The Imaging Chain in Optical Astronomy

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Beatrix Gardner
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1 The Imaging Chain in Optical Astronomy 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) Optical Imaging Chain 1: source 5: processing 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 2: object 3: collector 4: sensor 6: display 7: analysis Exceptions: Planets and the moon Dust and gas that reflects or absorbs starlight Optical Imaging Chain in Astronomy until 1980 or so Optical Imaging Chain in Modern Astronomy (post-1980) 1: source 2: object 5: processing 1: source 2: object 5: processing 3: collector 4: sensor 6: display 7: analysis 8: storage (stack of glass) 3: collector 4: sensor 8: storage 6: display 7: analysis 1

2 Transition ( Catch( Catch-up ) ) Phase: Digitize Plates 6: display 7: analysis Optical Imaging Chain in Radio Astronomy 1,2 + Scanner 3,4 radio waves receiver where waves are collected 8: storage waves converted into electro signals 5 computer received as signal 6,7 Specific Requirements for 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 Angular Resolution 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) Sensitivity vs. Dynamic Range Sensitivity ability to measure faint brightness Dynamic Range ability to image bright and faint sources in same system Wavelength Coverage 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) 2

$Optical Collector (Link #3) Optical Collection (Link #3): Refracting Telescopes Lenses collect light BIG$ Limits the Lens Diameter Largest is 40" at Yerkes Observatory, Wisconsin http://astro.uchicago.

Limits the Lens Diameter Largest is 40" at Yerkes Observatory, Wisconsin http://astro.uchicago.

jpg Optical Collection (Link #3): Reflecting Telescopes Mirrors collect light Chromatic Aberrations eliminated

$refractive lenses Optical Reflecting Telescopes Concave parabolic primary mirror to collect light from source modern$ 5 meter WIYN telescope mirror, Kitt Peak, Arizona Thin and Light (Weight) Mirrors Light weight Easier to point

5 meter WIYN telescope mirror, Kitt Peak, Arizona Thin and Light (Weight) Mirrors Light weight Easier to point

3 Optical Collector (Link #3) Optical Collection (Link #3): 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 Optical Collection (Link #3): 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 Optical Reflecting Telescopes Concave parabolic primary mirror to collect light from source modern mirrors for large telescopes are thin, lightweight & deformable, to optimize image quality 3.5 meter WIYN telescope mirror, Kitt Peak, Arizona Thin and Light (Weight) Mirrors Light weight Easier to point light-duty mechanical systems cheaper Hale 200" Telescope Palomar Mountain, CA Thin Glass Less Thermal Mass Reaches Equilibrium ( cools down to ambient temperature) quicker 3

4 200" " mirror (5 meters) 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 Keck telescopes,, Mauna Kea, HI 400" " mirror (10 meters) 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 Basic Designs of Optical 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 4. Nasmyth (or Coudé) focus (coudé French for bend or elbow ): uses a tertiary mirror to redirect light to external instruments (e.g., a spectrograph) Prime Focus Newtonian Reflector Sensor f Mirror diameter must be large to ensure that obstruction is not significant Sensor 4

Cassegrain Telescope Feature of Cassegrain Telescope Long Focal Length in Short Tube f Sensor Secondary Convex Mirror Location of Equivalent Thin Lens Coudé or

diameters (in use or under construction: 10-meter Keck (Mauna Kea, Hawaii) 8-meter Subaru (Mauna Kea) 8-meter Gemini (twin telescopes: Mauna Kea & Cerro Pachon,

Palomar (California) 4-meter NOAO (Kitt Peak, AZ & Cerro Tololo, Chile) Keck telescope mirror (note person) Why Build Large Telescopes? 1.

5 Cassegrain Telescope Feature of Cassegrain Telescope Long Focal Length in Short Tube f Sensor Secondary Convex Mirror Location of Equivalent Thin Lens Coudé or Nasmyth Telescope Optical Reflecting Telescopes Schematic of 10-meter Keck telescope (segmented mirror) Sensor Large Optical Telescopes Telescopes with largest diameters (in use or under construction: 10-meter Keck (Mauna Kea, Hawaii) 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) Keck telescope mirror (note person) Why Build Large Telescopes? 1. Larger Aperture Gathers MORE Light Light-Gathering Power Area Area of Circular Aperture = πd 2 / 4 D 2 D = diameter of primary collecting element 2. Larger aperture better angular resolution recall that: λ θ D Summit of Mauna Kea, with Maui in background 5

6 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 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# F# of Large Telescopes Hale 200" on Palomar: f/3.3 focal length of primary mirror is: " = 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: m = 17.5 m 58 m 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# Sensors (Link #4) Astronomical Cameras 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 6

7 Astronomical Sensors Most common detectors: Human Eye Photographic Emulsion film plates Electronic Sensors CCDs Angular Resolution Fundamental Limit due to Diffraction in Optical Collector (Link #3) λ θ D But Also Limited by Resolution of Sensor! Charge-Coupled Coupled Devices (CCDs( CCDs) Standard light detection medium for BOTH professional and amateur astronomical imaging systems Significant decrease in price numerous advantages over film: high quantum efficiency (QE) meaning most of the photons incident on CCD are counted linear response measured signal is proportional to number of photons collected fast processing turnaround (CCD readout speeds ~1 sec) NO development of emulsion! regular grid of sensor elements (pixels) as opposed to random distribution of AgX grains image delivered in computer-ready form Sensor Resolution Obvious for Electronic Sensors (e.g., CCDs) Elements have finite size Light is summed over area of sensor element ( integrated ) Light from two stars that falls on same element is added together stars cannot be distinguished in image! x Same Effect in Photographic Emulsions More difficult to quantify Light-sensitive grains of silver halide in the emulsion Placed randomly in emulsion Random sizes large grains are more sensitive (respond to few photons) small grains produce better resolution Photographic techniques: silver halide Film Emulsion on flexible substrate Still used by amateurs using sensitive film B&W and color Special treatment to increase sensitivity Photographic Plates Emulsion on glass plates Most common detector from earliest development of AgX techniques until CCDs in late 70 s 7

8 Eye as Astronomical Detector Eye includes its own lens focuses light on retina ( sensor ) When used with a telescope, must add yet another lens redirect rays from primary optic make them parallel ( collimated ) rays appear to come from infinity (infinite distance away) reimaging is performed by eyepiece Eye with Telescope Without Eyepiece With Eyepiece Light entering eye is collimated Eye as Astronomical Detector Point sources (stars) appear brighter to eye through telescope Factor is D P 2 2 D is telescope diameter P is diameter of eye pupil Magnification should make light fill the eye pupil ( exit pupil ) Extended sources (for example, nebulae) do not appear brighter through a telescope Gain in light gathering power exactly compensated by image magnification, spreads light out over larger angle. Atmospheric Effects on Image Large role in ground-based optical astronomy scintillation modifies source angular size twinkling of stars = smearing of point sources extinction reduces light intensity atmosphere scatters a small amount of light, especially at short (bluer) wavelengths water vapor blocks specific wavelengths, especially near- IR scattered light produces interfering background astronomical images are never limited to light from source alone; always include source + background sky light pollution worsens sky background Scattering Wavelength Dependent Depends on color of light Long wavelengths are scattered less Scattering by Molecules 1 "Rayleigh Scattering" 4 λ Molecules are SMALL Blue light is scattered MUCH more than red light Reason for BOTH blue sky (blue light scattered from sun in all directions) red sunset (blue light is scattered out of the sun s direct rays) 8

9 Scattering by Dust 1 "Mie Scattering" λ Dust particles are MUCH larger than molecules e.g., from volcanos, dust storms Blue light is scattered by dust somewhat more than red light Link #5: Image Processing Link #5: Image Processing Formerly: performed in darkroom e.g., David Malin s Unsharp Masking Subtract a blurred copy from a sharp positive (or, add a blurred negative to a sharp positive) Now performed in computers, e.g., contrast enhancement sharpening normalization (background division) Image Processing Once collected, images must be corrected for: Atmosphere (to extent possible) e.g., sequence of images obtained at a variety of telescope elevations usually can be corrected for atmospheric extinction CCD defects and artifacts dark current CCD pixel reports a signal even when not exposed to light bad pixels some pixels will be dead, hot, or even flickering variations in pixel-to-pixel sensitivity every pixel has its own QE can be characterized by flat field Image Display and Analysis Links #6 and #7 Image Display and Analysis This step often is where astronomy really begins. Type and extent of display and analysis depends on purpose of imaging experiment Common examples: evaluating whether an object has been detected or not determining total CCD signal (counts) for an object, such as a star determining relative intensities of an object from images at two different wavelengths determining relative sizes of an extended object from images at two different wavelengths 9

10 Link #8: Storage Storage Glass plates Lots of climate-controlled storage space expensive available to one user at a time now being digitized (scanned), as in the archive you use with DS9 Digital Images Lots of disk space cheaper all the time available to many users 10

The Imaging Chain in Optical Astronomy

The Imaging Chain in Optical Astronomy 1 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.