specification points Flashcards
draw a ray diagram to show the image formation in normal adjustment
and an equation for the angular magnification
M = angle subtended by image at eye/ angle subtended by object at unaided eye
= 𝜶 / β
what is the definition for the normal adjustment
The setting for a refracting telescope in which the light emerges parallel from the eyepiece lens and the image is viewed at infinity
what is the focal length of the lens
The distance between the principal focus of a lens and its optical centre.
give an equation for magnification with reference to the focal length of the eyepiece and objective lense
M = f₀ / fₑ
f₀ = focal length of objective lense
fₑ = focal length of eyepiece lense
draw a diagram showing the cassegrain arrangment for a reflecting telescope
list some disadvantages of refracting telescopes
Mounting of the lens and support can only be made using the edge of the lens
Using glass of sufficient clarity and purity and free from defects to make large-diameter telescopes is extremely difficult
Large-diameter lenses are heavy and tend to distort under their own weight
Suffer from chromatic aberration and spherical aberration
Heavy and difficult to manoeuvre quickly
Difficult to mount heavy observing equipment and associated electronics
Large magnifications require large objective lenses and very long focal lengths
list some advantages of reflecting telescopes
Large single mirrors can be made, which are light and easily supportable from behind
Mirror surfaces can be made just a few nanometres thick, giving excellent image properties
Mirrors use only the front surface for reflection, so removing many of the problems associated with lenses
No chromatic aberration, and no spherical aberration when using PARABOLIC MIRRORS, can still occur in normal reflecting telescopes
Relatively light mirrors allow rapid response to astronomical events
Smaller segmented mirrors can be used to form a large composite objective mirror
what is Spherical aberration
The distortion of an image due to imperfections in the mirror or lens causing differing focal lengths
what is chromatic aberation
An optical defect that causes light of different colours to be focused at different focal points.
list some similarities between radio telescopes and optical telescopes
Radio telescopes can be ground-based because the atmosphere is transparent to a large range of radio wavelengths. Optical telescopes can also be ground based
Radio dish antennae need large structures to support them and to steer them. Depending on the optical telescope, the structure can vary, but to attain a large magnification, they do need to be supported by large structures too.
list some differences between radio telescopes and optical telescopes
Radio telescopes can operate during the day and night and are situated away from artificial sources of radio interference. But optical telescopes are mainly used only at night
Optical telescopes collect visible light and magnify it for viewing, while radio telescopes collect invisible radio waves, amplify them, and record them for study.
optical telescopes are commonly used in satellites whereas radio telescopes are mainly used on earth
radio telescopes usually have a much lower resolution than a similarly sized optical telescope
Radio telescopes can detect faint galaxies which no optical telescope can, they can work even in cloudy conditions and they can work during the day and night
radio telescopes mainly have low resolving powers
what is the minimum angular resolution of a telescope
The minimum angle, θ, that an instrument can distinguish between two small objects for a particular wavelength of light or other electromagnetic radiation, as determined by the Rayleigh criterion: θ ≈ λ / D
what is the equation for thr rayleigh criterion and what does it mean
θ ≈ λ / D
This states that two point objects can be resolved by an optical instrument if their angular separation is at least λ/D, where λ is the wavelength of the radiation and D is the diameter of the objective mirror or lens
what is collecting power and what is it proportional to
A measure of a telescope’s ability to collect incident electromagnetic radiation, and which is directly proportional to the square of the diameter of its objective.
collecting power ∝ (objective diameter)²
compare the eye and CCD as detectors in terms of:
quantum efficiency, resolution and convenience of use
Whereas for CCDs, most photons will cause an electron to liberated (high quantum efficiency), the human eye has a much lower quantum efficiency for low-light regions, as well as losing colour, meaning CCDs are much more effective in low-light scenarios
A simple comparison between the number of pixels/light sensitive cells per unit area suggests that CCDs and the human eye have similar resolutions
(convenience is quite obvious i think, our eyes are just there init)
CCDs also allow for remote viewing, long exposure times, detection of waves outside the visible region and computer analysis
what is a CCD
Charge-coupled device (CCD): A semiconductor device in which light is converted directly into digital information, commonly used in cameras and in conjunction with telescopes for digital imaging.
what is quantum efficiency
( detected photons / incident photons ) x 100
in a CCD
definition for apparent magnitude, m
The apparent brightness of a star expressed on the magnitude scale.
what is absolute magnitude, M
The apparent magnitude a star would have if it were placed at a standard distance of 10 parsec from the Earth.
What is the Hipparcus scale
A scale describing the apparent magnitude (relative brightness) first devised by Hipparchus of Nicaea (190–20BC).
what is the brightest and dimmest apparent magnitude of a star that can be seen with the naked eye
brightest stars that can be seen with the naked eye as magnitude 1.0 and the faintest as magnitude 6.0
how does magnitude difference correspond to brightness/intensity
A magnitude difference of 1 corresponds to a brightness ratio of 2.51
what is luminousity
The total energy radiated by a star each second (also called power); units J s^−1 or W
what is the brightness and its equation
The amount of energy radiated per second per square metre (also called intensity or radiation flux); unit Wm−2.
b = L / 4𝝿r²
parsec (pc) definition
The astronomical distance at which the angle subtended by the mean distance of the Earth–Sun system, i.e.1 AU, is one arcsecond
in other words, the distance at which an object lies if its measured parallax angle is 1 arcsecond; 1 pc = 3.262 ly or 2.06×105 AU.
what is the definition of a light year
The distance light travels in a vacuum in one year: 9.46 × 1015m.
what is Stefan–Boltzmann law (Stefan’s law)
The relation that gives the total energy emitted per square metre per second from an object at a given temperature T to be proportional to T^4. The constant of proportionality is σ, the Stefan–Boltzmann constant.
what is Wien’s displacement law
(Wien’s law) and the constant of λmaxT
peak wavelength ∝ 1/temperature
For a hot object, the wavelength of the peak emission intensity is inversely proportional to the absolute temperature of the object:
λmaxT = 0.0029 mK .
draw the general shape of the black body curve for the sun (relative intensity against wavelength)
and draw the line for weins displacement law on this graph
what assumption must you make about a star in relation to black body radiation
a star is a perfect black body
reproduce the stellar spectral classes table including the values of:
- Spectral class letter
- Intrinsic colour
- Temperature / K
- Prominent absorption lines
what are Hydrogen Balmer lines
Hydrogen Balmer lines are absorption lines that are produced as a result of the excitation of electrons from the n=2 state of hydrogen atoms
explain the prominence of hydrogen balmer lines in the different spectral classes
The lines are most prominent in class A stars since at these temperatures there is a large amount of hydrogen in n=2 state
Classes O and B have weaker lines since most of the hydrogen in these classes is ionised due to the very high temperatures
Class F is quite cool so it is unlikely that hydrogen is excited, meaning the lines are weak
For classes G, K and M, the temperatures are too low for excitation to occur and so there are no, or extremely weak, Balmer lines
draw the hertzsprung russel diagram including:
general shape and label the groups
labelled axes with scales: absolute magnitude against temperature and spectral class
draw the general shape for the stellar evolution path for a star similar to the sun
explain the process the sun will go through from the main sequence
The sun currently has an absolute magnitude of 5, and is in spectral class G - this means it is a main sequence star
As it uses up hydrogen, the hydrogen in its outer regions will fuse, causing the star to expand and become a red giant
As it continues to use up fuel, it will collapse, causing an increase in temperature and pressure which will induce helium fusion
Following this, helium shell fusion will begin to occur, once again causing the sun to expand
Over time the outer material will be expelled and only the extremely hot but small core will be left - this is known as white dwarf
Once all fusion processes have stopped, the white dwarf will cool to become a black dwarf
what is a supernovae
The explosive death of a star, caused by the sudden onset of nuclear burning or energetic shock wave; one of the most energetic events in the Universe.
an object that undergoes a very quick and very large increase in its absolute magnitude
what is the nature of a type 1a super novae and draw a diagram for the typical light curve from a type 1a supernovae
They result from the explosion of a white dwarf star
The maximum peak value of absolute magnitude is the same for all type 1a supernovae
The peak value occurs at around 20 days and is -19.3 Since the peak value is always this value, type 1a are standard candles
The consequence of this is that the distance of a type 1a supernovae can be calculated since the absolute magnitude is known and the apparent magnitude can be measured
what is a neutron star and some of its properties
The highly dense remnant of a star after a supernova explosion, composed mainly of neutrons.
Their density is estimated to be the same as nuclear matter
They spin very rapidly so that angular momentum is conserved as the core collapses
Over time they lose energy, and so their speed of rotation will decrease
They can act as very strong radio sources due to their strong magnetic fields
why is a black hole formed and what are some of its properties
When the center of a very massive star collapses in upon itself
The escape velocity of a black hole is greater than the speed of light
The Event Horizon of a black hole is the boundary at which the escape velocity is exactly equal to the speed of light
Anything that passes the Event Horizon cannot escape the black hole
The density of a black hole is proportional to the inverse of the square of its mass - this means that the larger a black hole is, the lower its density
what are gamma ray bursts (GRBs) and why do they occure
Flashes of gamma rays lasting from a few milliseconds to tens of seconds coming from distant galaxies and thought to occur due to the collapse of supergiant stars to form neutron stars or black holes
what is at the centre of every galaxy
super massive black holes
what is a standard candle
An astronomical object of known intrinsic brightness, for example a supernova, that is used to determine astronomical distances.
explain the controversy concerning the findings from using Type Ia supernovae to measure cosmological distances
the data suggested, controversially, that the expanding Universe is accelerating and not slowing down.
For this to happen implies that there is some as-yet undetected energy permeating the Universe that acts in opposition to gravity.
This has been given the name dark energy and its origin is currently a mystery to astrophysicists
what is Schwarzschild radius and derive the equation for it
The radius of an imaginary sphere from the centre of a black hole at which the escape velocity is equal to the speed of light. It defines the event horizon.
explain the doppler effect in terms of wavelength
the apparent change in wavelength of a wave, when the source moves relative to the observer:
- When the source moves away from the observer the wavelength will appear to increase
- When the source moves towards the observer the wavelength will appear to decrease
explain doppler effect in terms of light sources
changes to wavelength consequently also result in a corresponding change in frequency. In the case of light, these changes mean that the colour of the light will visibly appear to change:
When a light source moves away from the observer, the light will be shifted towards the red end of the colour spectrum
When a light source moves towards an observer, the light will be shifted towards the blue end of the colour spectrum
what do positive and negative z values indicate in the doppler shift equation
when the source and observer are approaching each other, the relative speed should be a negative quantity, whereas when they are moving away from each other, the relative speed quantity should be positive.
This means that positive z values indicate red-shift whereas negative z values indicate blue-shift.
explain the diagram of the simplified light curve for a binary star system. explain what is happening at points 1-4
At position 1, the two stars are side by side and so no eclipse occurs, and the absolute magnitude is at its greatest
At position 2, the larger of the two stars is in front of the smaller one, resulting in all of the smaller star’s light being blocked out
At position 3, the two stars are once again side by side and so no eclipse occurs, and the absolute magnitude is again at its greatest
At position 4, the smaller star is in front of the larger one and so part of the larger star’s light is blocked and the intensity is at a secondary minimum
what are the two key conclusions about the universe concluded from the observation of redshift in the light emitted by distant galaxies
- All visible galaxies show redshift, suggesting that all galaxies are moving away from each other
- The more distant galaxies demonstrate a greater amount of redshift, suggesting they are moving away at a faster r
what can be suggested from Hubbles law
that the universe is expanding,
and also supports the idea that it started from a single point.
By dividing the distance to a galaxy by its recessional velocity, the time since it started receding can be approximated.
This leads to the currently accepted age of the universe being around 15 billion years.
what does the big bang theory state and what is the evidence that supports it
It states that the universe began from a single, very small, hot and dense point, from which it has expanded.
Evidence:
The redshift observed from distant galaxies suggests that they are receding, which demonstrates that the universe is expanding as predicted
Cosmic Microwave Background Radiation (CMBR) is radiation that can be observed throughout the universe - the Big Bang can explain its existence as being the result of high energy radiation produced in the big bang that has been stretched as the universe has expanded
The Big Bang theory predicts that early fusion reactions produced helium by fusing hydrogen, before the universe cooled and expanded to leave a relative abundance of those two elements - this agrees with observations of nuclear matter in the universe
what are quasars
The most distant and yet most powerful observable objects in the universe
Calculations using the inverse square law suggest that their power output is the same as that of several galaxies combined
From the variation in their output, they are estimated to be roughly the size of a solar system
where are quasars originated from and where do they appear
Believed to originate from supermassive black holes at the centre of early active galaxies
Appear at the centre of young galaxies and consequently often obscure the host galaxy due to their brightness
what were the first observations of quasars and later confirmations
They were first observed as being very strong radio sources
Later observations confirm that they emit electromagnetic radiation across the whole range of the spectrum
what is an exoplanet
a planet that is found outside of our own solar system.
what are the difficulties in the detection of exoplanets
the light they emit is usually obscured by the brighter star that they orbit
Resolving exoplanets that are too close to their star is often beyond current optical abilities
If the exoplanet and star can be resolved, they don’t reflect much light and so will only appear very dimly
what develpments have been made in detecting exoplanets
Large exoplanets with large orbits are easiest to observe
Hot exoplanets can be viewed from the infrared radiation that they emit, rather than relying on the limited amount of light that they reflect
Exoplanets that orbit brown dwarfs or other dimmer stars allow for easier observations
Masking techniques have been developed to prevent the bright light from stars obscuring the exoplanets when being imaged
what is the radial velocity method
A method of observing exoplanets:
The gravitational pull causes the star the exoplanet is orbiting to wobble
This wobble results in a Doppler shift in the light it emits
The magnitude of the Doppler shift, and the direction in which the shift occurs, is linked to the component of the star’s velocity away from and towards the Earth
The data can be analysed to calculate an approximate distance of how close an exoplanet is to the star
The analysis used is similar to that of a binary star system
what is the transit method
A method of observing exoplanets:
There is a small dip in the apparent magnitude of a star when a planet passes in front of it
A light curve can be plotted to show the dip in intensity as well the period of time that is is over
Dips like this can also be caused by processes such as variations in the star’s output itself as well as sun spots, however the observation of repeated transits like this, indicate the existence of an exoplanet
A light curve can be plotted for the transit method to show the dip in intensity as well the period of time that is is over, draw this curve
what is the equation for the angle, in radians, subtended by an object of height, h, and a distance, d, away
θ = h/d
the angle subtended by an object at a distance d away, and with a height h
what is an equation that links the focal length (f), image distance (v), and object distance (u)
(1 / f) = (1 / u) + (1 / v)
