Atmospheric Extinction

Transcription

1 Atmospheric Extinction Calibrating stellar photometry requires correction for loss of light passing through the atmosphere. Atmospheric Rayleigh and aerosol scattering preferentially redirects blue light relative to red light.

2 Atmospheric Extinction Calibrating stellar photometry requires correction for loss of light passing through the atmosphere.

3 Extinction Correction in Practice In each filter measure the star at a various airmass ( x below is (airmass 1)) and determine the extinction in units of magnitudes per airmass for each observing band. Alternatively, have a calibrated star in your field of view (easy in the era of sky surveys).

4 Time Variability of Extinction Systematics, I Extinction Rayleigh scattering (optical; proportional to static pressure and airmass) Ozone (optical) Water (IR) Volcanic aerosols Can vary by 0.1-1% Episodic problem SCTF 1/29/2014 Nabro eruption, 13 June 2011 (Bourassa, et al. (2012)) 4

5 Filter Bandpasses Calibrating observations precisely is dependent upon having well defined/known bandpasses. Bandpasses can be defined by atmospheric characteristics absorption, especially in the infrared. airglow emission sensitivity to distinguishing features in stellar atmospheres. Detector response plays a role in shaping the effective filter profile (e.g. HST 814) Slightly different filter/response profiles (e.g. B-R vs. g'-r') produce slightly different answers Photometric transformation equations permit the conversion of one magnitude/filter system into another (more later...).

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7 Infrared Bandpasses Atmospheric absorption provides natural boundaries for defining infrared filter bandpasses.

8 Stellar Photometry with Filters Differences between magnitudes (which are ratios when you think about it) measured in different filters are diagnostic of temperature of blackbodies (stars). V R I

9 Stellar Photometry with Filters Differences between magnitudes (which are ratios when you think about it) measured in different filters are diagnostic of temperature of blackbodies (stars).

10 The Electromagnetic Spectrum and Photon Energy hc E=hν= λ λ ν=c For reference 1um wavelength corresponds to 2x10-19J = 1.24 ev

11 Detectors Goal: Convert photons to an electronic signal (apologies to photography...) with as little accompanying noise as possible with as much conversion efficiency as possible ideally at the quantum/poisson limit enforced by the photons. 1 photon yields 1 electron (or ideally a bunch of electrons) Primary Detection Methods Bulk thermal response (bolometry) incident radiation changes the temperature of the detector electrical resistance changes with temperature Conversion of photons to ''free'' electrons quantum response photoelectric or solid state detection Coherent detection sense wave nature (phase) of the photons primarily through heterodyne conversion to lower frequencies

12 Detectors Goal: Convert photons to an electronic signal (apologies to photography...) with as little accompanying noise as possible with as much conversion efficiency as possible ideally at the quantum/poisson limit enforced by the photons. 1 photon yields 1 electron (or ideally a bunch of electrons) Primary Detection Methods Bulk thermal response (bolometry) incident radiation changes the temperature of the detector electrical resistance changes with temperature Conversion of photons to ''free'' electrons quantum response photoelectric or solid state detection Coherent detection sense wave nature (phase) of the photons primarily through heterodyne conversion to lower frequencies

13 A free electron is a detectable electron (via voltage or current) an electron can be free in space -- photoelectric effect or it can be ''free'' within a crystal lattice -- solid state detection The Photoelectric Effect E= Metals are characterized by a work function which determines the energy difference between the highest energy state for an electron within the metal and the energy of an electron in free space. A photon with energy in excess of this work function will liberate a free, detectable, electron -- the photoelectric effect Heated metals will emit free electrons -- those with thermal energy in excess of the material's work function -- thermionic emission via a Boltzmann law. hc λ

14 A free electron is a detectable electron (via voltage or current) an electron can be free in space -- photoelectric effect or it can be ''free'' within a crystal lattice -- solid state detection The Photoelectric Effect E = kt Metals are characterized by a work function which determines the energy difference between the highest energy state for an electron within the metal and the energy of an electron in free space. A photon with energy in excess of this work function will liberate a free, detectable, electron -- the photoelectric effect Heated metals will emit free electrons -- those with thermal energy in excess of the material's work function -- thermionic emission via a Boltzmann law.

15 Photomultipliers Photomultipliers are based on the cascade amplification of individual electrons liberated by the photoelectric effect Work functions for metals are typically a few electron volts - Remember 1 ev = 1240 nm

16 The Photoelectric Effect Photomultipliers are based on the cascade amplification of individual electrons liberated by the photoelectric effect Work functions for metals are typically a few electron volts 1 ev = 1240 nm Photocathodes can be engineered to have sensitivity out to 1.5 um (obviously not using pure elemental metals...)

17 Photomultiplier Shortcomings Shortcomings of photomultipliers poor wavelength coverage due to large work function of metals (limited to <1.5um operating wavelength) poor quantum efficiency (<20% conversion of photons to electrons) thermally emitted electrons (known as dark current) large single-detector area One big advantage photon counting

18 Electronic Energy Levels in Solids An alternative approach mimic photoelectric effect in a crystal lattice: At large separations, electronic orbitals have atomic characteristics. As atomic separation decreases these degenerate states must split under the interacting potential of all of the nuclei in the crystal. The ensemble of split energy levels is a band which may be full, partially filled and/or overlapping with other bands Electrons that have immediately adjacent available energy states can change state and thus conduct metal

19 Electronic Energy Levels in Insulators Insulators have filled energy bands which do not overlap with adjacent energy bands for the interatomic equilibrium spacing. At T=0K a material will either have overlapping energy states and is a conductor, or it will have a bandgap above a completely filled energy state and be an insulator.

20 Semiconductors Semiconductors are insulators where the bandgap is small. Thermal excitation at room temperature elevates some electrons into the conduction band.

21 Semiconductors At T=0K, the world contains only conductors and insulators. Above 0K, electrons at the top of the Fermi sea can be excited to higher energy states if the states are sufficiently ( ~kt ) close. Small bandgap materials are thus semiconductors with marginal electrical conductivity at room temperature due to thermally excited carriers. The conductivity of metals improves at low temperatures. The conductivity of semiconductors declines.

22 Semiconductor Detectors While the photoelectric effect creates free electrons semiconductors provide an analog in the solid state. Photoexcitation across the material's insulating bandgap produces free carries. E cutoff 1.24μ m = E gap (ev ) Resulting carriers produce a change in bulk material resistance (photoconductors) Carriers can also be directly detected as an electrical current in a diode configuration (photovoltaics) Photons can also change the bulk temperature of a small piece of semiconductor changing the electrical resistance (bolometers) Note cutoff is for room temperature. Cutoffs change at cryogenic temperature due to changing lattice spacing (e.g. InSb detectors have a 5.5um cutoff at 77K).

23 What Do You Actually Measure? Photons make electrons, but electronics of some sort must convert that signal into a detectable voltage. Electrons Voltage Analog to Digital Converter (ADC) Digital counts proportional to the voltage For example 1V = counts

24 What Do You Actually Measure? Photons make electrons, but electronics of some sort must convert that signal into a detectable voltage. Electrons Voltage Drive a photon-produced current through a resistor (Ohm's Law). V =I R Collect electrons in a (very small) capacitor. q V = C Analog to Digital Converter (ADC) Digital counts proportional to the voltage

25 What Do You Actually Measure? Photons make electrons, but electronics of some sort must convert that signal into a detectable voltage. Electrons Voltage Drive a photon-produced current through a resistor (Ohm's Law). V =I R Collect electrons in a (very small) capacitor. q V = C Analog to Digital Converter (ADC) Digital counts proportional to the voltage CCD gain: The number of electrons corresponding to one ADC count. V /count = V range 5V = ADC count range x10 19 coulombs / electron V /electron = C readout ( Farads)

26 The Ideal Imaging Device You tell me...

27 An Image

28 Non-Ideal Detector Behavior Much of image calibration involves correction for undesirable detector/array characteristics. Bias: A pattern intrinsic to the array which repeats readout-to-readout. Bias is additive in nature. Flat Field: Non-uniformity of response from detector-to-detector. Since this response is a scale factor the flat field is multiplicative in nature. Related is the quantum efficiency the fraction of incident photons actually detected. Dark Current: Unwanted production of electrons that can be significantly influenced by detector temperature (recall kt excitation). Dark current likely is different in every pixel. Cooling helps. Dark current is additive. Read Noise: Intrinsic random electronic noise that adds in quadrature with Poisson noise total = sqrt(p2 + RN2) Limited well-depth: Too many electrons saturate a pixel. Electronic pickup/pattern: Non-random signal reception from the local environment.

29 Non-Ideal Detector Behavior

30 Read Noise and SNR If source photons are the only source of noise, recall S SNR = S In the presence of background the unwanted background photons add to the Poisson noise. S SNR = S+B Read noise acts as an additional source of unwanted background. Read noise is characterized as the RMS fluctuation (in units of electrons) in the measurement (readout) of a detector. To convert the read noise into the equivalent number of electrons that would cause that noise one has to square read noise. SNR = S = source/star electrons S S + B+ RN B= background electrons 2 RN= read noise in RMS electrons

31 Read Noise and SNR If source photons are the only source of noise, recall SNR = S S In the presence of background the unwanted background photons add to the Poisson noise. SNR = S S+B Read noise acts as an additional source of unwanted background. Read noise is characterized as the RMS fluctuation (in units of electrons) in the measurement (readout) of a detector. To convert the read noise into the equivalent number of electrons that would cause that noise one has to square read noise. SNR = S S + B+ RN 2 Note, importantly, that we're talking about statistics from a counting experiment. The things you are counting are the electrons produced by the detected photons, not the photons themselves and certainly not the counts from the analog-to-digital converter. ADC counts must be turned into the number of electrons via the CCD gain. S = source/star electrons B= background electrons RN= read noise in RMS electrons

32 CCD Architecture Test open shutter closed shutter Note that bad things can happen when buckets overflow (saturation).

33 Indium Bump Bonds