The Imaging Chain The Imaging Chain 1. energy source 2. object in - PDF document

Review and Overview Review and Overview “Imaging Chain” includes these elements: The Imaging Chain The Imaging Chain 1. energy source 2. object in Optical Astronomy in Optical Astronomy 3. collector 4. detector (or sensor) 5. processor 6. display 7. analysis 8. storage (if any) Source and/or Object Source and/or Object Optical Imaging Chain Optical Imaging Chain • In astronomy, the source of energy (1) and the 1: source object (2) are almost always one and the same! • i.e., The object emits the light – Examples: • Galaxies 5: processing • Stars – Exceptions: • Planets and the moon • Dust and gas that reflects or absorbs starlight 6: display 7: analysis 3: collector 2: object 4: sensor Optical Imaging Chain in Optical Imaging Chain in Modern Optical Imaging Chain in Optical Imaging Chain in Modern Astronomy until 1980 or so Astronomy (post- -1980) 1980) Astronomy until 1980 or so Astronomy (post 5: processing 5: processing 1: source 1: source 2: object 2: object 6: display 6: display 7: analysis 7: analysis 3: collector 8: storage 3: collector 4: sensor 4: sensor 8: storage (stack of glass) 1

Transition (“ “Catch Catch- -up up” ”) Phase: ) Phase: Optical Imaging Chain in Radio Transition ( Optical Imaging Chain in Radio Digitize Plates Astronomy Digitize Plates Astronomy 6: display 7: analysis 1,2 + radio waves 3,4 receiver where Scanner waves are collected waves converted into electro signals 5 8: storage computer received as signal 6,7 Angular Resolution Angular Resolution Specific Requirements for Specific Requirements for vs. Field of View vs. Field of View Astronomical Imaging Systems Astronomical Imaging Systems • Requirements always conflict • Angular Resolution: ability to distinguish sources – Always want more than you can have that are separated by small angles ⇒ must “trade off” desirable attributes – Limited by: − Deciding the relative merits is a difficult task • Optical Diffraction • “general-purpose” instruments (cameras) may not be • Sensor Resolution sufficient • Want simultaneously to have: • Field of View: angular size of the image field – excellent angular resolution AND wide field of view – Limited by: – high sensitivity AND wide dynamic range • Optics • Dynamic range is the ability to image “bright” and “faint” • Sensor Size (area) sources – broad wavelength coverage AND ability to measure narrow spectral lines Wavelength Coverage Wavelength Coverage Sensitivity vs. Dynamic Range Sensitivity vs. Dynamic Range vs. Spectral Resolution vs. Spectral Resolution • Wavelength Coverage • Sensitivity – Ability to image over a wide range of wavelengths – ability to measure faint brightness – Limited by: • Spectral Transmission of Optics (Glass cuts off UV, far IR) • Dynamic Range – ability to image “bright” and “faint” sources in same • Spectral Resolution system – Ability to detect and measure narrow spectral lines – Limited by: • “Spectrometer” Resolution (number of lines in diffraction grating) 2

Optical Collection (Link #3): Optical Collection (Link #3): Refracting Telescopes Refracting Telescopes • Lenses collect light • BIG disadvantages – Chromatic Aberrations (due to dispersion of glass) – Lenses are HEAVY and supported only on periphery Optical Collector (Link #3) Optical Collector (Link #3) • Limits the Lens Diameter • Largest is 40" at Yerkes Observatory, Wisconsin http://astro.uchicago.edu/vtour/40inch/kyle3.jpg Optical Collection (Link #3): Optical Collection (Link #3): Optical Reflecting Telescopes Optical Reflecting Telescopes Reflecting Telescopes Reflecting Telescopes • Concave • Mirrors collect light parabolic primary mirror to collect • Chromatic Aberrations eliminated light from source • Fabrication techniques continue to improve – modern mirrors • Mirrors may be supported from behind for large ⇒ Mirrors may be made much larger than telescopes are refractive lenses thin, lightweight & 3.5 meter deformable, to WIYN optimize image telescope quality mirror, Kitt Peak, Arizona Hale 200" " Telescope Telescope Hale 200 Thin and Light (Weight) Mirrors Thin and Light (Weight) Mirrors Palomar Mountain, CA Palomar Mountain, CA • Light weight ⇒ Easier to point – “light-duty” mechanical systems ⇒ cheaper • Thin Glass ⇒ Less “Thermal Mass” – Reaches Equilibrium (“cools down” to ambient temperature) quicker http://www.cmog.org/page.cfm?page=374 http://www.astro.caltech.edu/observatories/palomar/overview.html 3

200" mirror (5 meters) " mirror (5 meters) Keck telescopes , Mauna Kea, HI 200 Keck telescopes , Mauna Kea, HI for Hale Telescope for Hale Telescope • Monolithic Mirror (single piece) • Several feet thick • 10 months to cool • 7.5 years to grind • Mirror weighs 20 tons • Telescope weighs 400 tons • “Equatorial” Mount – follows sky with one motion http://www2.keck.hawaii.edu/geninfo/about.html 400" mirror (10 meters) 400 " mirror (10 meters) Basic Designs of Optical Basic Designs of Optical for Keck Telescope for Keck Telescope Reflecting Telescopes Reflecting Telescopes • 36 segments 1. Prime focus: light focused by primary mirror alone • 3" thick 2. Newtonian: use flat, diagonal secondary mirror to • Each segment weighs 400 kg (880 pounds) deflect light out side of tube – Total weight of mirror is 14,400 kg (< 15 tons) 3. Cassegrain: use convex secondary mirror to reflect light back through hole in primary • Telescope weighs 270 tons Nasmyth (or Coudé) focus (coudé ⇒ French for 4. • “Alt-azimuth” mount (left-right, up-down “bend” or “elbow”): uses a tertiary mirror to redirect motion) light to external instruments (e.g., a spectrograph) – follows sky with two motions + rotation Prime Focus Newtonian Reflector Prime Focus Newtonian Reflector f Sensor Mirror diameter must be large to ensure that obstruction is not significant Sensor 4

Feature of Cassegrain Feature of Cassegrain Cassegrain Telescope Cassegrain Telescope Telescope Telescope • Long Focal Length in Short Tube f Sensor Location of Secondary Equivalent Thin Lens Convex Mirror Optical Reflecting Telescopes Optical Reflecting Telescopes Coudé é or or Nasmyth Nasmyth Telescope Telescope Coud Schematic of 10-meter Keck telescope (segmented mirror) Sensor Why Build Large Telescopes? Why Build Large Telescopes? Large Optical Telescopes Large Optical Telescopes Telescopes with largest diameters 1. Larger Aperture ⇒ Gathers MORE Light Keck (in use or under construction : telescope Light-Gathering Power ∝ Area mirror – – 10-meter Keck (Mauna Kea, (note Area of Circular Aperture = π D 2 / 4 ∝ D 2 Hawaii) person) – – 8-meter Subaru (Mauna Kea) • D = diameter of primary collecting element – 8-meter Gemini (twin telescopes: 2. Larger aperture ⇒ better angular Mauna Kea & Cerro Pachon, Chile) resolution – 6.5-meter Mt. Hopkins (Arizona) λ ∆ θ ≅ D – 5-meter Mt. Palomar (California) – recall that: – 4-meter NOAO (Kitt Peak, AZ & Cerro Tololo, Chile) Summit of Mauna Kea, with Maui in background http://seds.lpl.arizona.edu/billa/bigeyes.html 5

Why Build Small Telescopes? Why Build Small Telescopes? F Ratio: F# F Ratio: F# 1. Smaller aperture ⇒ collects less light • F# describes the ability of the optic to ⇒ less chance of saturation • “deflect” or “focus” light (“overexposure”) on bright sources – Smaller F# ⇒ optic “deflects” light more than 2. Smaller aperture ⇒ larger field of view system with larger F# (generally) – Determined by “F ratio” or “F#” f # ≡ F D f = focal length of collecting element Small F# Large F# D = diameter of aperture F# of Large Telescopes F Ratio: F# F# of Large Telescopes F Ratio: F# • Two reflecting telescopes with different F# • Hale 200" on Palomar: f/3.3 and same detector have different “Fields of – focal length of primary mirror is: View”: 3.3 × 200" = 660" = 55' ≅ 16.8 m – Dome must be large enough to enclose • Keck 10-m on Mauna Kea: f/1.75 – focal length of primary mirror is: large ∆θ small ∆θ 1.75 × 10m = 17.5 m ≅ 58 m Small F# Large F# Astronomical Cameras Astronomical Cameras Usually Include: Usually Include: 1. Spectral Filters – most experiments require specific wavelength range(s) – broad-band or narrow-band Sensors (Link #4) Sensors (Link #4) 2. “Reimaging” Optics – enlarge or reduce image formed by primary collecting element 3. Light-Sensitive Detector: Sensor 6