AST 111 Study Guide (Ch.6)

Optical telescopes that can operate as interferometers:

VLT (called VLTI) Keck LBT

Infrared (IR) radiation: 700 nm to 1 mm

felt as heat

Why is interferometry easier to carry out with radio telescopes than with optical telescopes?

"The advantage of interferometry for optical astronomers is that it can provide measurements of stars with a higher angular resolution than is possible with conventional telescopes.

Particle Astronomy

Cosmic rays: high-velocity subatomic particles balloons space telescopes underground detectors

Spectrographs or spectrometers

Devices that separate light into different colors or wavelengths ✔ Spread out the light according to wavelength/frequency prism (outdated) grating

Stream of particles: photons (equation)

E = hc / λ • E → energy of the photon • h → Planck's constant • c → speed of light • λ → wavelength • shorter-wavelength photons carry more energy

What form of radiation has the lowest energy?

Radio waves, have the lowest energies, longest wavelengths, and lowest frequencies of any type of EM radiation.

Are most modern telescopes refractors or reflectors? Why?

Reflector telescopes do not suffer from chromatic aberration because all wavelengths will reflect off the mirror in the same way. ... Reflector telescopes are cheaper to make than refractors of the same size

Ultraviolet (UV) radiation:

Short-wavelength electromagnetic energy emitted by the Sun. suntan and sunburns

How does a telescope with an equatorial mount follow the stars?

When properly aligned with the Earth's poles, equatorial mounts can allow the telescope to follow the smooth, arc-like motion of a star across the sky.

Where would you put an x-ray telescope? Why?

X-ray telescope, instrument designed to detect and resolve X-rays from sources outside Earth's atmosphere. Because of atmospheric absorption, X-ray telescopes must be carried to high altitudes by rockets or balloons or placed in orbit outside the atmosphere.

Light Gathering Power (LGP)

ability of a telescope to collect light proportional to the area of the primary lens/mirror: A = π D^2/4 • A → area • D → diameter compare the LGP of telescopes A and B: LGPA /LGPB = ( DA /DB )^2

Resolving power:

ability of the telescope to reveal fine detail diffraction fringes larger telescopes have better resolving power

What type of telescopes was invented first?

refracting telescope

Magnifying power(equation)

the ability of a telescope to make an image larger M = ( Fvp/ Fve ) • M → magnifying power • Fp → focal length (primary mirror) • Fe → focal length (eyepiece)

X-rays and gamma rays

electromagnetic waves used in medicine to kill cancer cells and view bones Gamma: shortest wave lengths

What forms of radiation can reach the surface of Earth?

infrared radiation makes up 49.4% of while visible light provides 42.3% Ultraviolet radiation makes up just over 8% of the total solar radiation.

Microwaves: a few mm to a few cm

microwave ovens radar long-distance telephone communications

True or False? "The energy of a photon is directly proportional to its wavelength"

true

What technologies are available to astronomers to increase the resolution of astronomical images?

Combining the light collected by two or more telescopes in a technique known as interferometry can boost the resolution beyond what a single telescope can accomplish

Interferometry

Connecting multiple telescopes together to work as a single telescope ✔ Common in radio astronomy ✔ Examples: VLA (New Mexico, USA) → 27 dish antennas VLBA (USA, Virgin Islands) → effective diameter almost as large as Earth ALMA (Chile) → 66 dish antennas SKA (future, Earth's southern hemisphere)

If Telescope A has a diameter of 10 m and Telescope B has a diameter of 5 m, how do their LGP compare?

4 times better

radio waves

: wavelengths higher than a few cm FM TV cell phones

How do astronomers avoid light pollution?

Astronomers avoid cities observatories are outside the cites

What are some advantages and disadvantages of space telescopes compared to ground-based telescopes?

Ground-based telescopes can't do the same, because the Earth's atmosphere absorbs a lot of the infrared and ultraviolet light that passes through it. Nevertheless, space-based telescopes are expensive to build and difficult to maintain

What form of light do our eyes perceive?

The human eye can only see visible light

What are photons?

a particle representing a quantum of light or other electromagnetic radiation. A photon carries energy proportional to the radiation frequency but has zero rest mass.

Why are false-color images useful?

composites allow us to visualize the wavelengths the human eye does not see (near the infrared range).

True or False? "Radio telescopes usually have better resolving power than optical telescopes"

false Radio waves have long wavelengths, so radio telescopes have poor resolution

True or False? "Light always behaves like a stream of particles"

false it behaves like particles and waves

True or False? "Poor-quality reflecting telescopes show a lot of chromatic aberration"

false refractor telescopes experience aberration

What are the two most important powers of a telescope?

light gathering and resolving power

atmospheric windows:

wavelength region in which Earth's atmosphere is transparent - at visual, infrared, and radio wavelengths visible light • some short-wavelength IR • some radio waves

Visible radiation:400 nm (blue/violet) to 700 nm (red)

wavelengths of light that human eyes can detect

telescope resolution equation

α (arc seconds) = 2.06 x10^5 ( λ/D ) • α → resolving power • λ → wavelength studied • D → diameter of the telescope small α means

Electromagnetic radiation (equation)

λ ν = c • c → speed of light λ → wavelength ν → frequency • the higher the frequency, the shorter the wavelength

Particle and wave properties are related:

• short wavelength / high frequency / high energy • long wavelength / low frequency / low energy

Fill in the blanks with the words "cosmic rays" or "gravity waves": • ______________ travel at the speed of light while ______________ are slower. • ______________ have mass. • ______________ are particles.

•cosmic waves travel at the speed of light while gravity waves are slower •gravity waves have mass •cosmic rays are particles

Space Telescopes

✔ Advantages: above Earth's atmosphere can observe 24/7 ✔ Hubble Space Telescope (HST) visible light, some IR and UV launched in 1990, but upgraded several times ✔ James Webb Space Telescope (JWST) IR far away from Earth will launch in 2021 requires coolant → lower lifespan ✔ Other examples: Herschel Planck WFIRST (future)

High Energy Astronomy

✔ Gamma-ray, X-rays, UV ✔ High in Earth's atmosphere or in space ✔ Difficult to focus photons ✔ Examples: Chandra (X-rays) Compton (gamma-ray) Fermi (gamma-ray)

Modern Radio Telescopes

✔ Large dish but small antenna: single large dish arrays of small dishes ✔ Challenges: low resolution low signal intensity ✔ Examples: Arecibo (Puerto Rico) FAST (China)

"Adaptive optics" refers to

✔ More sophisticated than active optics ✔ Increases resolution ✔ Uses a real or laser guide star as reference

airborne telescopes are

✔ Only some IR accessible from the ground or high mountains ✔ Earth's atmosphere emits IR ✔ Observations possible at higher altitudes balloons aircraft

Cameras and Photometers used

✔ Photographic plates (outdated) ✔ Photometers ✔ CCDs (visible light) record images photometer ✔ Array detectors (IR) ✔ False-color images