Chapter 6:The Tools of the Astronomer

Telescopes come in two general types

Refractors use lenses to bend the light to a focus

Reflectors use mirrors to reflect the light to a focus

The most important property of any telescope is to gather large amounts of light and concentrate it to a

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Refraction is the bending of light when it goes from one

medium to another

“n” is the index of refraction.

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The refracting telescope uses two lenses

Since the eye already has a lens, the eyepiece is needed to bring the light rays back to parallel for the eye to see

Large refractors can be very long

and bulky

The Largest Lens is 40”

Lenses and refractors suffer from Chromatic

Aberration

This applies to camera lenses, your eye, telescopes and anything else that uses a lens to focus light

Correcting for Chromatic aberration can be

expensive

The compound lens takes two lenses of different materials and combines them to correct for color

distortion

Color separation is useful in a prism so that we can obtain a

spectrum of lightSince it is meant to be separated we don’t call it an aberration. Instead, it is called dispersion

A diffraction grating works on interference of light waves

Diffraction is much more efficient at separating light into its colors than dispersion

Unfortunately, diffraction also leads to problems

Look closely enough and points aren’t just points but rings, too

Refractors have many problems

They are large and bulky and

difficult to maneuver

They suffer from

chromatic aberration

Even the best glass cuts off the IR and UV wavelengths

Reflection is the bouncing of light off a surface

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Mirrors do not suffer from chromatic aberration and they do not cut off long or short

wavelengths

A concave mirror

focuses light to a focal

pointTelescope mirrors are made so that the focus is a plane instead of a point

There are several types of

reflecting telescope

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The resolution of a telescope depends on its size and the

wavelength of the light

D 51006.2 Better resolution

means smaller angle

The atmosphere limits the resolving power of a

ground-based telescope

Adaptive Optics can clear up most of the distortions caused by the atmosphere

The distortions are caused

by differences in the air above the telescope

Once a sight with good “seeing” is found everyone

wants to use itKitt

Peak Arizona

Many of the worlds largest optical telescopes are on top

of Mauna Kea Hawaii

The Largest Optical Telescope

Keck I and II (for now)

The 10 meter Keck Mirror

How do we use telescopes?

Early telescopic observations were done by eye using the “view and sketch” system

An early 19th century sketch of the head of a comet

By the early 1900’s photographic plates were the dominant scientific way to observe

One of the earliest images of the Moon

An early image of the Great Nebula in Orion

Today scientific observations are done with

a CCD Camera

A CCD converts photons into electrons and then counts the

electrons

Each pixel acts like a light bucket, catching photons, converting them to electrons and storing them until they are read out by the electronics. Check out the

CCD Simulator in the Animations section of the Telescope and Astronomical Instruments module on

the ClassAction website

Another common measuring device is the spectrograph

Early spectrographs used prisms to separate the light into the colors

Modern spectrometers use diffraction gratings

The CCD takes a black & white picture of a spectrum

Spectra can be displayed as a graph or rainbow of color

Visible light is only a small part of the

electromagnetic spectrum

Observing In Radio WavesRadio waves have long

wavelengths so they have poor resolution

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To compensate for the long of radio waves, radio telescopes have very large diameters

Under the dish at Arecibo

The Very Large Array (VLA)

The VLA can tie many radio dishes together in an interferometer which gives much higher resolution

To view in IR you need to get above the atmosphere

The Spitzer was placed in orbit to view IR

Observing Neutrino’s opens up a new window on the universe

Neutrino’s are very hard to detect since they don’t interact with normal matter very much

Looking for gravity waves is another new technique

LIGO uses an interferometer to detect the passing gravity waves and has two sites

Some wavelengths require observing in space

The Chandra Observatory sees in x-rays The GALEX

mission observes the sky in ultraviolet

WMAP looks in the

microwave

The sky looks different in each wavelength we observe

The branch of astronomy called Cosmology is meeting up with high energy particle

physics

We also use computer models to simulate astronomical

eventsWatch Galaxy Merger Simulation, Binary

Pulsar Merger and Hypernova Swift 1 videos