telescopes

thin lens equation

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

what are CCD’s

sensitive light detectors which capture images digitally

how does a CCD detect photons

a CCD is a silicon chip, when light is incident on it, they emit electrons which get trapped in the potential wells

advantages of CCDs

digital images can be saved and shared, quantum efficiency of 70% meaning it can detect fainter objects, can see parts of the EM spectrum outside of visible light

resolving power of telescope

The measure of how much detail you can see, dependent on minimum angular velocity

what causes chromatic aberration

each wavelength has a different refractive index so different colours are refracted by different amounts, edge of lens refracts light most, causing chromatic aberration to occurs at the edge

disadvantages of refracting telescopes 

glass must be pure which is very difficult to achieve with larger diameter, large lenses can bend and distort under their own weight due to how heavy they are, chromatic and spherical aberration both affect lenses, refracting telescopes are very heavy so difficult to manoeuvre, large magnifications require very large diameter objective lenses with very long focal lengths, lenses can only be supported from edges which can be an issue when they are very large and heavy

advantages of reflecting telescopes

mirrors that are just a few nanometres thick can be made so excellent image quality, mirrors are unaffected by chromatic aberration and spherical aberration can be solved using parabolic mirrors, mirrors are not as heavy as lenses so are easier to handle, large primary mirrors are easy to support from behind since you don’t need to be able to see through them

radio and optical telescope similarities

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 built on the ground,

radio and optical telescope differences

radio telescopes have to be much larger in diameter for same resolving power (as radio waves are larger than visible wavelengths), radio waves have larger collecting power due to larger objective diameter, construction of radio telescopes is cheaper and simpler as wire mesh is used instead of mirror, radio telescope must move across an area to build up an image, for radio telescopes longer wavelengths are less affected by imperfections in the dish

how do IR and UV telescopes work

they work the same way as optical telescopes

x-ray telescopes

they have grazing mirrors to reflect light. x-rays are detected using geiger counters, fine wire mesh or highly sensitive x-ray CCD cameras

collecting power

the ability of a lens or mirror to collect incident EM radiation, directly proportional to the areas of objecting lens/ squre of the diameter