astro

• Convex/converging lens


  Focuses incident light

  
• Concave/diverging lens


  Spreads out incident light

  
• Principal axis


  The line passing through the centre of the lens at 90º to its surface

  
• Principal focus (F) in a converging lens


  The point where incident beams passing parallel to the principal axis will converge

  
• Principal focus (F) in a diverging lens


  The point from which the light rays appear to come from. This is the same distance either side of the lens

  
• Focal length (f)


  The distance between the centre of a lens and the principal focus
• Real image


  Formed when light rays cross after refraction. Can be formed on a screen

  
• Virtual image


  Formed on the same side of the lens. The light rays do not cross, so a virtual image cannot be formed on a screen

  
• Lens formula


  1/u + 1/v = 1/f, where u is the distance of the object from the centre of the lens, v is the distance of the image from the centre of the lens, and f is the focal length of the lens

  
• Power of a lens

  A measure of how closely a lens can focus a beam that is parallel to the principal axis (in other words, how short the focal length is). Positive for converging lenses, negative for diverging lenses. Measured in Dioptres (D)

  
• Objective lens


  • Collects light and creates a real image of a very distant object. Should have a long focal length and be large to collect as much light as possible

  
• Eyepiece lens


  • Magnifies the image produced by the objective lens so the observer can see it. Produces a virtual image at infinity to reduce eye strain

  
• Normal adjustment for a refracting telescope

  The distance between the objective lens and the eyepiece lens is the sum of their focal lengths (fo + fe)

  
• Magnifying power (M) of a telescope

  M = angle subtended by the image at the eye / angle subtended by the object at the unaided eye = β/α

  
• Cassegrain reflecting telescope


  • Involves a concave primary mirror with a long focal length and a small convex secondary mirror in the centre. Allows the telescope to be shorter than other configurations

  
• Chromatic aberration


  For a given lens, the focal length of red light is greater than that of blue light, which means they are focused at different points. This can cause coloured fringing in the image

  
• Spherical aberration


  The curvature of a lens or mirror can cause rays of light at the edge to be focused in a different position to those near the centre, leading to image blurring and distortion

  
• Achromatic doublet


  Consists of a convex lens made of crown glass and a concave lens made of flint glass cemented together to bring all rays of light into focus in the same position

  
• Disadvantages of refracting telescopes

  • Glass must be pure and free from defects, which is difficult for large diameter lenses
  • Large lenses can bend and distort under their own weight
  • Chromatic and spherical aberration affect lenses
  • Refracting telescopes are incredibly heavy and difficult to manoeuvre
  • Large magnifications require very large diameter objective lenses with very long focal lengths
  • Lenses can only be supported from the edges

  
• Advantages of reflecting telescopes

  • Mirrors that are just a few nanometres thick can be made and give excellent image quality
  • Mirrors are unaffected by chromatic aberration, and spherical aberration can be solved by using parabolic mirrors
  • Mirrors are not as heavy as lenses, so they are easier to handle and manoeuvre
  • Large composite primary mirrors can be made from lots of smaller mirror segments
  • Large primary mirrors are easy to support from behind

  
• Radio telescopes


  • Use radio waves to create images of astronomical objects. The atmosphere is transparent to a large range of radio wavelengths so they can be ground-based. The simplest uses a parabolic dish to focus radio waves onto a receiver

  
• Similarities between radio and optical telescopes

  • Both intercept and focus incoming radiation to detect its intensity
  • Both can be moved to focus on different sources of radiation, or to track a moving source
  • Both can be ground-based since the atmosphere is transparent to the relevant wavelengths

  
• Differences between radio and optical telescopes

  • Radio telescopes have to be much larger in diameter than optical telescopes to achieve the same resolving power
  • Radio telescopes have a larger collecting power due to their larger objective diameter
  • Construction of radio telescopes is cheaper and simpler using a wire mesh instead of a mirror
  • Radio telescopes must move across an area to build up an image, unlike optical telescopes
  • Radio telescopes experience a large amount of man-made interference, unlike optical telescopes
• Infrared telescopes


  • Use infrared radiation to create images. Consist of large concave mirrors that focus radiation onto a detector. Must be cooled to almost absolute zero and well shielded to avoid thermal contamination. Placed in space to avoid atmospheric absorption
• Ultraviolet telescopes


  • Use ultraviolet radiation to create images. Utilise the Cassegrain configuration to bring UV rays to a focus. Detect UV photons using solid state devices that convert them into electrons

  
• 1. ray telescopes


  • Use X-rays to create images. Since X-rays are absorbed by the atmosphere, these telescopes must be in space. Use a combination of parabolic and hyperbolic mirrors to focus the high-energy X-rays onto CCDs

  
• Gamma telescopes


  • Use gamma radiation to create images. Do not use mirrors as gamma rays would just pass through. Instead use a detector made of layers of pixels that detect the gamma photons

  
• Types of gamma ray bursts


  • Short-lived (0.01 to 1 second), associated with merging neutron stars or a neutron star falling into a black hole
  • Long-lived (10 to 1000 seconds), associated with a Type II supernova
• Collecting power


  A measure of the ability of a lens or mirror to collect incident EM radiation. Directly proportional to the area of the objective lens/mirror, and hence the square of the objective diameter

  
• Resolving power


  The ability of a telescope to produce separate images of close-together objects. Determined by the minimum angular resolution θ = λ/D, where λ is the wavelength and D is the objective diameter

  
• Charge-coupled devices (CCDs)


  • An array of light-sensitive pixels that become charged when exposed to light by the photoelectric effect
• CCDs can be compared to the human eye in terms of quantum efficiency and spectral range
• Minimum angular resolution (θ)


  Where θ is in radians
• λ


  Wavelength of radiation

  
• D


  Diameter of the objective lens or objective mirror

  
• Rayleigh Criterion


  States that two objects will not be resolved if any part of the central maximum of either of the images falls within the first minimum diffraction ring of the other

  
• As light enters a telescope, it is diffracted in a target-like shape called an 'airy disc'

  
• Central maximum


  The bright white circle in the centre of the airy disc

  
• Minimum rings


  The dark rings around the central maximum of the airy disc

  
• Charge-coupled devices (CCDs)


  An array of light-sensitive pixels, which become charged when they are exposed to light by the photoelectric effect