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April 23, 2008 Astro 890 Detectors Wide, High, Deep, and Sensitive
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April 23, 2008Astro 890 The Plan Wax poetic (and silly) about the landscape Get real and focus on stuff I actually know something about Photon Detection Ways of organizing large numbers of Pixels Processing signal ADCs System Examples of real detectors
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April 23, 2008Astro 890 Radar Detector Radon Detector Smoke Detector Lie Detector Leak Detector Snake Detector Paul said talk about Detectors
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April 23, 2008Astro 890 Even if I stop being silly… Neutrino Detector? Cosmic Ray Detector? Photon Detector? Yeah, Photon, that’s the one
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April 23, 2008Astro 890 Still a Tall Order DC to Daylight doesn’t cut it –(daylight is only 10 15 Hz) ~10 8 Hz to 2x10 28 Hz 4 x10 -7 to 10 14 eV (~100 ergs)
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April 23, 2008Astro 890 Stick to what you know? We “do” ground based optical/NIR instruments –Ground based means to atmospheric cutoff at ~300 nm (3000 Å) caused by O 3 in upper atmosphere. –NIR, for us, means to 2.5 µ or J H and K bands. At any longer wavelength the background fills the wells so fast additional hardware is required for real time co-adding.
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April 23, 2008Astro 890 First Astronomical CCD Picture Fairchild 100x100 CCD, 1974
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April 23, 2008Astro 890 Photon Detection (0.3 to 2.5µ) Electron into vacuum (photocathode). Silver halide to silver or other photo chemistry (Call Kodak) Create hole electron pair(s)
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April 23, 2008Astro 890 Silicon Works Great up to ~1µm 1µ=1.24 eV Si band gap =1.1 eV Beyond that … –InSb out to ~5 µ –HgCdTe which can be tuned to 2.5 µ, 5 µ or even 12 µ
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April 23, 2008Astro 890 Avoid Solid State Physics at all Cost
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April 23, 2008Astro 890 Care for (half of) the Pair Don’t let it spend any time in an area with a lot of holes (electrons) Move a packet of them around without loss
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April 23, 2008Astro 890 QE (DQE) Get Photon into detector –Fresnel losses, Si has index of 3.4! R~30% Anti-reflection coatings can help –Semi-transparent electrodes. Losses ~50% This is the main reason for thinning. Recombines near surface P channel has field in the right direction Not letting it get out of detector –Si becomes transparent much beyond 1µ
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April 23, 2008Astro 890 Absorption length in Si
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April 23, 2008Astro 890 Moving charge Two ways, really the same –Make a moving potential well (CCD) –Make an array of switches (Most IR detectors)
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April 23, 2008Astro 890 CCD wells Buried “channel stops” to define columns Array of insulated electrodes to define pixels Additional array of electrodes to shift a line at a time
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April 23, 2008Astro 890 132 Output 1 3 2 132132 CCDs 3 Phase Serial Register
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April 23, 2008Astro 890 Three 2D Archtectures CCD with Parallel and Serial shifts CCD with only Parallels and an output per column X-Y array of switches
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April 23, 2008Astro 890 RAIN (PHOTONS) BUCKETS (PIXELS) VERTICAL CONVEYOR BELTS (CCD COLUMNS) HORIZONTAL CONVEYOR BELT ( SERIAL REGISTER ) MEASURING CYLINDER (OUTPUT AMPLIFIER) CCD Analogy
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April 23, 2008Astro 890 Exposure finished, buckets now contain samples of rain.
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April 23, 2008Astro 890 Conveyor belt starts turning and transfers buckets. Rain collected on the vertical conveyor is tipped into buckets on the horizontal conveyor.
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April 23, 2008Astro 890 Vertical conveyor stops. Horizontal conveyor starts up and tips each bucket in turn into the measuring cylinder.
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April 23, 2008Astro 890 ` After each bucket has been measured, the measuring cylinder is emptied, ready for the next bucket load.
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April 23, 2008Astro 890 A new set of empty buckets is set up on the horizontal conveyor and the process is repeated.
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April 23, 2008Astro 890 Eventually all the buckets have been measured, the CCD has been read out.
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April 23, 2008Astro 890 Q=CV V=Q/C High responsivity means very small C 1cm x 1cm x 1mm ~ 1pF (10^-12 F) Good CCDs now have node capacity of ~10 fF (10^-14F) →10 µV/electron
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April 23, 2008Astro 890 Turning Charge Packet into a Voltage
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April 23, 2008Astro 890 IR array Architecture
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April 23, 2008Astro 890 Signal Processing 1 “kTC” noise –Any time you reset a capacitor the resulting voltage is uncertain by √kTC. –Good CCD will have C~10 -14 F, T~150K, noise ~30 electrons RMS –Must use “dual slope integrator” or “double correlated sampling” (DCS) to reject kTC noise. –DCS also rejects “noise” at any frequency less than 1/ (integration time)
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April 23, 2008Astro 890 Dual slope Integrator
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April 23, 2008Astro 890 Signal Processing 2 Optimal S/N for White Noise is Simple differential averager (aka dual slope integrator) As with all white noise problems S/N grows as √t where t is the integration time Noise from amplifiers is always 1/f like If 1/f knee is below 1/t then noise is essentially White If 1/f knee is well above 1/t and noise is really 1/f then S/N is independent of t.
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April 23, 2008Astro 890 Dark Current (silicon) Room Temperature ~1/40 eV Band gap ~1.2 eV Leads to a dark current of ~20pA/cm 2 Or about 300 electrons/second for 15µ x 15µ pixel. Must cool, typically 190K for 1 electron/hour, although surface effects can cause problems. 2.5 µ HcCdTe requires 77K or lower
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April 23, 2008Astro 890 The next step Systems have typically had 16 bit ADC Not enough resolution for low noise CCDs with a full well of 200,000 electrons 18 bit ADCs now work (and are a lot cheaper than old style 16 bit units)
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April 23, 2008Astro 890 Telescopes are harsh Fiber optic coupling between telescope focus and computers helps with noise problems Old=120 Mbits/sec, New=2 Gbits/sec
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April 23, 2008Astro 890 System (Warm Part) Computer “Sequencer” –Generates clock patterns –Receives data from the cold –Fiber interface
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April 23, 2008Astro 890 System (Cold Part) Head Electronics –Drive electronics for detector –Signal processing and analog to digital conversion –Fiber interface
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April 23, 2008Astro 890 Real World Detectors 800 x 800 Texas Instruments, Original HST detector 3 x 8 k e2v, Worlds Largest IR array in Barbie Coffin
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