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Astronomical Imaging

Astronomical Imaging. Telescopes and Detectors. Astronomical Imaging. GOAL: image large objects at VERY large distances (typically measured in light years, ly) Nearest star: alpha Centauri, 4.3 ly Nearest galaxy: Andromeda, 3 million ly “Edge” of universe: 15 billion ly REQUIREMENTS:

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Astronomical Imaging

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  1. Astronomical Imaging Telescopes and Detectors Chester F. Carlson Center for Imaging Science

  2. Astronomical Imaging • GOAL: image large objects at VERY large distances (typically measured in light years, ly) • Nearest star: alpha Centauri, 4.3 ly • Nearest galaxy: Andromeda, 3 million ly • “Edge” of universe: 15 billion ly • REQUIREMENTS: • High angular resolution (where possible) • High telescope/detector sensitivity Chester F. Carlson Center for Imaging Science

  3. Angular resolution = ability to distinguish detail Easy yardstick for grasping resolution: the Moon Moon’s disk: 1/2 degree across (same for Sun) 1 degree = 60 arc minutes; 1 arc minute = 60 arc seconds unaided eye can distinguish shapes/shading on Moon’s surface (resolution: ~1 arc minute) w/ small telescope can distinguish large craters (resolution: a few arc seconds) w/ large telescope can see craters 1/2 mile (~1 arc second) across Angular Resolution Chester F. Carlson Center for Imaging Science

  4. Factors determining angular resolution: Diameter of main light collecting surface (mirror or lens) of telescope determines diffraction limit of telescopic imaging system Quality of telescope collecting surface smoother surface = better resolution Atmospheric effects turbulence smears image essentially same effect as stars’ “twinkling” Angular Resolution Chester F. Carlson Center for Imaging Science

  5. Sensitivity • Sensitivity = ability to detect faint sources of electromagnetic radiation • Telescope sensitivity: proportional to its light collecting area (area of mirror or lens surface) • Detector sensitivity: measured by its quantum efficiency (fraction of input photons that generate signal in detector) • Also, need the ability to expose the detector (integrate) for very long periods of time Chester F. Carlson Center for Imaging Science

  6. Refractor telescopes exclusively use lenses to collect light have big disadvantages: aberrations & sheer weight of lenses Reflector telescopes use mirrors to collect light relatively free of aberrations mirror fabrication techniques steadily improving Telescopes: Basic Flavors Chester F. Carlson Center for Imaging Science

  7. Use parabolic, concave primary mirror to collect light from source modern mirrors for large telescopes are lightweight & deformable, to optimize image quality Optical Reflecting Telescopes 3.5 meter WIYN telescope mirror, Kitt Peak, Arizona Chester F. Carlson Center for Imaging Science

  8. Basic optical designs: Prime focus: light is brought to focus by primary mirror, without further deflection Newtonian: use flat, diagonal secondary mirror to deflect light out side of tube Cassegrain: use convex secondary mirror to reflect light back through hole in primary Nasmyth focus: use tertiary mirror to redirect light to external instruments Optical Reflecting Telescopes Chester F. Carlson Center for Imaging Science

  9. Optical Reflecting Telescopes Schematic of 10-meter Keck telescope Chester F. Carlson Center for Imaging Science

  10. Largest telescopes in use or under construction: 10 meter Keck (Mauna Kea, Hawaii) 8 meter Subaru (Mauna Kea) 8 meter Gemini (Mauna Kea & Cerro Pachon, Chile) 6.5 meter Mt. Hopkins (Arizona) 5 meter Mt. Palomar (California) 4 meter NOAO (Kitt Peak, AZ & Cerro Tololo, Chile) Big Optical Telescopes Keck telescope mirror (note person) Summit of Mauna Kea, with Maui in background Chester F. Carlson Center for Imaging Science

  11. Usually Cassegrain in design primary “mirror” is replaced by parabolic reflector “dish” secondary is called subreflector Radio Telescopes 12 meter radio telescope, Kitt Peak, Arizona Chester F. Carlson Center for Imaging Science

  12. Since wavelength of interest is longer, must increase telescope aperture to achieve good angular resolution alternative is to use an array of radio telescopes Radio Telescopes Very Large Array, New Mexico Chester F. Carlson Center for Imaging Science

  13. Use grazing incidence optics to defeat tendency for X-rays to be absorbed by mirrors Tiny wavelength, so exceedingly difficult to produce “smooth” mirrors for tight focus Chandra is first X-ray telescope to achieve<1 arcsecond resolution X-ray Telescopes Chandra X-ray telescope mirror design Chester F. Carlson Center for Imaging Science

  14. Detectors • Optical: CCDs rule • film replaced by CCDs by early 80’s • detector formats (sizes) continually growing • 1024x1024: industry standard • 4096x4096, CCD arrays: no longer uncommon • IR: CIDs (near-IR), bolometers (far-IR) • CIDs: similar to CCDs but each pixel addressed independently • bolometers: directly measure heat input Chester F. Carlson Center for Imaging Science

  15. Detectors • Radio: receivers • original (50’s-60’s) technology similar to that of home stereo use • now emphasize extremely high sensitivity and extremes in radio frequency range • X-ray: proportional counters, CCDs • prop. counters efficiently convert X-ray energies to voltages • CCDs provide better X-ray position & energy determination Chester F. Carlson Center for Imaging Science

  16. Observatory Sites • The best telescope/detector is useless at a bad site! • Factors for consideration of appropriate site: • atmospheric transparency at wavelength of interest • atmospheric turbulence • sky brightness • accessibility Chester F. Carlson Center for Imaging Science

  17. Observatory Sites • Optical work: • need dark, cloud-free site • helps to remove atmosphere from system (e.g., Hubble)! • IR work: • need cold site • dry site very important at certain wavelengths • radio work: • need dry site (shorter wavelengths) • need interference-free site (longer “) • X-ray work: • need to be above atmosphere Chester F. Carlson Center for Imaging Science

  18. Optical/IR Telescopes • Dark, high, & dry: most big optical/IR telescopes are placed on mountaintops in deserts Kitt Peak, Arizona Mauna Kea, Hawaii Gemini South, Chile Chester F. Carlson Center for Imaging Science

  19. IR Telescopes • For optimum IR work, need high, dry, cold site • South Pole works well, but accessibility an issue Center for Astronomical Research in Antarctica Chester F. Carlson Center for Imaging Science

  20. IR Telescopes • Helps to go into space, or at least above the bulk of the atmosphere SIRTF: NASA’s Space Infrared Telescope Facility SOFIA: NASA’s Stratospheric Observatory for IR Astronomy Chester F. Carlson Center for Imaging Science

  21. Must go above atmosphere to detect celestial objects! (X-rays are absorbed by Earth’s atmosphere) X-ray Telescopes Chandra is in high Earth orbit Chester F. Carlson Center for Imaging Science

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