3/27/17. Detector Basics. Quantum Efficiency (QE) and Spectral Response. Quantum Efficiency (QE) and Spectral Response

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1 3/27/17 Detector Basics The purpose of any detector is to record the light collected by the telescope. All detectors transform the incident radiation into a some other form to create a permanent record, such as particles (photographic plates), molecules (eyes), or electrons (CCDs). Astronomical Detectors Image courtesy Nick Raines and Steve Eikenberry There are eight important properties by which to gauge the utility of a detector: 1. Quantum Efficiency and Spectral Response 2. Temporal Response and Resolution 3. Dynamic Range 4. Linearity and Stability 5. Noise 6. Spatial Resolution and Field of View 7. Ease of Conversion to Digital Signal 8. Spectral Resolution This lecture draws upon the lecture notes of Steve Majweski ( and C. S. J. Pun ( Several slides are taken, with permission, directly from the last of these sources. Quantum Efficiency (QE) and Spectral Response Quantum Efficiency (QE) and Spectral Response QE curve for HST WFPC2 How much light are you wasting? Note that for ground-based data, you have to account for both the QE curve and the atmospheric transmission. Quantum efficiency is defined as the percentage (or fraction) of incident photons that are detected [QE = # photons detected / # incident photons] A detection can be defined in any number of ways, such as crystals formed (photographic emulsion) or charge-pairs created (CCD). The point is that the photon is recorded. Ideally you want QE=100%. No detector is efficient at all wavelengths, but rather has a QE that varies with wavelength. The spectral response is the dependence of QE upon wavelength. HST Cycle 14 WFPC2 Instrument Handbook 1

2 3/27/17 Quantum Efficiency (QE) and Spectral Response Temporal Response QE curve for Chandra HRC-I detector How quickly can you take images, and how long do you have to wait? What is the shortest integration possible? How long can you integrate? Time variability Faint sources (related to dynamic range and detector noise properties) How quickly can you take consecutive images (readout time)? Time variability Observing efficiency Detector Basics The purpose of any detector is to record the light collected by the telescope. All detectors transform the incident radiation into a some other form to create a permanent record, such as particles (photographic plates), molecules (eyes), or electrons (CCDs). Astronomical Detectors Image courtesy Nick Raines and Steve Eikenberry There are eight important properties by which to gauge the utility of a detector: 1. Quantum Efficiency and Spectral Response 2. Temporal Response and Resolution 3. Dynamic Range 4. Linearity and Stability 5. Noise 6. Spatial Resolution and Field of View 7. Ease of Conversion to Digital Signal 8. Spectral Resolution This lecture draws upon the lecture notes of Steve Majweski ( and C. S. J. Pun ( Several slides are taken, with permission, directly from the last of these sources. 2

3 Dynamic Range Linearity Can I look at bright and faint things at the same time? Dynamic range = largest possible signal / smallest possible signal The dynamic range is relevant because it tells you for a single image the range of object brightnesses you can observe. A related quantity for digital detectors is the full well depth (capacity), which tells you how many detections your detector can record before saturation. Concrete example: CCD # of detections is an integer (so min=1) Assume full well depth = 2 16 (65,536) counts Dynamic range = 65,536/1 = =3? Ideally, you want the response of your detector to be linearly proportional to the number of photons (1 photon 1 detection). Human eye and photographic plates highly nonlinear CCDs and most other modern detectors are nearly linear In some cases, must still apply a linearity correction. Stability Will it count the same tomorrow as today? How reliable is your photometry? Is the efficiency of your detector stable in time? Sensitivity of photographic plates degrades with time, especially in high humidity. Sensitivity of detectors on satellites can degrade with time due to hard radiation and cosmic rays. In magnitudes, this is mag of dynamic range. Linearity Comparison for Photographic Plate and CCD Noise It is a sad fact of life that detectors introduce additional noise into observations CCD Types of noise Poisson noise (aka shot noise): Goes as N 1/2, where N is the number of photons. For an ideal detector this is the only source of noise. Two components that contribute to the Poisson noise are (1) source photons, and (2) background sky photons. Read noise (RN): Some detectors, like CCDs, generate additional noise when the signal is read out Thermal noise (dark noise): thermal agitation of electrons in detector. Noise in electronics: self-explanatory Photographic plate (Note that the scale is logarithmic) Noise components add in quadrature, so Total noise = [N + RN 2 + dark noise 2 + electronic noise 2 ] 1/2 For research instruments, Poisson and read noise are usually the dominant sources of noise. Figures courtesy John P Oliver, 3

4 Angular Resolution and Field of View Angular Resolution and Field of View Angular resolution Recall that the image scale (in arcsec/mm) is determined by the design of the telescope Two pixels are required to resolve an object. Consequently, for a detector the angular resolution = image scale 2 pixel size In which cases is the detector well matched to the PSF? Field of view FOV = image scale N pix pixel size Want the resolution to be well-matched to the telescope. If the detector resolution is worse than the seeing, you re sacrificing performance. If the detector resolution is much better than the seeing, you re sacrificing field of view. Nyquist sampling Want at least 2 pixels within FWHM for the PSF to be well sampled Ease of Conversion to Digital Signal Automated Plate Measuring Machine Ease of Conversion to Digital Signal For quantitative analysis, you want to convert your data into a digital format. Eye: out of luck (for now) Photographic plates: densitometer CCDs and other modern detectors: direct digital output For quantitative analysis, you want to convert your data into a digital format. Eye: out of luck (for now) Photographic plates: densitometer CCDs and other modern detectors: direct digital output Spectral Resolution Spectral resolution is a moot point for current optical and infrared detectors, but at some wavelengths (particularly high energies like X-rays and Gamma rays) the detectors return energy information for the individual photons. In these cases it is relevant to quantify the precision of this energy information. Spectral Resolution Spectral resolution is a moot point for current optical and infrared detectors, but at some wavelengths (particularly high energies like X-rays and Gamma rays) the detectors return energy information for the individual photons. In these cases it is relevant to quantify the precision of this energy information. 4

5 Chemical Human Eye Photographic plates Electronic Electron emission Photomultiplier tube Photocell Voltage Photovoltaic cell [solar collector, near-infrared InSb detectors] Pyroelectric cell Thermocouple Resistance Bolometer [commonly used in submillimeter and microwave instruments] Photoconductive cell [NIR HgCdTe detectors] Charge Charge Coupled Device (CCD) Cooper pairs (excess quasiparticles) Superconducting Tunnel Junction (STJ) The Human Eye A thin convex lens Focal length ~ 14mm (near vision) 17mm (far vision) Aperture ~ 2 8mm Dynamic range: 100,000:1 Photon sensor on the retina: cones (photopic vision) in daylight, rods (scotopic vision) at night QE ~ 3% (cone) 10% (rod) Cones: 6-7 million Rods: 100 million rod Thermal detectors respond to temperature rise due to absorption of radiant energy. [Example: bolometer] Quantum detectors respond to incident photons. [Examples: photographic plate, CCD] cone Photographic Plates Photography was invented in 1840s, but use in astronomy only started to become popular in around Micron size crystals of soluble silver halide salts (such as silver bromide, AgBr) are suspended in a gelatin emulsion on a glass plate When a photon strikes, the silver ion can then combine with the electron to produce a silver atom. The free silver produced in the exposed silver halide makes up the latent image The latent image is later amplified in the developing process. The deposit of silver produces a dark area in the film. Disadvantages of photographic plates Nonlinear and not completely stable medium Low QE (~4%) Not easy to convert to digital output Advantages: Angular resolution and field of view Ag + Br (crystal) + hν(radiation) Ag + + Br + e Ag + + e Ag(atom) Electronic Detector Basics: Photoelectric Effect Physical basis for most detectors in astronomy Photons of sufficient energy hitting surface of metal releases electrons (photoelectrons) Energy of released electrons depends NOT on intensity of light (if we think of light as a wave), but rather on the frequency of light (particle nature of light). There is a minimum frequency of light before any photo-electrons can be emitted from a particular metal: KE e = E photon W = hv W = h(v-v min ) where KE e is the KE of photoelectron, is photon energy, W is the work function of the metal, h is Planck s constant, v is the photon frequency, v min is the minimum photon frequency of the metal. sol.sci.uop.edu/~jfalward/particlesandwaves/ 5

6 3/27/17 Photovoltaic Photomultiplier Tube Combines photoelectric effect with amplification of electric signal. 1. Photon comes in. 2. Photoelectric effect generates electron 3. Electrons, just like photons, when moving with sufficient KE, not only release electrons from metals, but there is also amplification, with more electrons coming out of the metal than entering. 4. Signal recorded. Incident photons generate voltage difference across a p-n junction Indium Antimonide (InSb) Used in near-infrared Good QE (30%) Linear Low Noise Other applications: Medical Imaging Missile Guidance Infrared Microscopy Solar Panels Fast read out Spitzer IRAC InSb Detector micro.magnet.fsu.edu/primer/digitalimaging/concepts/concepts.html Photoconductive Bolometer Incident photons increase electrical conductivity, or equivalently decrease resistance (absorbed photons generate current) Heating of material by incident light changes resistance of material. Differs from photoconductive in that this is a thermal rather than photoelectric effect Mercury Cadmium Telluride (HgCdTe) Widely used at submillimeter and microwave wavelengths. Used in near-infrared Each bolometer is equivalent to a single pixel. Courtesy Nick Raines and Steve Eikenberry The Max-Planck Millimeter Bolometer array (MAMBO) for the IRAM 30m 6

7 3/27/17 Comparison of Various Optical Detectors Charge Coupled Device (CCD) Detector QE Advantages Disadvantages Eye 10% Quick reset between images Large dynamic range (100,000:1) Fixed integration time Nonlinear Conversion to Digital Photographic Plate 4% Good angular resolution & FOV No electronic noise Nonlinear Stability issues Low QE Conversion to Digital Slow to change plates Photomultiplier Tube 30% Decent QE Linear Digital Angular resolution/fov CCD >90% in some cases High QE over wide wavelength range Large dynamic range Linear Digital Somewhat limited FOV (but can mosaic) Readout can be slow A CCD is a two-dimensional quantum detector that outputs a digital image. (We ll talk about the details shortly). Photoelectric effect; photons captured as charge. Detector consists of an array of "pixels" (picture elements) laid out in rows and columns Charge filled pixels are manipulated to move the charge to the output and into a control computer. QE can be >90%, and response is reasonably linear. Limited number of pixels, so must often make a tradeoff between resolution and FOV. The above illustration is of an 800x800 pixel CCD made by Texas Instruments (TI) for the Hubble Space Telescope WFPC. The inset shows the output amplifier. Charge Coupled Device (CCD) CCDs Image courtesy Nick Raines and Steve Eikenberry A CCD is a two-dimensional quantum detector that outputs a digital image. (We ll talk about the details shortly). Photoelectric effect; photons captured as charge. Detector consists of an array of "pixels" (picture elements) laid out in rows and columns Charge filled pixels are manipulated to move the charge to the output and into a control computer. QE can be >90%, and response is reasonably linear. Limited number of pixels, so must often make a tradeoff between resolution and FOV. The above illustration is of an 800x800 pixel CCD made by Texas Instruments (TI) for the Hubble Space Telescope WFPC. The inset shows the output amplifier. 7

8 Charge Coupled Device (CCD) Charge Coupled Device (CCD) Silicon-based integrated circuits consisting of a matrix of photodiodes which convert light energy in the form of photons into an electronic charge Silicon-based integrated circuits consisting of a matrix of photodiodes which convert light energy in the form of photons into an electronic charge Invented in 1960 s Standard detector for UV and optical Photoelectrons are released by semiconductors, but freed photoelectrons stay inside device Invented in 1960 s Standard detector for UV and optical HST WFC3 4k x 4k detector HST WFPC3 800 x 800 detector The above illustration is of an 800x800 pixel CCD made by Texas Instruments (TI) for the Hubble Space Telescope WFPC. The inset shows the output amplifier. Semi-conductors Typical semiconductors behave as insulators at low temperatures, but are relatively conductive at higher T. Band Theory Means of modelling behavior of electrons in a crystal solid. For a semiconductor crystal lattice, the allowed quantum states occupy bands of closely packed energy levels. These bands are called energy bands. There are no states between these bands; hence these regions are called energy gaps. The minimum distance between allowed states is represented by this energy gap (E g ) for insulators and semiconductors. Image source: Dr Steven R. Majewski Elemental semiconductors: column IVa of periodic table. Most popular: Si, Ge Compound semiconductors: elements in columns Ib, IIb, IIIa, Va, Via, VIIa of periodic table Compound semiconductors are made from diatomic molecules symmetrically spanning column IVa in the periodic table, e.g. GaAs, InSb, HgCdTe. They have similar behavior as column IVa semiconductors. wikipedia Image source: Dr Steven R. Majewski Valence band: ground states that are normally completely filled Conduction band: excited states that are normally completely unfilled An electron must absorb a mimimum of E g energy to be excited from the valence to conduction band. 8

9 3/27/17 Electric Conduction in Semiconductors The conduction bands in semiconductors are normally unfilled. Electrons in the valence band need to absorb photon energy to lift them into unpopulated energy levels in the conduction band. The key for the usefulness of semiconductors for visible and infrared photon detectors is that their band gap energies match those of visible/ir photons. For each semiconductor, long wavelength cutoff λlong,cut = hc/egap CCDs Semiconductor Egap(eV) Detection InSb 0.18 IR Ge 0.67 IR Si 1.11 NIR, Opt GaAs 1.43 Opt AgBr 2.81 Opt SiC 2.86 Opt Image courtesy Nick Raines and Steve Eikenberry 5000A=2.48eV Band Theory Doping of a Semiconductor Gratzel, Nature 414, When electrons are excited to the conduction band, they leave behind empty positions, or holes. Moving the electrons along one direction is equivalent to moving the holes in the opposite direction. You can drastically change the conductivity of semiconductor by preloading it with excess of electrons and holes (doping) N-type: Dope the semiconductor with a second material that gives away electrons to the semiconductor, leaving the semiconductor with a surplus of electrons. The net effect is to reduce the bandgap, thereby changing the spectral bandwidth (i.e. make the semiconductor sensitive to longer wavelengths). This can be accomplished by adding elements in column Va to semiconductors in column IVa. P-type: Dope the semiconductor with a material that absorbs electrons, creating a surplus of holes in the semiconductor. This doping increases the conductivity. Can be accomplished by adding elements in column IIIa to semiconductors in column IVa. Courtesy: Dr Steven R. Majewski 9

10 Metal Oxide Semiconductor (MOS) Capacitor Foundation of a single CCD pixel Made of semiconductor covered with thin layer of insulator, e.g. SiO 2, with an electrode (gate) on top. If semiconductor is P-doped and you put the gate at voltage +V, then holes will move away from the gate, but no free electrons exist to move towards the SiO 2 (depletion zone/region). The depletion region act as a well, or photon bucket, where if there are no thermally created electron/hole pairs, only photoelectrons will be stored 10µm depletion region MOS Capacitor as a electron well The depletion region can therefore be thought of as a potential well, where photoelectrons can be stored. The size of the bucket is proportional to the voltage of the gate electrode (bias voltage) Maximum charge a pixel can hold is called full well capacity, or full well depth, (~ 100k 300k e - /pixel). It affects the dynamic range of the CCD. photoelectron Image Source: Molecular Expressions TM digitalimaging/concepts/concepts.html Courtesy: Dr Steven R. Majewski Methods of collecting charges in each pixel 1. Charge Injection Device: Switch the gate voltage to negative, thus repelling the electrons collected into the Si, where they can be collected and measured as a current. Methods of collecting charges in each pixel 2. Charge coupling: the principle behind CCD By building multiple gates on the same piece of Si, we can generate a series of depletion zones. If the gates are far enough apart (> 1mm), then the wells will be independent of each other. Image source: Dr Steven R. Majewski 10

11 Methods of collecting charges in each pixel 2. Charge coupling: the principle behind CCD By adjusting voltages, we can transfer charges from one zone to another because electrons are attracted to the larger gate voltage (or, seek the deeper well) Therefore we are able to move charge on Si along the rows of gates. Charge Transfer in CCD Each pixel in a CCD will have a group of gates (at least two) called phases. Each phase alters its voltage with a repeat pattern of high (opening a well) and low (closing a well) states Need accurately timed sequence to drive stored electrons collected. Voltage of P(1) V 1 V 2 V 3 V 1 V 2 V Courtesy: Dr Steven R. Majewski Courtesy: Molecular Expressions TM Full Frame Architecture Putting it all together... The last row of a CCD is called the serial register (also called multiplexer) Line address readout: 1. Shift all columns by one pixel into multiplexer 2. Readout all the multiplexer pixel electrons to an amplifier by shifting charges 3. When multiplexer is empty, repeat 1 Problem: Still collecting photons while array being emptied! Resulting in smearing of image Solution: Cover CCD with shutter during readout SiO 2 : 10µm Depletion zone: ~5µm Si thickness: mm Courtesy: Dr Steven R. Majewski Image source: Molecular Expressions TM 11

12 3/27/17 Quantum Efficiency Photon Penetration in Si Consider incoming photon flux F(0) on the surface of a CCD chip. The flux at depth z is given by F ( z ) = F0 e αz QE of CCD varies with wavelength, mostly due to the different penetrating power through Si of different energy photons. The long wavelength cutoff λlong,cut is caused by the energy bandgap in semiconductors. The short wavelength cutoff λshort,cut is caused by the weak penetration of photons, leading to many photons absorbed before reaching the depletion zone. where α is called the coefficient of intrinsic absorption, which is a function of temperature T and wavelength λ. Wavelength α (µm-1) α (µm-1) (Å) T=300K T=77K Photon Penetration in Si The distance 1/α, is called a scale height (or optical depth). Fraction of photons transmitted at N scale heights F ( z ) = F0 e αz :.368 :.135 :.050 :.018 Therefore blue photons are almost totally absorbed by ~ 1µm To increase efficiency in blue, decrease thickness of Si For infrared photons of 1µm, one scale height > 200 µm. Sensitivity in red requires Si thick enough to have enough opportunity to absorb the photons. However, Si that is too thick can cause loss of resolution (i.e. photoelectrons generated may travel to depletion zones of other pixels). Also, more Si means higher dark current. Wavelength α (µm-1) α (µm-1) (Å) T=300K T=77K CCDs Image courtesy Nick Raines and Steve Eikenberry Reminders and Announcements: HW #3 due on Thursday. Next week is the last chance to observe. If you don t know when you are giving your presentation, talk to me. Bring me your CTO data before reducing it with IRIS Tomorrow during the lab period I will be in room 7 to help people with data reduction. 12

13 Full Frame Architecture Putting it all together... The last row of a CCD is called the serial register (also called multiplexer) Line address readout: 1. Shift all columns by one pixel into multiplexer 2. Readout all the multiplexer pixel electrons to an amplifier by shifting charges 3. When multiplexer is empty, repeat 1 Problem: Still collecting photons while array being emptied! Resulting in smearing of image Solution: Cover CCD with shutter during readout SiO 2 : 10µm Depletion zone: ~5µm Si thickness: mm Courtesy: Dr Steven R. Majewski Image source: Molecular Expressions TM Photon Penetration in Si Consider incoming photon flux F(0) on the surface of a CCD chip. The flux at depth z is given by F( z) αz = F 0 e where α is called the coefficient of intrinsic absorption, which is a function of temperature T and wavelength λ. To improve QE (1) Backlit CCD: Photons incident from the back, need to be built very thin (~15µm versus ~300mm for regular front-lit CCD) Difficult to make (fragile and need even thickness of Si to order of 1µm or get large QE variation over the chip) and therefore expensive Wavelength α (µm -1 ) α (µm -1 ) (Å) T=300K T=77K Left: Molecular Expressions TM Right: 13

14 To improve QE (2) Florescent coating: applications of substances to CCD that act as a wavelength converter, i.e. release a longer wavelength photon upon an incident photon, to improve QE in blue and even UV. Common UV coating include molecules called Polycyclic Aromatic Hydrocarbons (PAHs) Minor note: PAHs are usually carcinogenic Image source: Dr Steven R. Majewski Top: Molecular Expressions TM Right: Charge Transfer Efficiency (CTE) Some electrons are lost during transfer from one pixel to another CTE = No. of charges transferred/ No. of original charges Charge Transfer Efficiency (CTE) Some electrons are lost during transfer from one pixel to another CTE = No. of charges transferred/ No. of original charges Is a CTE of 99.9% Is it good enough? Is a CTE of 99.9% Is it good enough? Consider a CCD with a 3 phase charge transfer, recording data from a 1024x1024 CCD require ~3x1024 transfers Ratio of charges left: (0.999) 3072 = 0.05! Need CTE of more like % ( yields ratio of 0.97)

15 Binning An effective method to reduce readout noise is pixel binning. Combine signals from adjacent pixels before arriving at the readout amplifier, improve sensitivity at low signal level. Reduces angular resolution of final image, i.e. increase plate scale Reduces total readout time for the whole CCD chip, For ACS/WFC the readout time is about 2.5 minutes For general CCDs, common binning modes are 1x1 (no binning); 2x2, 3x3 (for imaging); 2x1, 3x1 (for spectroscopy) Noise Considerations for CCD Read Noise: also known as readout noise For a given CCD circuitry, the read noise is a constant, independent of the signal received, expressed in terms of e - root-mean-square (e - RMS) ACS/WFC has a read noise of 5 e - RMS All sources of noise can be combined in quadrature, i.e., total noise σ 2 Total = σ 2 Poisson+ σ 2 RN + σ 2 Dark Effect of binning: Consider 4 pixels and we are interested with the combined signal from the addition of these pixels (a) With no binning and readout of 2x2 pixels: there are four readouts: σ 2 Total = Σσ 2 Poisson+ 4σ 2 RN+ σ 2 Dark (b) With 2x2 binning of those same pixels, there is only one readout: σ 2 Total = Σσ 2 Poisson+σ 2 RN+ σ 2 Dark S remains the same, but N. Therefore S/N Courtesy: Molecular Expressions TM digitalimaging/concepts/concepts.html 2x2 binning 15