraction grating and lenses to form the A spectrograph uses a diffraction grating and lenses to form the spectrum of an astronomical object.spectrum of an astronomical object.

A spectrum of a star

Observation of hydrogen in the atmosphere of a star

A radio telescope uses a large concave dish A radio telescope uses a large concave dish to reflect radio waves to a focusto reflect radio waves to a focus

Radio telescopes use large Radio telescopes use large reflecting antennas or reflecting antennas or dishes to focus radio dishes to focus radio waveswaves

Very large dishes provide Very large dishes provide reasonably sharp radio reasonably sharp radio imagesimages

Higher resolution is achieved with interferometry techniques that Higher resolution is achieved with interferometry techniques that link smaller dishes together.link smaller dishes together.

On the Plains of San Agustin

Optical and Radio Views of SaturnOptical and Radio Views of Saturn

Telescopes in orbit around the Earth detect Telescopes in orbit around the Earth detect radiation that does not penetrate the radiation that does not penetrate the

atmosphereatmosphere

The Earth’s atmosphere absorbs much of the radiation that The Earth’s atmosphere absorbs much of the radiation that arrives from spacearrives from space

The atmosphere is transparent chiefly in two wavelength The atmosphere is transparent chiefly in two wavelength ranges known as the optical window and the radio windowranges known as the optical window and the radio window

A few wavelengths in the near-infrared also reach the A few wavelengths in the near-infrared also reach the groundground

For observations at For observations at wavelengths to which the wavelengths to which the Earth’s atmosphere is Earth’s atmosphere is opaque, astronomers opaque, astronomers depend on telescopes depend on telescopes carried above the carried above the atmosphere by rockets or atmosphere by rockets or spacecraft spacecraft

The James Webb Space Telescopewill replace the Hubble Space Telescope

Satellite-based observatories provide new Satellite-based observatories provide new information about the universe and permit information about the universe and permit coordinated observation of the sky at all coordinated observation of the sky at all

wavelengthswavelengths