Astrophysics

• 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

  
• Shorter the focal length


  Stronger the lens
• 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 parallel to the principal axis (how short the focal length is)

  
• Power unit


  Dioptres (D)
• Power formula


  P = 1/f
• 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

  Distance between objective and 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

  
• Magnifying power formula (when α and β are both less than 10°)

  M = fe/fo
• Cassegrain reflecting telescope


  • Concave primary mirror with long focal length and small convex secondary mirror. 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, causing coloured fringing in the image

  
• Chromatic aberration has very little effect on reflecting telescopes as it only occurs in the eyepiece lens

  
• 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, 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
• The reasons above are why reflectors are preferred in modern telescopes
• Simple radio telescope


  • Uses a parabolic dish to focus radio waves onto a receiver

  
• Similarities between radio and optical telescopes

  • Both function by intercepting and focusing incoming radiation to detect its intensity, both can be moved to focus on different sources of radiation or track a moving source, both can be ground-based since the atmosphere is transparent to their respective 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 larger collecting power, construction of radio telescopes is cheaper and simpler using a wire mesh, radio telescopes must move across an area to build up an image, radio telescopes experience more man-made interference while optical telescopes experience interference from weather and light pollution
• Infrared telescopes


  • Use large concave mirrors to focus infrared radiation onto a detector, must be cooled using cryogenic fluids and well shielded, must be launched into space as the atmosphere absorbs most infrared radiation
• Ultraviolet telescopes


  • Use the Cassegrain configuration to bring ultraviolet rays to a focus, detect UV photons using solid state devices that convert them into electrons

  
• 1. ray telescopes


  • Use a combination of parabolic and hyperbolic mirrors that must be extremely smooth, as X-rays would just pass straight through ordinary mirrors, convert X-rays into electrical pulses using CCDs

  
• Gamma telescopes


  • Do not use mirrors at all as gamma rays have too much energy, instead use a detector made of layers of pixels that detect the gamma photons as they pass through

  
• 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

  
• Resolving power


  The ability of a telescope to produce separate images of close-together objects, related to 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
• Features of CCDs compared to the human eye

  • Quantum efficiency (percentage of incident photons that cause electron release)
  • Spectral range

  
• Minimum angular resolution (θ)


  Where θ is in radians

  
• λ

  Wavelength of radiation