Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science 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 3.5 meter WIYN telescope mirror, Kitt Peak, Arizona

Imaging Science Fundamentals 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) Summit of Mauna Kea, with Maui in background Keck telescope mirror (note person)

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

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 –Tiny wavelength, so exceedingly difficult to produce “smooth” mirrors for tight focus –Chandra is first X-ray telescope to achieve <1 arcsecond resolution Chandra X-ray telescope mirror design

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 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

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 Mauna Kea, Hawaii Kitt Peak, Arizona Gemini South, Chile

Imaging Science Fundamentals 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