3.9 Astrophysics Flashcards
How do you calculate the magnification of a refracting telescope?
(Angle subtended by image at eye) / (angle subtended by object at unaided eye)
What is normal adjustment?
Eye views parallel rays as if they’ve come from an object at infinity.
What are the disadvantages of refracting telescopes?
Chromatic aberration - Different wavelengths are refracted by different amounts so, the edges of an image appear coloured.
Spherical aberration
Light, usually blue/UV, are absorbed by the lens
Tube length must be very long for a good magnification
Distortion of lens
Lenses are heavy
What is spherical aberration?
When the shape of a lens isn’t fully parabolic, then light doesn’t converge at a single point.
What are the advantages of a cassegarian telescope?
Lighter and more compact
The mirror has no chromatic aberration
The objective can be made much bigger- better resolution
What are the disadvantages of a cassegarian telescope?
The secondary mirror obstructs light- there’s less light entering the telescope which decreases the brightness of the image.
Costly to maintain- recoating the mirror and cleaning
The eyepiece lens has spherical and chromatic aberration
What is the relationship between collecting power and the diameter objective lens?
Collecting power ∝ diameter^2
What property of the telescope does collecting power effect?
The higher the collecting power, the brighter the image.
What is angular resolution?
The smallest angle of separation between 2 objects which can be distinguished.
What happens to a wave when it passes through an aperture?
The wave is diffracted which can result in circular interference fringes. This leads to a loss of detail.
What is an airy disc?
The large central maximum on a diffraction pattern when light is diffracted. It is twice as wide as the other maxima in the pattern.
What is the Rayleigh criterion?
2 objects are resolvable when the first minimum of one airy disc coincides with the central maximum of the other.
What is quantum efficiency?
A measure of the effectiveness of an imaging device to convert incident photons into electrons
What is the quantum efficiency of a CCD?
> 70%
What is the quantum efficiency of the eye?
1% - 10%
What is the resolution of a CCD?
> 200 MP
What is the resolution of the eye?
Approximately 100m light sensitive cells in the retina.
What is the ‘convenience of use’ of a CCD?
It can be used in low temperatures of space
It can be used remotely
Detects a wider range of wavelengths
Exposure can last longer
What is the ‘convenience of use’ of the eye?
You experience a loss of colour vision at low light levels
Why are CCDs better than the naked eye?
Quantum efficiency is higher so, dimmer stars can be seen more easily.
CCDs can detect a wider range of wavelengths so, objects emitting non-visible light can be seen.
More convenient- image can be viewed remotely and recorded
Pixel size is smaller so, stars can be more easily seen as separate.
How does a single dish radio telescope work?
It uses a big parabolic dish as a reflector and an aerial picks up the signal.
The larger the dish, the more energy per second it collects. Collecting power ∝ diameter^2
What are the similarities between a radio telescope and an optical telescope?
The larger the diameter, the larger the collecting power, the larger the resolving power.
What are the differences between a radio telescope and an optical telescope?
Wavelength used be a radio telescope are much longer so, is resolution is much lower than an optical telescope.
What are the advantages of a radio telescope?
They are manoeuvrable, allowing the source of the wave to be tracked.
How can radio telescopes be modified to increase resolution?
Multiple telescopes are linked together and their data can be combined to produce a single image. So, a resolution that is a thousand times better than an optical telescope can be produced.
What are the disadvantages of radio telescopes?
They are affected by man-made interference from:
Radio transmissions
Mobile phones
Radar systems
Microwave ovens
What is an alternative way in which radio telescopes can be made to make them cheaper?
Instead of a mirror, a wire mesh can be used where the precision of the dish must be about lambda/20 to avoid spherical aberration.
This makes construction easier and cheaper than optical telescopes.
What is the structure of an IR or UV telescope?
Similar to an optical reflecting telescope.
But, CCDs are used as radiation detectors.
The mirrors have to be more perfectly made as the light has a shorter wavelength and therefore more affected by imperfections.
Where are IR telescopes located and why?
They must be positioned at high altitudes in dry places as most IR is absorbed by water vapour in the atmosphere. But, ideally they are sent to pace.
Where are UV and x-ray telescopes located and why?
They can be strapped to high altitude weather balloons, aeroplanes or sent into space as the radiation is absorbed by most of the upper atmosphere.
What are the disadvantages of IR telescopes?
Can only be built in specific locations
They must be heat resistant and have its components cooled by liquid nitrogen to operate effectively.
What are IR telescopes used for?
They’re useful for imaging relatively cool objects that don’t emit much visible light, like clouds of gas and dust where new stars are forming.
What are UV telescopes used for?
Used to observe very hot stars with temperatures above 1000k that are dim and mostly emit UV.
What are the disadvantages of x-ray telescopes?
They’re difficult to manufacture.
X-rays are either fully absorbed by or fully penetrate most materials. So, you need special components.
They must be outside the earths atmosphere.
What are x-ray telescopes used for?
Observing extremely high energy emissions of matter entering a black hole.
What is the structure of an x-ray telescope?
Grazing telescope. X-rays reflect if they graze the surface of a mirror. You can use multiple mirrors to gradually focus x-rays.
What is the Hipparcos scale?
A first magnitude star is the brightest
A sixth magnitude star is the dimmest
What is the difference in intensity between a first and sixth magnitude star?
A magnitude 1 star has an intensity 100 times greater than a magnitude 6 star.
What is the intensity of a star?
The effective brightness
Which factors affect the brightness of a star?
Luminosity- total energy emitted per second
Distance from earth
How do you calculate the difference in brightness using the Hipparcos scale?
2.51^n
What is a parsec?
The distance a star is from earth whne the angle of parallax is 1 arc second.
