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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. - PowerPoint PPT Presentation
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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. 2radiusAreaPowerGatheringLight
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
ri
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
52.06 10D
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