Chapter 6 Astronomical Instruments

Ch. 3: Why do we need telescopes? Humans can only see 6000 stars at night and cannot resolve (discern) the detail of objects in the sky if they are too close together. Humans also cannot see anything but visible light. We also need some way to record images, like in photography.

A Reflecting Mirror brings light to a focus

A Refracting Lens can form an image by using the principle of the refraction of light see this link for a marching band analogy

A Refracting Lens can be thought of as being built up from a collection of prisms that deviate the light, so that it comes to a focus.

Refraction of light occurs when light passes from one type of material into another, in which the speed of light is different.

Image Formation can also occur with a concave mirror

Two types of Telescope: Reflectors and Refractors

Refractors are no longer used for much research The Yerkes observatory (near Chicago) has the largest-ever refractor, with a 40 inch diameter lens. See their web site.

Types of Reflecting Telescopes

The Palomar Telescope was the largest for many years

Palomar Telescope (notice the person near the focus)

Telescopes should be large to give lots of sensitivity to dim light Longer exposures also produce better images. But you have to quit before dawn, so the exposure time of the photo is obviously limited.

Mauna Kea Observatory in Hawaii

Keck telescopes on Mauna Kea, Hawaii

Keck telescopes – cutaway view (website)

Very Large Telescopes Observatory in Chile

Telescopes on the Canary Islands (Atlantic Ocean)

Telescopes on Kitt Peak (near Tucson, AZ)

Reflectors can have larger apertures than Refractors

Types of Reflecting Telescopes

Resolution is critical for seeing image detail

Diffraction is an effect that limits resolution in images

Atmospheric Turbulence is reduced at high altitudes

CCD Chips are used, similar to those in your digital camera

Image processing can sharpen an image, though not quite like you see in some of the spy shows on TV

Active Optics are used in the Keck telescopes and VLT (web) Without active optics With active optics

Adaptive Optics use a laser beam as a “guide star” and continually change the shape of the mirror to compensate for atmospheric turbulence and distortion.

The Hubble Space Telescope

The Hubble Space Telescope has several cameras

Telescopes are available for most of the Electromagnetic Spectrum Radio (and microwave), infrared, visible, ultraviolet, X-ray, Gamma ray

Electromagnetic Waves Waves are characterized by their Wavelength – distance between crests Frequency – number of cycles per second Amplitude – displacement from equilibrium Type – transverse, longitudinal, etc. Polarization – direction of electric field in an electromagnetic wave Dan Russell demos here for class discussion

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Frequency f (in Hz) Wavelength l

Radio telescopes usually “look” at microwave signals Frequencies of some radio waves used in commercial service in the US (you don’t need to know these): AM radio: 535 kHz to 1610 kHz FM radio: 88 MHz to 108 MHz TV channels 2-6 are 54-88 MHz (before the digital era) TV channels 7-13 are 174-220 MHz TV channels 14-69 are 470-806 MHz (these are changing) Microwave oven: 2.45 GHz Cell phone: 824-894 MHz or 1850-1990 MHz (PCS) Most radio astronomy uses much higher frequencies Parabolic reflectors are often used for higher frequencies

Radio Telescope at Green Bank, West Virginia

Radio Telescope at Green Bank, West Virginia

Radio Telescope at Green Bank, West Virginia

Arecibo Observatory (Puerto Rico)

Radio “image” of a Radio Galaxy, along with the visible image

Very Large Array Interferometer, near Socorro, New Mexico

Very Large Array Interferometer, near Socorro, New Mexico

Very Large Array Interferometer, near Socorro, New Mexico

ALMA = Atacama Large Millimeter Array These are somewhat more compact, and will study millimeter waves, which require a precise surface. Now installed in the Atacama desert, Chile, South America

The National Radio Astronomy Observatory https://public. nrao The National Radio Astronomy Observatory https://public.nrao.edu/ has more detail on some of the newer radio telescopes This picture shows an ALMA telescope being moved up to the high desert in the Atacama desert of Chile, S. America.

Comparison of radio image (left) and optical image (right)

Infrared Telescopes are hoisted by balloons or put in orbit

Spitzer Infrared Telescope (Spitzer web site)

Infrared Image of the constellation Orion, compared to the visible image on the right

The Hubble Space Telescope can obtain ultraviolet pictures The Hubble Space Telescope can obtain ultraviolet pictures. Ultraviolet images are shown using false color.

X-ray Telescopes are made of mirrors of special layered metal

Chandra Observatory, being prepared for launch

Chandra Observatory, mounted in the shuttle Columbia (Eileen Collins, commander) (Harvard web site) (NASA web site)

X-ray Image

Compton Gamma Ray Observatory, during deployment from the space shuttle

Gamma-ray Image

Multiple Wavelengths give us a fuller picture of the cosmos

The great orbiting observatories of the late 20th century: Hubble Space Telescope Spitzer Space Infrared Telescope Chandra X-ray Observatory Compton Gamma Ray Observatory XMM-Newton X-Ray Observatory (ESA) Far Ultraviolet Spectroscopic Explorer links to websites of these observatories