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Astronomical Detectors

Astronomical Detectors. The First Detector in Astronomy. Eye: complete system of telescope (variable aperture, ~1cm max), detector, and data reduction processor. Retina Light sensitive, chemical process Rods 100 million 2 m m wide can detect low light levels averted vision Cones

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Astronomical Detectors

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  1. Astronomical Detectors

  2. The First Detector in Astronomy Eye: complete system of telescope (variable aperture, ~1cm max), detector, and data reduction processor • Retina • Light sensitive, chemical process • Rods • 100 million • 2mm wide • can detect low light levels • averted vision • Cones • color information • 7 million • cluster near axis • 1000 per square mm • Reusable • integration times 100 ms Quantum efficiency ~1% Resolution 80 arcsecs

  3. Discoveries made with the Eye • Naked Eyes • Nomenclature: Constellations, star names • Astrometry: Stellar positions • Magnitude scale • Planets in solar system, planet orbits (Tycho, Kepler) • Comets • Supernovae explosions, variable stars • Sunspots (by Chinese 800 B.C.) • Solar corona (eclipse) • Lunar Phases • Star Clusters (e.g. Pleiades)

  4. Discoveries made with the Eye • With telescopes • Sunspot cycle • Saturn‘s rings, Jupiter‘s Red Spot • Nebulae, Spiral Nebulae • Parallax, astrometric measurements • Asteroids, moons of most planets • Craters on Moon

  5. Observations with the eye can be unreliable • No record of observations so as to check results • Precise measurements cannot be made (position, magnitude, etc) because observations are not recorded or digitized • Integration time of eye is short so faint objects cannot be seen without using larger and larger telescopes Example of unreliability of eye: Canals on Mars

  6. Eta Carinae Images over the past 100 years Ground-based CTIO 4m Hubble Space Telescope

  7. A Revolution in Detectors: Photographic Plates • 1840 J.W. Draper makes a photograph of the moon. Followed by photographs of the Sun by Foucault and Fizeau • Sunspots photographed in 1858 by W. De La Rue • Jansen and Lockyer in the 1870s photographed the solar spectrum and discovered the spectral lines of Helium. • Ainsee Common photographed Orion Nebula and these revealed stars and details you could not see in a telescope • Photographs by Hubble in the early 1900‘s established that some nebula where „island universes“ (i.e. galaxies). His spectral observations of galaxies (exposures of more than one night) led to the discovery of the expansion of the Universe. • For 100 years photographic plates/film dominated the field of astronomical detectors.

  8. Single Channel Detectors: Photomultiplier Tubes • Photomultiplers are photon counting devices that consist of vacuum tube with a photocathode and focusing electrodes for amplification. • Photocathode: light striking the photocathode produces an electron through the photoelectric effect • Focusing electrodes focus the electron toward electron multiplier or dynode. Each dynode is held at a more positive voltage as the previous one • Electrons are accelerated as they approach the first dynode arriving with greater energy. • More low energy electrons are released and acclerated by the second dynode. • A cascade amplifies the number of electrons (1 photon can produce up to 108 electrons) • At anode an electric current is produced

  9. In 1907 Joel Stebbins pioneered the use of photoelectric devices in Astronomy

  10. Spectral response of some commonly used photocathode materials

  11. Photomultiplier tubes: pile up errors Each detected photon produces a pulse of finite duration, t0, which causes a dead time in the detector. The number of pulses (exposure time) is reduced by the amount of overlapping deadtimes. N = n/(1–t0n) N is the true rate, and n the apparent rate Pile-up errors System blocks completely at high light levels

  12. Disadvantages of Photomultiplier tubes: • Must be operated at high voltages • High light levels can destroy the tube • Not very efficient in optical regions: 3500 to 10.000 Ang. • Single channel devices (more on this later)

  13. Multi-Channel Detectors (Arrays): Photographic Plates Photographic plates were the first two-dimensional detectors • Advantages of photographic plates: • Wide field of view. TLS Schmidt plates are 3 deg x 3 deg. • Resolution, more so than a CCD of the same area

  14. So why do we not still use plates? • Disadvantages of photographic plates: • Low quantum efficiency. The best plates have a QE of about 3% • Long exposure times, inefficient use of time • Reciprocity failure. It becomes less effective as exposure time increases • Non-linear color sensitivity. Plates are more sensitive to blue light • Hypersensitising and Developing. Hypersensitising involves baking plates to increase efficiency (up to 10%). You cannot see results until after developing usually many hours later. • Storage. They are fragile and take up space. They also decay with age. • Digitisation. Must be scanned to put the data in digital form • Cost and availability. In 1996 a single 30 cm x 30 cm plate costs $100 USD. Kodak no longer makes plates.

  15. Reciprocity Failure of Photograhic Plates Cross section of a photographic plate

  16. CCD: The Digital Revolution Nomenclature: • CCD = Charge Coupled Device • A photon detecting device that exploits the photoelectric effect and the semiconducting properties of silicon • The voltage generated is coupled to the intensity of the incident light • Pixel = picture element. Each pixel is an independent photon detector • DN = Data Number (ADU = Analog to Digital Unit) • The output signal from a CCD. Value and range depend on the nature of the voltage digitization. E.g. an 8-bit CCD produces DNs from 0 to 28 (0 to 255). Most CCDs are 16 bit (maximum = 65535) or higher

  17. CCD Properties and Definitions

  18. lmax = 12400 Å/Eg

  19. n-type: a material with more than 4 valence electrons is added (As, from group Va). The extra electrons cannot be accommodated in the valence band and so occupy the conduction band. They represent a persistent set of negative carriers p-type: a material with fewer than 4 valence electrons is added (e.g. B, from group IIIa). This has one fewer electron than normal and creates a small "vacuum" in the electron sea of the valence band. This is called a "hole." As valence electrons shift to fill it, the hole propagates like a positive charge in the opposite direction. The holes represent a persistent set of positive carriers. From http://www.astro.virginia.edu/class/oconnell/astr511/lec11-f03.html

  20. In the depletion region a photon produces an electron. This migrates into the conduction band

  21. The Basic Unit of a CCD The basic element of a CCD consists of a Metal Oxide Semiconductor. The bulk material is p-silicon on which an insulating layer of silicon oxide has been grown as well as thin conducting electrodes of transparent polysilicon. The central electrode is set to a positive bias while the two flanking electrodes are negative. This creates a "depletion" region under the central electrode containing no holes but a deep potential well to trap electrons. The region shown is about 10µ thick. During exposure light enters through the "front-side" electrodes. Photoelectrons generated under the central electrode will be attracted toward the electrode and held below it. The corresponding holes will be swept away into the bulk silicon. From http://www.astro.virginia.edu/class/oconnell/astr511/lec11-f03.html

  22. A „3-phase CCD“ Reading out a CCD Parallel registers shift the charge along columns There is one serial register at the end which reads the charge along the final row and records it to a computer For last row, shift is done along the row Columns

  23. Figure from O‘Connell‘s lecture notes on detectors The CCD is first clocked along the parallel register to shift the charge down a column The CCD is then clocked along a serial register to readout the last row of the CCD The process continues until the CCD is fully read out.

  24. How much charge is lost in the this charge transfer process? Typical Charge Transfer Efficiency of a CCD is >99.999 % Suppose you have a 4096 x 4096 CCD and detect 40.000 photons (electrons). Signal to Noise ratio = √N = 200 Charge recorded = 40.000 x 0.999994096 = 38.394 1605 electrons „lost“ S/N decreased to 195

  25. Typical CCD readout times are 90 – 240 secs, depending on the size of the CCD. This is for single amplifier CCDs. To reduce the readout time some devices can have 4 channels (amplifiers) for readout: Serial register with one amplfier 4 Serial registers with 4 amplfiers 4 Channel CCD Normal readout 4 channel CCD cuts readout time by a factor of 4. Problem: each quadrant usually behaves differently, with its own bias, flat field response, etc. In the data reduction 4 channel CCDs have to be reduced as if they were 4 independent frames.

  26. Quantum Efficiency of some science grade CCDs from ESO: The real power of CCDs is their high quantum efficiency

  27. Quantum Efficiency of CCDs compared to other devices

  28. Two basic types of CCDs: Front illuminated CCD: A CCD whose gate structure is located in front of the potential wells. In other words the light has to pass through the gates Back illuminated CCD: A CCD whose thickness is reduced to 10 mm so that it can be focused on the front where there are no gates. A back illuminated CCD is just a front illuminated CCD flipped and thinned. It is more efficient, particularly in the blue. Risk: There is a risk that in the thinning process the CCD will be destroyed

  29. Thick Front-illuminated CCD Thin Back-illuminated CCD

  30. Front side (thick) versus Back side (thin) CCDs For wavelengths shorter than 4000 Å electrons are generated close to the surface. Thus backside CCDs have a much higher potential for ultraviolet sensitivity than front side thick devices

  31. Front side (thick) versus Back side (thin) CCDs Thin CCDs have a much higher quantum efficiency, particularly in the blue. Most CCDs in use are thinned as the thinning process is fairly reliable and it is rare when a CCDs is destroyed in the thinning process.

  32. CCD Parameters Important for Observations • Gain: Converts ADU to number of photons detected. Important for Signal-to-Noise estimate. Typically 0.5–10 e–1/ADU • Linearity: Detected counts should be proportional to the exposure time. If a CCD has a non-linear regime these level of counts should be avoided • Readout Noise: Noise introduced by CCD readout electronics. negligible for High Signal-to Noise observations • Dark: Thermal noise. Neglible for High Signal-to-Noise Observations Most science grades CCDs are kept at –120 C or cooler. • Bias level: Constant level added to the data by the electronics to ensure that there are no negative numbers McDonald CCD for coude spectrograph: Gain = 0.56 ± 0.015 e–1/ADU Readout Noise = 3.06 electrons Bias level = 1024

  33. Mean Intensity 1.5 x 105 Noise Tests for CCD: Linearity Take a series of frames of a low intensity lamp and plot the mean counts as a function of exposure time If the curve followed the red line at the high count rate end (and some CCDs do!) then you would know to keep your exposure to under 150.000

  34. Noise Tests for CCD: Gain For Photon statistics the variance, s = √Photons. Therefore s2 should be a measure of the number of detected photons • Take a series of frames at with a constant light level • Compute s for frames • Change the exposure time and take another series of frames calculating a new s • Plot the observed mean intensity versus the variance squared (s2) • The slope is a measure of the gain

  35. 1.5 x 105 Mean Intensity

  36. Problems and Pitfalls of CCD Usage Saturation If too many electrons are produced (too high intensity level) then the full well of the CCD is reached and the maximum count level will be obtained. Additional detected photons will not increase the measured intensity level: 65535 16-bit AD converter ADU Exposure time

  37. Problems and Pitfalls of CCDs Blooming: If the full well is exceeded then charge starts to spill over in the readout direction, i.e. columns. This can destroy data far away from the saturated pixels.

  38. Blooming columns Saturated stars

  39. Anti-blooming CCD can eliminate this effect: No blooming Blooming

  40. One solution: Anti-blooming CCDs Anti-blooming CCDs have additional gates to bleed off the overflow due to saturation The problem is these gates cover 30% of the pixel. This results in reduced sensitivity, smaller well depth, and lower resolution (gaps between pixels has increased)

  41. Residual Images If the intensity is too high this will leave a residual image. Left is a normal CCD image. Right is a bias frame showing residual charge in the CCD. This can effect photometry Solution: several dark frames readout or shift image between successive exposures

  42. Fringing CCDs especially back illuminated ones are bonded to a glass plate SiO2 10 mm Glue 1 mm Glass When the glass is illuminated by monochromatic light it creates a fringe pattern. Fringing can also occur without a glass plate due to the thickness of the CCD

  43. l (Å) 6600 6760 6920 7080 7280 7460 7650 7850 8100 8400 Depending on the CCD fringing becomes important for wavelengths greater than about 6500 Å

  44. Readout Noise Signal-to-Noise Ratio Readout noise in electrons 1 0 3 10 Intensity High readout noise CCDs (older ones) could seriously affect your Signal-to-Noise ratios of observations

  45. Basic CCD reductions • Subtract the Bias level. The bias level is an artificial constant added in the electronics to ensure that there are no negative pixels • Divide by a Flat lamp to ensure that there are no pixel to pixel variations • Optional: Removal of cosmic rays. These are high energy particles from space that create „hot pixels“ on your detector. Also can be caused by natural radiactive decay on the earth.

  46. Overscan region Bias Pixel Most CCDs have an overscan region, a portion of the chip that is not exposed so as to record the bias level. The prefered way is to record a separate bias (a dark with 0 sec exposure) frame and fit a surface to this. This is then subtracted from every frame as the first step in the reduction. If the bias changes with time then it is better to use the overscan region

  47. Flat Field Division Flat Field Raw Frame Raw divided by Flat Every CCD has different pixel-to-pixel sensitivity, defects, dust particles, etc that not only make the image look bad, but if the sensitivity of pixels change with time can influence your results. Every observation must be divided by a flat field after bias subtraction. The flat field is an observation of a white lamp. For imaging one must take either sky flats, or dome flats (an illuminated white screen or dome observed with the telescope). For spectral observations „internal“ lamps (i.e. ones that illuminate the spectrograph, but not observed through the telescope are taken. Often even for spectroscopy „dome flats“ produce better results, particularly if you want to minimize fringing.

  48. CCD versus Photographic Plate CCD 1/9 coverage Cost: 120.000 Euros, ~1.000.000 Euros for same FOV Cost per exposure over lifetime: < 1 Euro Plate 3 x 3 deg (less since image is trimmed) in one exposure Cost 100 Euros Requires 20 m telescope to detect same number of photons as CCD

  49. CCD versus CMOS CCD

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