TelescopesTelescopes

AnalyzingAnalyzing electromagnetic electromagnetic spectra to spectra to search for understanding of celestial search for understanding of celestial

objects.objects.

What is a Telescope?What is a Telescope?

A A telescopetelescope is simply a device that is simply a device that captures as many photons as possible captures as many photons as possible from a given region of sky and from a given region of sky and concentrates them into a focused beam for concentrates them into a focused beam for analysis.analysis.Optical telescopesOptical telescopes are ones designed to are ones designed to

collect visible wavelengthscollect visible wavelengthsOther “Other “exotic” or high energy telescopesexotic” or high energy telescopes are are

used to collect “used to collect “invisibleinvisible” (high freq.) ” (high freq.) wavelengths.wavelengths.

OPTICAL TELESCOPESOPTICAL TELESCOPESLight gathering instruments Light gathering instruments that allow us to see objectsthat allow us to see objects

Galileo was the first to make practical Galileo was the first to make practical application of the telescope.application of the telescope.

Refracting telescopeRefracting telescope - gathers light with a - gathers light with a convex lensconvex lens

Reflecting telescopeReflecting telescope - gathers light with a - gathers light with a concave mirrorconcave mirror

REFRACTING TELESCOPESREFRACTING TELESCOPES

Refraction refers to the bending of light as it Refraction refers to the bending of light as it passes from one medium to another.passes from one medium to another.

Convex objective lens gathers lightConvex objective lens gathers light Upper limit of 40 inches diameter objective lensUpper limit of 40 inches diameter objective lens Commonly small telescopes for amateursCommonly small telescopes for amateurs Most terrestrial telescopes are refractorsMost terrestrial telescopes are refractors

Largest (40 inch) is at Largest (40 inch) is at Yerkes ObservatoryYerkes Observatory in in WisconsinWisconsin

REFRACTING TELESCOPEREFRACTING TELESCOPE

REFLECTING REFLECTING TELESCOPESTELESCOPES

Concave mirror gathers and focuses lightConcave mirror gathers and focuses light Mirrors may be much larger than the objective lenses of Mirrors may be much larger than the objective lenses of

refracting telescopesrefracting telescopes Most really serious observatory telescopes are reflectorsMost really serious observatory telescopes are reflectors The largest reflector in the US is the The largest reflector in the US is the 6.5 m Mt Hopkins 6.5 m Mt Hopkins

telescopetelescope The famous Hale telescope is located at Palomar The famous Hale telescope is located at Palomar

observatory in San Diego, CA.observatory in San Diego, CA. A technology called adaptive optics allows computers to A technology called adaptive optics allows computers to

control several small mirrors to gather and focus lightcontrol several small mirrors to gather and focus light

ReflectionReflection

One problem that must be surmounted with a reflecting telescope is how to place an observer at the focus. In the example shown above, the focus is inside the telescope. This is called the prime focus, and in some large telescopes observations can be made at the prime focus. More commonly, various mirror arrangements are used to transport the light from the focus to an external observer. Two common ones are a Cassegrain focus and a Newtonian focus.

Types of Reflector DesignsTypes of Reflector DesignsEach design uses a primary mirror at the bottom of the telescope to capture radiation, which is then directed along different paths for analysis. Notice that the secondary mirrors shown in (c) and (d) are actually slightly diverging, so that they move the focus outside the telescope.

Reflector or RefractorReflector or Refractor

A number of factors tend to favor reflecting over A number of factors tend to favor reflecting over refractorsrefractors1.1. Passing light through lens creates a deficiency Passing light through lens creates a deficiency

known as chromatic aberrationknown as chromatic aberration

2.2. Some light is actually absorbed as by the lens as Some light is actually absorbed as by the lens as light passes through itlight passes through it

3.3. Large lens can be heavy and hard to supportLarge lens can be heavy and hard to support

4.4. Lens have two surfaces (to be polished and Lens have two surfaces (to be polished and machined) where a mirror has only one.machined) where a mirror has only one.

Imaging & Non-imaging Imaging & Non-imaging DetectorsDetectors

Camera – photographsCamera – photographsPhotometer – device to measure the Photometer – device to measure the

amount of light received (intensity)amount of light received (intensity)Spectrometer – analyze and study of Spectrometer – analyze and study of

spectrumspectrum

Image ProcessingImage Processing

Use of cameras becoming rareElectronic detectors known as charge-

coupled devices (CCDs) are widespread. Their output goes directly to a computer. A CCD consists of a wafer of silicon divided into a

two-dimensional array of many tiny picture elements, known as pixels.

Pixels detect more photons; and thus light intensity. Much more efficient than photographic plates

Telescope SizeTelescope Size

Larger telescopes are preferred for two reasonsGreater light gathering powerGreater detail can be seen (resolving power)

Bigger = BetterBigger = Better

Light Gathering PowerLight Gathering Power

The standard measure of a telescope is its The standard measure of a telescope is its Light Gathering Power: Expressed as : Expressed as the total collecting area of the telescope. the total collecting area of the telescope. The greater the area, the more light collected. The greater the area, the more light collected. We usually We usually express the size of a telescope in

terms of the diameter of its primary collecting optic:

Light Gathering Power increases as the square of the diameter of the primary collecting optic.

Resolving PowerResolving PowerLarger telescopes have finer angular

resolution Resolution refers to the ability of any device, such as a camera

or telescope, to form distinct, separate images of objects lying close together in the field of view. The finer the resolution, the better we can distinguish the objects and the more detail we can see

Diffraction of light as it enter a telescope causes the rays to spread out, making it impossible to focus the beam into a sharp point and introduces a “fuzziness” into the optical system.

For light of any given wavelength, large telescopes produce less diffraction than small ones.

Resolution Detail becomes clearer in the Andromeda Galaxy as the angular resolution is improved some 600 times, from (a) 10', to (b) 1', (c) 5", and (d) 1". (AURA)

Atmospheric BlurringAtmospheric Blurring

The resolution of ground-based optical telescopes is actually limited by seeing – the blurring effect of Earth’s atmosphere , which smears the stars image into what is referred to a seeing disk. Atmospheric turbulence effects long waves lessAtmospheric turbulence effects long waves less Higher elevation experience less atmospheric Higher elevation experience less atmospheric

turbulenceturbulence

Radio TelescopesRadio Telescopes

Use antennas to detect cosmic radio waves:Use antennas to detect cosmic radio waves: Radio-wavelength lines of Hydrogen and Molecules Radio-wavelength lines of Hydrogen and Molecules

(CO) from cold interstellar gas (CO) from cold interstellar gas Radio waves are emitted by hot electrons, or Radio waves are emitted by hot electrons, or

electrons accelerated by strong magnetic fields. electrons accelerated by strong magnetic fields.

Interferometry: Trick of ganging together many smaller telescopes to achieve the higher resolution of a large single dish (aperture synthesis).

Radio vs. OpticalRadio vs. Optical

(a) VLA radio “photograph” (or radiograph) of the spiral galaxy M51, observed at radio frequencies with an angular resolution of a few arc seconds; (a) shows nearly as much detail as (b), an actual (light) photograph of that same galaxy made with the 4-m Kitt Peak optical telescope. (NRAO/AURA)

Why Do We Have to Go to Space to See All of the Electromagnetic Spectrum?

Detecting Other Radiation Detecting Other Radiation From SpaceFrom Space

Infrared telescopes are located on tall, dry Infrared telescopes are located on tall, dry mountains, or in airplanes! Why???mountains, or in airplanes! Why???

Ultraviolet, gamma, and x-ray telescopes Ultraviolet, gamma, and x-ray telescopes must be located in outer space because must be located in outer space because the atmosphere absorbs much of this the atmosphere absorbs much of this radiation.radiation.Used to detect planets, newly formed stars, Used to detect planets, newly formed stars,

quasars, pulsars, super novas and black quasars, pulsars, super novas and black holes.holes.