Astrophysics 1
- Created by: Sagaana
- Created on: 28-01-20 07:14
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- Astrophysics
- Telescopes
- Optical Telescopes
- Refracting Telescopes
- normal adjustment =when lenses are placed that their principal foci are at the same position
- they are made up of two converging lenses
- Magnification= angle image /angle object
- Magnification in normal adjustment = focal length of objective lens/focal length of eyepiece
- fo must be greater then fe for sufficient magnification so fe must be very small
- -ves
- Chromatic aberration
- Colours have different refractive index so they are focused to different points.This cause the image to blur.
- can be fixed with achromatic aberration
- large lens without impurities are expensive to make
- impurites cause the light to scatter and decrease the collective power of telescope
- They must be very big for the focal length to be large.
- Large lens need a lot of support.This can distort the lens
- Chromatic aberration
- Reflecting Telescope
- parallel light beams reflected and are focused through the concave mirror
- Light collected by eyepiece
- -ves
- Secondary mirror can diffract light
- Decrease in image clarity
- Spherical Aberation
- when it is not spherical/parabolic they rays are not reflected to the same focus
- Secondary mirror can diffract light
- +ves
- Large high quality mirrors can be cheaply made
- Large mirrors can be supported from underneath so it does distort
- Don't suffer from chromatic aberration
- It has a parabolic concave mirror and a convex parabolic secondary mirror.
- Refracting Telescopes
- Non Optical Telescopes
- Radio telescopes
- made up of parabolic mesh of wire which reflects onto the detector
- +ves
- Radio waves are not absorbed by atmosphere so can be placed at sea level
- doesn't need to be accurate as a mirror reflector
- -ves
- Wavelength cant be smaller than the holes in mesh
- they have a low resolving power
- IR & UV telescopes
- similar structure to the Cassegrain
- +ves
- it has a good resolving power
- -ves
- IR radiation can be absorbed by the H2O & CO2
- Telescopes produce IR radiations themselves so they need to be cooled
- X-rays Telescopes
- +ves
- resolving power is high due to small wavelength
- -ves
- they must be in orbit as atmosphere blocks X-rays
- They are difficult to reflect
- They use grazing mirrors as x-rays are difficult to reflect.
- +ves
- Telescopes in space
- +ves
- No absorption of radiation
- no light pollution from Earthly sources
- Natural cooling of the telescope
- -ves
- Difficult to service
- Needs its own power source
- +ves
- Radio telescopes
- lenses
- they change the direction of light rays by refraction
- Axial rays = converge to the principal focus
- Non-Axial rays=converge on the focal plane but of the principal axis
- Focal length is the distance between the lens axis and the focal plane
- converging lenses can make real and virtual image
- image is real when object is further away than the focal length from the lens
- the image is virtual when the object is closer to the lens
- Angular Resolution
- The smallest angular separation at which two objects can be distinguished
- aka resolving power
- Gives you are clearer image
- larger apertures the less diffraction so more resolving power
- diffraction happens when the objective lens aperture has a similar to a single slit.
- The central maximum is called the Airy disc this is twice as wide as the further maxima
- diffraction happens when the objective lens aperture has a similar to a single slit.
- Rayleigh Criterion
- when two airy discs can be just resolved when the central maximum of one pattern coincides with the first mininmum
- angle greater than = object can be distinguished
- angle is equal to = object can be just distinguished
- angle less than = Object cannot be distinguished
- The smaller the angle the better the resolution
- collecting power
- proportional to dish diameter squared
- Higher collecting power = fainter objects that can be seen
- CCDs
- they capture light digitally at the eyepiece lens
- they are a silicon chip which are split in to square
- When photons hit a pixel electrons are released and charged which were trapped into a potential well
- this is proportional to the light intensity
- the charge is processed to give image
- Quantum efficiency
- The ratio of number of photons detected to the total number of photons falling on device
- eye quantum efficiency = 1%
- CCD quantum effciency > 70%
- +ves (over the eye)
- CCD has a higher resolution
- more pixels per unit area
- Long exposure time
- allows us to see fainter objects
- The photos are digital
- can be shared and stored
- CCD has a higher resolution
- not restricted to just the visible spectrum
- Optical Telescopes
- Stars 1
- Absolute Magnitude
- Light year= Distance that a star travels in one year
- 9.46*10^15
- Stellar Parallax
- It is the apparent movement of a star against the background
- We can measure parallax as an angle and deduce the distance to a star.
- Parsec
- The distance to an object at which 1 AU subtends one second of arc
- Light year= Distance that a star travels in one year
- Classification of Luminosity
- Luminosity = The total power output of a star
- Brightness = intensity of stars that we perceive at Earth (apparent magnitude)
- Apparent Magnitude
- this is how bright a star appears on earth
- It depends on both luminosity and how far away it is
- It is measured using the Hipparcos scale
- The larger the value the dimmer the star
- Initially 1-6 (this is to the visible eye)
- it is a logarithmic scale with a factor of 2.51 between 1
- Absolute Magnitude
- The brightness a star would exactly 10 parsecs away
- Classification by temp black body radiations
- Standard Candles
- an object where absolute magnitude and apparent magnitude can be measured
- This allows us to work out the distance in pasecs
- Type 1a supernova light curve
- the slope on LHS must be steeper then slope on RHS
- an object where absolute magnitude and apparent magnitude can be measured
- Black Body curves
- zero intensity at zero wavelength
- steeper on LHS than on RHS
- as temp increase the peak wavelength is higher in energy so the graph shifts to the left
- intensity at all wavelengths is higher
- Wien's Displacement
- assumes that star is a back body
- as temp increases the peak wavelength decreases
- Stefan's Law
- assumes star is a black body
- gives us the total power from a star
- inverse square law
- assumes that an even amount of power is irradiated in every directions
- inverse square law
- Standard Candles
- Stellar Spectral Classes
- Stars are divided into spectral classes based on temperature
- O is the hottest and M is the coldest
- Hydrogen Balmer series
- These lines in emission and absorption spectra are caused by electrons only being able the exist at certain energy level
- atomic hydrogen where the electron is usually in ground level
- This is for low temperatures
- The Visible part of the spectrum is caused by the electron moving from n=2 to higher levels
- Strength of the spectral lines show the temperature of a star
- when temperature is too high electron will reach a high energy level
- intensity of balmer line depends on the temperature
- for some balmer line two temperature are possible so astronomer look at absorption lines of other atoms and molecules as well.
- intensity of balmer line depends on the temperature
- when temperature is too high electron will reach a high energy level
- Absolute Magnitude
- Telescopes
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