Astronomical Imaging Telescopes and Detectors Imaging Science Fundamentals

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

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 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science

Angular Resolution • 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 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science

Angular Resolution • 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” Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science

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 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science

Telescopes: Basic Flavors • 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 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science

Optical Reflecting Telescopes • Use parabolic, concave primary mirror to collect light from source – modern mirrors for large telescopes are lightweight & deformable, to optimize image quality Imaging Science Fundamentals 3. 5 meter WIYN telescope mirror, Kitt Peak, Arizona Chester F. Carlson Center for Imaging Science

Optical Reflecting Telescopes • 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 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science

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

Big Optical Telescopes • 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) Imaging Science Fundamentals Keck telescope mirror (note person) Summit of Mauna Kea, with Maui in background Chester F. Carlson Center for Imaging Science

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

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

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

Detectors • Optical: CCDs rule – film replaced by CCDs by early 80’s – detector formats (sizes) continually growing • 1024 x 1024: industry standard • 4096 x 4096, 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 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science

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 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science

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 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science

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 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science

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

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 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science

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 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science

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