The Imaging Chain The Imaging Chain in Optical Astronomy in Optical Astronomy 1
Review and Overview Review and Overview “Imaging Chain” includes these elements: 1. energy source 2. object 3. collector 4. detector (or sensor) 5. processor 6. display 7. analysis 8. storage (if any) 2
Optical Imaging Chain Optical Imaging Chain 1: source 5: processing 6: display 7: analysis 3: collector 2: object 4: sensor 3
Source and/or Object Source and/or Object • In astronomy, the source of energy (1) and the object (2) are almost always one and the same! • i.e., The object emits the light – Examples: • Galaxies • Stars – Exceptions: • Planets and the moon • Dust and gas that reflects or absorbs starlight 4
Optical Imaging Chain in Optical Imaging Chain in Astronomy until 1980 or so Astronomy until 1980 or so 5: processing 1: source 2: object 6: display 7: analysis 3: collector 4: sensor 8: storage (stack of glass) 5
Optical Imaging Chain in Modern Optical Imaging Chain in Modern Astronomy (post- -1980) 1980) Astronomy (post 5: processing 1: source 2: object 6: display 7: analysis 3: collector 8: storage 4: sensor 6
Transition (“ “Catch Catch- -up up” ”) Phase: ) Phase: Transition ( Digitize Plates Digitize Plates 6: display 7: analysis + Scanner 8: storage 7
Optical Imaging Chain in Radio Optical Imaging Chain in Radio Astronomy Astronomy 1,2 radio waves 3,4 receiver where waves are collected waves converted into electro signals 5 computer received as signal 6,7 8
Specific Requirements for Specific Requirements for Astronomical Imaging Systems Astronomical Imaging Systems • Requirements always conflict – Always want more than you can have ⇒ must “trade off” desirable attributes − Deciding the relative merits is a difficult task • “general-purpose” instruments (cameras) may not be sufficient • Want simultaneously to have: – excellent angular resolution AND wide field of view – high sensitivity AND wide dynamic range • Dynamic range is the ability to image “bright” and “faint” sources – broad wavelength coverage AND ability to measure narrow spectral lines 9
Angular Resolution Angular Resolution vs. Field of View vs. Field of View • Angular Resolution: ability to distinguish sources that are separated by small angles – Limited by: • Optical Diffraction • Sensor Resolution • Field of View: angular size of the image field – Limited by: • Optics • Sensor Size (area) 10
Sensitivity vs. Dynamic Range Sensitivity vs. Dynamic Range • Sensitivity – ability to measure faint brightness • Dynamic Range – ability to image “bright” and “faint” sources in same system 11
Wavelength Coverage Wavelength Coverage vs. Spectral Resolution vs. Spectral Resolution • Wavelength Coverage – Ability to image over a wide range of wavelengths – Limited by: • Spectral Transmission of Optics (Glass cuts off UV, far IR) • Spectral Resolution – Ability to detect and measure narrow spectral lines – Limited by: • “Spectrometer” Resolution (number of lines in diffraction grating) 12
Optical Collector (Link #3) Optical Collector (Link #3) 13
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 • Limits the Lens Diameter • Largest is 40" at Yerkes Observatory, Wisconsin http://astro.uchicago.edu/vtour/40inch/kyle3.jpg 14
Optical Collection (Link #3): Optical Collection (Link #3): Reflecting Telescopes Reflecting Telescopes • Mirrors collect light • Chromatic Aberrations eliminated • Fabrication techniques continue to improve • Mirrors may be supported from behind ⇒ Mirrors may be made much larger than refractive lenses 15
Optical Reflecting Telescopes Optical Reflecting Telescopes • Concave parabolic primary mirror to collect light from source – modern mirrors for large telescopes are thin, lightweight & 3.5 meter deformable, to WIYN optimize image telescope quality mirror, Kitt Peak, Arizona 16
Thin and Light (Weight) Mirrors Thin and Light (Weight) Mirrors • Light weight ⇒ Easier to point – “light-duty” mechanical systems ⇒ cheaper • Thin Glass ⇒ Less “Thermal Mass” – Reaches Equilibrium (“cools down” to ambient temperature) quicker 17
Hale 200" " Telescope Telescope Hale 200 Palomar Mountain, CA Palomar Mountain, CA http://www.cmog.org/page.cfm?page=374 http://www.astro.caltech.edu/observatories/palomar/overview.html 18
200" mirror (5 meters) " mirror (5 meters) 200 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 19
Keck telescopes , Mauna Kea, HI Keck telescopes , Mauna Kea, HI http://www2.keck.hawaii.edu/geninfo/about.html 20
400" mirror (10 meters) " mirror (10 meters) 400 for Keck Telescope for Keck Telescope • 36 segments • 3" thick • Each segment weighs 400 kg (880 pounds) – Total weight of mirror is 14,400 kg (< 15 tons) • Telescope weighs 270 tons • “Alt-azimuth” mount (left-right, up-down motion) – follows sky with two motions + rotation 21
Basic Designs of Optical Basic Designs of Optical Reflecting Telescopes Reflecting Telescopes 1. Prime focus: light focused by primary mirror alone 2. Newtonian: use flat, diagonal secondary mirror to deflect light out side of tube 3. Cassegrain: use convex secondary mirror to reflect light back through hole in primary Nasmyth (or Coudé) focus (coudé ⇒ French for 4. “bend” or “elbow”): uses a tertiary mirror to redirect light to external instruments (e.g., a spectrograph) 22
Prime Focus Prime Focus f Sensor Mirror diameter must be large to ensure that obstruction is not significant 23
Newtonian Reflector Newtonian Reflector Sensor 24
Cassegrain Telescope Cassegrain Telescope Sensor Secondary Convex Mirror 25
Feature of Cassegrain Feature of Cassegrain Telescope Telescope • Long Focal Length in Short Tube f Location of Equivalent Thin Lens 26
Coudé é or or Nasmyth Nasmyth Telescope Telescope Coud Sensor 27
Optical Reflecting Telescopes Optical Reflecting Telescopes Schematic of 10-meter Keck telescope (segmented mirror) 28
Large Optical Telescopes Large Optical Telescopes Telescopes with largest diameters Keck (in use or under construction : telescope mirror – 10-meter Keck (Mauna Kea, (note Hawaii) person) – 8-meter Subaru (Mauna Kea) – 8-meter Gemini (twin telescopes: 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 http://seds.lpl.arizona.edu/billa/bigeyes.html 29
Why Build Large Telescopes? Why Build Large Telescopes? 1. Larger Aperture ⇒ Gathers MORE Light Light-Gathering Power ∝ Area – Area of Circular Aperture = π D 2 / 4 ∝ D 2 – • D = diameter of primary collecting element 2. Larger aperture ⇒ better angular resolution λ ∆ θ ≅ D – recall that: 30
Why Build Small Telescopes? Why Build Small Telescopes? 1. Smaller aperture ⇒ collects less light ⇒ less chance of saturation • (“overexposure”) on bright sources 2. Smaller aperture ⇒ larger field of view (generally) – Determined by “F ratio” or “F#” f # ≡ F D f = focal length of collecting element D = diameter of aperture 31
F Ratio: F# F Ratio: F# • F# describes the ability of the optic to “deflect” or “focus” light – Smaller F# ⇒ optic “deflects” light more than system with larger F# Small F# Large F# 32
F# of Large Telescopes F# of Large Telescopes • Hale 200" on Palomar: f/3.3 – focal length of primary mirror is: 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: 1.75 × 10m = 17.5 m ≅ 58 m 33
F Ratio: F# F Ratio: F# • Two reflecting telescopes with different F# and same detector have different “Fields of View”: large ∆θ small ∆θ Small F# Large F# 34
Sensors (Link #4) Sensors (Link #4) 35
Astronomical Cameras Astronomical Cameras Usually Include: Usually Include: 1. Spectral Filters – most experiments require specific wavelength range(s) – broad-band or narrow-band 2. “Reimaging” Optics – enlarge or reduce image formed by primary collecting element 3. Light-Sensitive Detector: Sensor 36
Astronomical Sensors Astronomical Sensors • Most common detectors: – Human Eye – Photographic Emulsion • film • plates – Electronic Sensors • CCDs 37
Angular Resolution Angular Resolution • Fundamental Limit due to Diffraction in “Optical Collector” (Link #3) λ ∆ θ ≅ D • But Also Limited by Resolution of Sensor! 38
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