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 focus.

Refraction is the bending of light when it goes from one medium to another “n” is the index of refraction.

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 light Since 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 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 point Telescope mirrors are made so that the focus is a plane instead of a point

There are several types of reflecting telescopes

The resolution of a telescope depends on its size and the wavelength of the light 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 it Kitt 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 19 th 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 Waves Radio waves have long wavelengths so they have poor resolution 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 events Watch Galaxy Merger Simulation, Binary Pulsar Merger and Hypernova Swift 1 videos