Telescopes

• A convex lens, also known as a converging lens, focuses incident light.
• A concave lens, also known as a diverging lens, spreads out incident light.
• The principal axis is the line passing through the centre of the lens normal to its surface.
• In a convex lens, the principal focus is the point where incident beams passing parallel to the principal axis will converge.
• In a concave lens, the principal focus is the point from which the light rays appear to come from.
• Focal length is the distance between the centre of a lens and the principal focus.
• The shorter the focal length, the stronger the lens.
• A real image can be formed on a screen, while a virtual image cannot.
• 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, f is the focal length of the lens and P is the power of the lens, P = 1/f = 1/u + 1/v.
• The power of a lens is a measure of how closely a lens can focus a beam that is parallel to the principal axis, and is measured in Dioptres (D).
• In convex lenses power is positive, and in concave lenses power is negative.
• Refracting telescopes are devised of two convex lenses: the objective lens which collects light and creates a real image of a very distant object, and the eyepiece lens which magnifies the image produced by the objective lens, producing a virtual image at infinity.
• The objective lens in a refracting telescope should have a long focal length and be large so as to collect as much light as possible.
• The eyepiece lens for a refracting telescope should produce a virtual image at infinity to reduce eye strain for the observer as the rays are parallel.
• Normal adjustment for a refracting telescope is when the distance between the objective and eyepiece lens is the sum of their focal lengths, so the principal focus for both lenses is in the same place.
• The angular magnification, M, of a telescope is the angle subtended by the image at the eye divided by the angle subtended by the object at the unaided eye (larger angle divided by smaller angle).
• When both angles are less than 10 degrees, angular magnification can also equal focal length of the objective lens divided by focal length of the eyepiece lens.
• The cassegrain reflecting telescope has a concave primary mirror with a long focal length and a small convex secondary mirror in the centre which focuses light onto an eyepiece lens.
• Chromatic aberration is caused by red light having a greater focal length than blue light as it is refracted less, causing an image with coloured fringing in a refracting telescope.
• Spherical aberration is caused by the curvature of a lens or mirror causing light at the edge to be focused in a different position to light near the centre, leading to image blurring and distortion. The effect is most pronounced in lenses with a large diameter, and can be avoided by using parabolic objective mirrors in reflecting telescopes.
• One way of minimising spherical and chromatic aberration is by using an achromatic doublet, which 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.
• Reflecting telescopes are generally preferred over refracting telescopes as mirrors are easier to keep free of deficits, are not as heavy, can be supported from behind, and can be made from smaller mirror segments, unlike lenses.
• Radio telescopes are used to create images of astronomical objects and can be used on earth as the atmosphere is transparent to radio waves, but must be in isolated areas to avoid interference from radio sources. They must also be large in diameter as radio waves are long, but are cheap and simple to build as they only need a wire mesh instead of a mirror.
• Infrared telescopes consist of large concave mirrors which focus radiation onto a detector and are used to view cooler areas of space. They must be cooled using cryogenic fluids to almost absolute zero and be well shielded from thermal contamination and the atmosphere absorbs most infrared radiation so they must be operated from space.
• Ultraviolet telescopes are used to create images of astronomical objects, in particular interstellar medium and star formation regions, but need to be positioned in space as the atmosphere absorbs all UV of wavelength less than 300 nm.
• X ray telescopes are used to observe high energy events such as active galaxies, black holes and neutron stars,. They must be positioned in space as the atmosphere absorbs all X - rays, and need to be made from very smooth parabolic and hyperbolic mirrors as they would pass through regular mirrors.
• Gamma telescopes are used to observe gamma ray bursts, quasars, black holes and solar flares. Gamma rays are too high energy to use mirrors as they would pass through, so a detector made of pixels which cause a signal when they come into contact with gamma rays is used.
• Collecting power is a measure of the ability of a lens or mirror to collect incident electromagnetic radiation. Collecting power is directly proportional to the objective diameter squared.
• Resolving power is the ability of a telescope to produce separate images of close objects. For an image to be resolved, the minimum angular resolution in radians is equal to wavelength divided by the diameter of the objective lens or mirror.
• The rayleigh criterion states that two objects will not be resolved if any part of the central maximum of either images falls within the first minimum diffraction ring of the other.
• Charge couple devices are an array of light sensitive pixels which become charged when they are exposed to to light by the photoelectric effect.
• Quantum efficiency is the percentage of incident photons which cause an electron to be released, and is 70 % for a CCD and 4 % for the eye.
• Spectral range is the detectable range of wavelengths of light, and is infrared, ultraviolet and visible for a CCD and visible only for the eye.
• Pixel resolution is the total number of pixels used to form the image on a screen and is 50 megapixels for a CCD and 500 megapixels for the eye.
• Spatial resolution is the minimum distance two object must be apart in order to be distinguishable, and is 10 micrometres for a CCD and 100 micrometres for the eye.