Radio Astronomy

• A. Gamma rays
• B. Visible light
• C. Radio waves ✓
• D. X-rays

Why: Radio waves have the longest wavelengths in the electromagnetic spectrum, ranging from about a millimetre to over 100 kilometres, which is why they are named for their use in radio communication and astronomy.

• A. 300 metres per second
• B. 340 metres per second
• C. 300,000 kilometres per second ✓
• D. 3,000 kilometres per second

Why: All electromagnetic waves travel at the speed of light in a vacuum, approximately 300,000 kilometres per second (about 3 x 10^8 m/s), regardless of their frequency or wavelength.

• A. They are directly proportional
• B. They are unrelated
• C. They are inversely proportional ✓
• D. They are equal when measured in the same units

Why: For electromagnetic waves travelling at a constant speed c, the relationship c = f x wavelength means that higher frequency corresponds to shorter wavelength, so they are inversely proportional.

• A. Radio waves are blocked by the Earth's ionosphere and atmosphere at certain frequencies ✓
• B. Radio waves are too weak to detect on the ground
• C. The ground produces radio waves that drown out signals
• D. Radio telescopes can only work in zero gravity

Why: The ionosphere reflects or absorbs many low-frequency radio waves, and water vapour and oxygen absorb certain higher frequencies, so some observations are only possible from high altitudes or space.

• A. Their very short wavelengths
• B. Their ability to be absorbed by dust
• C. Their long wavelengths, which are much larger than dust grains ✓
• D. Their high energy per photon

Why: Interstellar dust scatters and absorbs short-wavelength light strongly, but radio waves have wavelengths much larger than dust grains, so they pass through the dust largely unaffected and can reveal hidden regions of space.

• A. To amplify visible light from distant stars
• B. To collect radio waves from celestial sources and focus them toward a receiver ✓
• C. To store astronomical data for later analysis
• D. To filter out all background noise from the atmosphere

Why: A radio telescope's antenna collects incoming radio waves from celestial sources and concentrates them onto a receiver, which converts them into a measurable electrical signal.

• A. Yagi antenna
• B. Dish (parabolic reflector) antenna ✓
• C. Dipole antenna
• D. Helical antenna

Why: Parabolic dish reflectors are the classic radio telescope design. Their curved surface focuses incoming parallel radio waves to a single point, the focal point, where the receiver is located.

• A. Radio waves travel more slowly than light
• B. Radio wavelengths are much longer than visible light wavelengths, which limits resolution for a given aperture size ✓
• C. Radio signals are weaker and need more collecting area to detect
• D. Radio telescopes must be placed in space to function

Why: Angular resolution scales with wavelength divided by aperture diameter. Because radio wavelengths are many orders of magnitude longer than visible light, a radio dish must be very large to match the resolution of a much smaller optical mirror.

• A. It is where the dish is anchored to the ground
• B. It is where incoming parallel radio waves converge and the receiver is placed ✓
• C. It is the point where the signal is transmitted back into space
• D. It is where the telescope's cooling system is located

Why: A parabolic surface reflects all incoming waves parallel to its axis so they arrive in phase at the focal point, where the feed or receiver is positioned to capture the concentrated signal.

• A. It combines signals from multiple separated antennas to simulate a much larger effective aperture ✓
• B. It increases the wavelength of incoming radio waves
• C. It removes the need for any antenna at all
• D. It converts radio waves into visible light

Why: An interferometer correlates signals from widely separated antennas. The maximum separation between them acts as the effective aperture, greatly improving angular resolution without building a single enormous dish.

• A. To amplify the signal after it has been digitized
• B. To collect incoming radio waves from the reflector and direct them to the receiver
• C. To store recorded observations for later analysis
• D. To point the telescope at the target source

• A. Bandwidth
• B. Dynamic range
• C. Sensitivity
• D. Gain stability

• A. To prevent the telescope's mirror from warping
• B. To reduce thermal noise generated within the receiver electronics
• C. To increase the frequency of the incoming signal
• D. To allow the antenna to rotate more smoothly

• A. To remove interference from terrestrial sources
• B. To boost weak signals while adding as little extra noise as possible
• C. To convert the signal from analog to digital form
• D. To filter out frequencies above the observing band

• A. To alternate the telescope between two sky positions to cancel gain fluctuations
• B. To switch the polarization of the received signal
• C. To change the observing frequency during a scan
• D. To disconnect the receiver during periods of high solar activity

• A. It collects more total energy from a single point in the sky than any single dish could
• B. It achieves an angular resolution set by the maximum separation between antennas rather than the diameter of a single dish ✓
• C. It eliminates the need for receivers and amplifiers in the signal chain
• D. It removes all atmospheric noise from the observations

Why: An interferometer's angular resolution is approximately λ/B, where B is the baseline (the separation between antennas), so it can resolve fine detail far beyond what a single dish of practical size could achieve.

• A. The absolute brightness temperature of the source
• B. A single Fourier component of the sky brightness distribution, related to the visibility ✓
• C. The total flux density of all sources in the field of view
• D. The redshift of the observed object directly

Why: Each baseline samples one spatial frequency of the sky brightness distribution. The correlated signal, called the visibility, is the Fourier transform of the sky brightness at that spatial frequency.

• A. Building a single telescope dish from many smaller mirrors bolted together
• B. Combining signals from many antenna pairs over time, using Earth's rotation to fill in the uv-plane and simulate a much larger aperture ✓
• C. Synthesizing radio signals in a laboratory to calibrate receivers
• D. Using computer software to add noise to images so they appear sharper

Why: Aperture synthesis uses multiple antennas and the rotation of the Earth to sample many baselines over time, effectively creating a virtual telescope as large as the maximum antenna separation.

• A. It cools the receivers so they operate with lower noise
• B. It changes the projected baselines as seen from the source, filling in more points in the uv-plane ✓
• C. It causes the radio sources to move, which increases their brightness
• D. It aligns all antennas to point at the same position in the sky

Why: As the Earth rotates, the projected orientation and length of each baseline relative to the source change, sampling different spatial frequencies. This improves image quality by filling gaps in uv-coverage.

• A. The uv-plane is a spatial map of the sky, identical to the final image
• B. The uv-plane contains the Fourier transform of the sky brightness, so inverse Fourier transforming the sampled uv data produces the image ✓
• C. The uv-plane shows the locations of the antennas on the ground
• D. The uv-plane is a record of atmospheric conditions during the observation

Why: The visibilities sampled in the uv-plane are Fourier components of the sky brightness. An inverse Fourier transform of the (gridded) visibilities produces the dirty image, which is then deconvolved to remove sampling artifacts.

• A. A supermassive black hole at the center of a galaxy
• B. A rapidly rotating neutron star that emits beams of radio waves ✓
• C. A cloud of ionized hydrogen gas in a nebula
• D. A distant galaxy with an extremely active core

Why: Pulsars are rapidly spinning neutron stars left over from supernova explosions. Their magnetic poles emit beams of radio waves, which sweep past Earth like a lighthouse, producing regular pulses.

• A. They physically expand and contract over time
• B. Their radio beams sweep across Earth as the neutron star rotates ✓
• C. They are orbited by a regularly eclipsing companion star
• D. Their brightness varies due to changing internal temperature

Why: A pulsar's radio emission comes from beams aligned with its magnetic poles. When the misaligned rotation axis brings a beam across our line of sight, we detect a pulse, much like a rotating lighthouse beam.

• A. A small star forming region inside the Milky Way
• B. A nearby dwarf galaxy with few stars
• C. An extremely luminous active galactic nucleus powered by a supermassive black hole ✓
• D. A type of pulsar that rotates more than 1000 times per second

Why: Quasars (quasi-stellar radio sources) are the very bright cores of distant galaxies, powered by gas falling onto supermassive black holes. They can outshine their entire host galaxy.

• A. A galaxy that is invisible in optical light but emits strong radio waves ✓
• B. A galaxy that only forms stars in radio-frequency bands
• C. A galaxy located within a radio telescope's dish
• D. A galaxy that transmits signals to other galaxies

Why: Radio galaxies are galaxies that emit intense radio emission, often from large lobes or jets driven by a central supermassive black hole. They can be strong radio sources even when they appear faint or unremarkable at optical wavelengths.

• A. Radio waves are blocked by interstellar dust, so telescopes can see through it ✓
• B. Radio waves are the only form of light these objects emit
• C. Radio waves can penetrate Earth's atmosphere without distortion
• D. Radio waves travel faster than visible light, allowing earlier detection

Why: Interstellar dust scatters and absorbs visible and infrared light, but radio waves pass through it largely unimpeded. This makes radio astronomy an excellent way to observe pulsars and distant quasars that would otherwise be hidden.

• A. 1.42 GHz ✓
• B. 14.2 GHz
• C. 142 MHz
• D. 4.2 GHz

Why: The hydrogen hyperfine transition emits at a rest frequency of about 1420.4 MHz, which corresponds to a wavelength of roughly 21 cm, and this frequency lies in the L-band of the radio spectrum.

• A. Electron transitions between energy levels of the Lyman series
• B. Spontaneous flipping of the electron spin relative to the proton spin (hyperfine transition) ✓
• C. Nuclear fusion of deuterium in the core of the atom
• D. Vibrational transitions of the hydrogen molecule

Why: The 21 cm line comes from a hyperfine transition in which the relative orientation of the electron and proton spins changes from parallel to antiparallel, releasing a photon of very low energy.

• A. It is strongly absorbed by interstellar dust, revealing only nearby stars
• B. Radio waves at this wavelength pass through dust clouds with little absorption, letting us map neutral hydrogen across the galaxy ✓
• C. It is emitted only by stars, not by gas
• D. It can only be observed from within the Earth's atmosphere

Why: Radio wavelengths of 21 cm are barely affected by interstellar dust, so astronomers can observe neutral hydrogen throughout the galactic disk, including regions hidden from optical telescopes.

• A. Its intensity, which increases with distance
• B. Its polarization direction
• C. The Doppler shift of the observed frequency relative to the rest frequency ✓
• D. The time delay between pulses

Why: Motion of a gas cloud toward or away from us shifts the observed frequency of the 21 cm line through the Doppler effect, so the frequency offset gives the line-of-sight velocity.

• A. Hydrogen atoms rarely have free electrons
• B. The spontaneous emission probability is very low, so the excited state has a lifetime of about 10 million years ✓
• C. The transition is forbidden by quantum mechanics entirely
• D. Hydrogen atoms in space are always fully ionized

Why: The spontaneous emission rate of the hyperfine transition is extremely small, giving an excited-state lifetime of roughly 10 million years; the line is observable only because the Milky Way contains a huge number of hydrogen atoms.

• A. To increase the physical size of the telescope dish
• B. To correct instrumental and atmospheric effects so measured signals reflect true sky brightness
• C. To convert radio signals into visible light images
• D. To remove all noise from the observed data permanently

• A. The maximum distance at which a source can be detected
• B. The complex Fourier component of the sky brightness measured by a pair of antennas
• C. The brightness of the brightest object in the field of view
• D. The amount of time a telescope can observe a single target

• A. It converts the measured visibilities into a sky image, since visibilities sample the Fourier transform of the sky brightness
• B. It amplifies weak signals from distant galaxies
• C. It synchronizes the clocks of all antennas in an array
• D. It removes radio frequency interference from the data

• A. It is the point spread function determined by the uv-coverage, which convolves with the true sky and must be deconvolved
• B. It is a filter that permanently removes all sidelobes from the image
• C. It is the physical shape of the antenna reflector surface
• D. It describes the atmospheric distortion over the observatory site

• A. It iteratively models the sky as a set of point sources and subtracts their dirty beam contributions
• B. It calibrates the phase of each antenna using a known reference source
• C. It flags and deletes all data affected by radio frequency interference
• D. It combines optical and radio images into a single composite