Astrophysics 1

?
  • Created by: Sagaana
  • Created on: 28-01-20 07:14
View mindmap
  • 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
        • 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
          • +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.
      • 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.
        • 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
      • 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
        • 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
        • not restricted to just the visible spectrum
    • 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
      • 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
        • 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
      • 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.

Comments

No comments have yet been made

Similar Physics resources:

See all Physics resources »See all Astrophysics resources »