# Chapter 6: The Tools of the Astronomer

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Chapter 6: The Tools of the Astronomer

Telescopes gather light and concentrate it.
They come in two types: Reflectors and Refractors Reflectors use mirrors to reflect the light to a focus Refractors use lenses to bend the light to a focus

The most important property of any telescope is to gather lots of light and concentrate.
The Light Gathering Power (LGP) of a telescope depends on the area and the area is proportional to the square of the diameter

Refraction is the bending of light when it goes from one material to another
The Law of Refraction relates the incident angle, q1, and the refracted angle, q2, to the index of refraction of each of the two materials Recall that the index of refraction is the ratio of the speed of light in vacuum to the speed of light in the material

If we curve the surfaces of a piece of glass, we can get parallel light rays to focus to a point

A 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” The large refractor at the Yerkes Observatory in Wisconsin was built in the 1897.

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

Chromatic Aberration causes rainbows around objects

Correcting for Chromatic aberration can be expensive
The compound lens takes two lenses of different materials and combine them to correct for color distortion

Chromatic aberration is used in a prism to separate light into its colors
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

Reflection is the bouncing of light off a surface

A concave mirror focuses light to a focal point
Telescope mirrors are made so that the focus is over a small area called the focal plane rather than a point

There are several types of reflecting telescopes

The resolution of a telescope depends on its size and the wavelength of the light

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

Mauna Kea Hawaii

The Largest Optical Telescope Keck I and II (for now)

The 10 meter Keck Mirror

The Earth’s Atmosphere Blocks Many Wavelengths
Only visible light, radio waves and some Infrared can be “seen” from the ground. To see the rest, you must go get outside Earth’s atmosphere by going into space

Early telescopic observations were done by eye and sketch

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

Today all 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. Watch ClassAction Telescopes and Astronomical Instruments module CCD Simulator animation

CCD’s only take black & white images
To make a color image we either take images with red, green and blue filters or use microfilters

Some CCD Cameras are HUGE
The LSST telescope is being built in Chile

Another common measuring device is the spectrograph

Spectrographs can use prisms or diffraction gratings
The diffraction grating spectrometer just takes a picture of the spectrum produced by the grating The mirror just before the prism rotates to put the different colors on the photomultiplier tube

The spectrograph produces a spectrum of the object
The CCD only takes a black and white image so it must be calibrated. The lines at the top and bottom are produced by a calibration light source and are used to calibrate the wavelengths.

Spectra can be displayed as a graph or rainbow of color

Radio waves have long wavelength and so have poor resolution

Under the dish at Arecibo
As long as the spacing in the mess is much less than the wavelength, the “light” sees the mesh as a solid surface

The Very Large Array (VLA)
Instead of building one huge dish, the VLA ties many smaller radio dishes together in an interferometer which gives much higher resolution

The individual dishes of the VLA are still large

Optical Interferometry is extremely difficult but it is being tried
The Very Large Telescope in Chile combines several telescopes together to form an interferometer.

The atmosphere blocks most IR so you have to get above it to see in IR
The SOFIA is an IR telescope mounted in a 747

Some wavelengths require observing in space
The GALEX mission observes the sky in far ultraviolet The Chandra Observatory sees in x-rays The Spitzer Space Telescope looks at the universe in far infrared

To learn about a planet we need to go there
The Cassini spacecraft is currently orbiting Saturn The Voyager I and II missions were fly-by missions. Voyager I flew by Jupiter and Saturn. Voyager made the grand tour and flew by Jupiter, Saturn, Uranus and Neptune

We learn even more when we land on a planet
Huygens landed on Titan in 2005 The Mars Curiosity Rover is roaming around on Mars The Russian Venera missions landed on Venus in the 1970’s and 1980’s

We learn the most if we go there and bring something back
The US Apollo missions brought back a total of 382 kilograms of moon rocks. The Soviet Luna robotic missions brought back 362 grams NASA plans to launch a Mars sample return mission in 2018

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

We also use computer models to simulate astronomical events
The collision of two galaxies takes billions of years but can be simulated with a supercomputer