Chapter 5 Outline

Chapter 5 Outline

• Optical Telescopes
• telescope: "light bucket", captures photons and concentrates them into a beam
• Optical telescopes collect visible wavelengths
• Refracting and Reflecting Telescopes
• refraction: bending of a beam of light as it passes from one medium to another (straw in water looks bent)
• refracting telescope: uses lens to concentrate beam of light and pass it through the focus (single point), distance between primary mirror and focus = focal length
• Reflecting telescope: uses mirror (not lens) to focus light. Mirror called primary mirror, focus of primary mirror = prime focus
• telescopes make images of their field of view, often very small
• Comparing Refractors and Reflectors
• reflecting better because:
• lens focuses red and blue light differently (chromatic abberation)
• some light absorbed by glass of lens (worse for infared)
• large lenses heavy, lenses deform under own weight
• lens has 2 surfaces (versus 1 of mirror) to polish and maintain
• Largest refractor at Yerkes Obs. in Wisconsin, lens 1m diameter, relfecting have 10m
• Types of Refracting Telescope
• light often intercepted on path to focus so light intercepted on way to secondary mirror
• Newtonian Telescope: light intercepted before it reaches prime focus and deflected 90 degrees to eyepiece, uncommen in large instruments
• Cassegrain telescope: light reflected by primary mirror to prime focus, intercepted by 2ndary mirror, reflected down small hole back to primary; point behind primary mirror where light ultimately converges = Cassegrain focus
• with more than 2 mirrors, light reflected to Nasmyth focus or into coude ("bent") room which is a separate observatory
• Hubble Space Telescope (HST) is a Cassegrain telescope with all instruments behind primary mirror
• Telescope Size
• size has increased because that increases the amount of light it can collect (light-gathering power) and the detail (resolving power)
• Light-Gathering Power
• larger telescope = greater collecting area (total area capable of collecting radiation), have larger reflecting mirrors (refracting lens)
• observed brightness directly proportional to area of telescope's mirror (square of mirror diameter) (5m telescope 25X brighter than 1m)
• also direct. prop. to time it takes telescope to gather light
• Mauna Kea has small atmospheric interference, good observation point
• largest telescope in European Southern Observatory: optical-infared Very Large Telescope (VLT) - 4 different 8.2m mirrors
• Resolving Power
• resolution: ability of device to form distinct images of objects close together
• diffraction (tendency of light to bend) limits resolution, creates fuzziness
• Circular mirror:  angular resolution (arcsec) = .25(wavelength/diameter) when 1 micron = 10^-6 m
• diffraction increases in prop. to wavelength used (diffraction-limited resolution)
• Images and Detectors
• Image Acquisition
• charge-coupled devices (CCDs): electronic detectors used to take "pictures", made up of pixels, charges build up on pixels and 2-D image results
• advantages of photos: more efficient (90% of photons recorded), quicker, in digital format
• Image Processing
• computers reduce "background noise", can correct instrumental problems
• Wide-Angle Views
• as angle light enters increases, accuracy of focus decreases (effect = coma)
• Photometry
• measurement of brightness
• add up values in CCD pixels
• use colored filters to limit wavelengths measured
• can determine objects temperature
• photometer: used for high accuracy rapid measurements of light intensity
• Spectroscopy
• works with optical telescopes