Noisein&& Detectors - ANUamedling/obstech/obstech_9_detector...Noisein&yourdata& •...
Transcript of Noisein&& Detectors - ANUamedling/obstech/obstech_9_detector...Noisein&yourdata& •...
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Noise in Detectors 8 March 2016
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Remember: • Noise in your data propagates to any measurements you make from it
• Most astrophysical data have noise following a Gaussian distribu@on, so errors can be added in quadrature • σtot2 = σ12 + σ22
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Noise in your data • You should be able to calculate your noise theore&cally AND empirically
• Empirical noise es&mates look at your data and es@mate noise directly from that
• Theore&cal noise es&mates use proper@es of the detector and your data (measured either by you or by the instrument team) • First we have to talk about how detectors work
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CCDs: Charge-‐coupled devices WiFeS Fairchild 4kx4k detector
• Op@cal detectors • Silicon-‐based wafers with pixel paRerns etched in • MOS = Metal-‐oxide-‐semiconductor layers
• Photons hit a pixel’s surface, and the photoelectric effect causes an electron to be released into the poten@al well
Image: M
icroscop
yu.com
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Not all photons create an e-‐
• A detector’s quantum efficiency (QE) describes how efficient it is at collec@ng photons • # read out electrons / # incident photons
• Factors causing QE to take a hit: • Reflec@on off the surface of the pixel • Absorp@on in intermediate layers of detector • “Fill-‐factor”, where part of the surface blocks photons • Internal losses during readout
• QE is wavelength dependent!
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Not all photons create an e-‐
Image: M
IT/Lincoln Labs
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CCD Readout • To shid pixels to the edge to be read out, voltage gates shuffle the electrons along
• Read one row, then move all rows down “bucket brigade” style
Image: J. Ja
nesic
k, Pixelvisio
n, Inc
Image: Schmid, Wikipedia
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CCD Readout • To shid pixels to the edge to be read out, voltage gates shuffle the electrons along
• Read one row, then move all rows down “bucket brigade” style
Image: C. Tremon
@
Image: Schmid, Wikipedia
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CCD Readout • To shid pixels to the edge to be read out, voltage gates shuffle the electrons along
• Read one row, then move all rows down “bucket brigade” style
Image: C. Tremon
@
Image: Schmid, Wikipedia
Reading out your chip can take a long &me!
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CCD Readout • Final stored value is in analog-‐to-‐digital-‐units “ADU” (or digital numbers “DN”) with a typical conversion (“GAIN”) of 2-‐8 e-‐/ADU
• “Readnoise” = noise introduced by the reading out of your detector • How well does your reported signal reflect the # of electrons in the well? Image: C. Tremon
@
Image: Schmid, Wikipedia
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In pairs… • If the noise in your image in photo-‐electrons is σe-‐ , what is σADU, the noise in your image now that it is in digital units?
• Put your answer in terms of the gain, G
• (3 minutes)
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Orthogonal Transfer CCDs • Tonry et al. 1997, PASP, 109, 1154 • Can shid horizontally and ver@cally at the same @me
• Allows on-‐chip image mo@on correc@on • Charge shuffle science image in X and Y during exposure
• Requires high charge transfer efficiency (CTE)
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On-‐chip binning • Some@mes it may be worth sacrificing spa@al resolu@on in order to save on readnoise
• With on-‐chip binning, you tell the detector to combine adjacent pixels and then only read out the sum of them
• This can be worthwhile e.g. when the seeing is so bad that you wouldn’t have the spa@al resolu@on anyway
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Nod-‐and-‐ShufAle
1. Object spectra are recorded in alterna@ng strips
2. Then the shuRer is closed and the spectra are shuffled to the adjacent (previously empty) strip
3. The telescope moves (“nods”) to a sky region
4. Sky spectra are recorded in the original (now empty) strips
5. Shuffle spectra back around on chip and repeat
This spectroscopy technique uses half the detector to record photons and the other half as storage
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Nod-‐and-‐ShufAle
1. Object spectra are recorded in alterna@ng strips
2. Then the shuRer is closed and the spectra are shuffled to the adjacent (previously empty) strip
3. The telescope moves (“nods”) to a sky region
4. Sky spectra are recorded in the original (now empty) strips
5. Shuffle spectra back around on chip and repeat
This spectroscopy technique uses half the detector to record photons and the other half as storage
This is great for sky subtrac&on but can kill you on readnoise!
Why?
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IR Detectors – not CCDs! • Hybrid devices
• Silicon CCD mul@plexers • Indium bonds to detector array (HgCdTe or InSb)
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IR Detectors – not CCDs! • Hybrid devices
• Silicon CCD mul@plexers • Indium bonds to detector array (HgCdTe or InSb)
Unlike CCDs, IR detectors can be read out non-‐destruc&vely!
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Non-‐destructive readout • Two main kinds of readout
• Fowler Sampling: Sample the counts a few @mes each at the beginning and end of integra@on
• Linear fisng: Least-‐squares fit to slope of integra@on ramp (up-‐the-‐ramp sampling)
• Slope = photon rate in both cases
Fowler Sampling Linear Fitting
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Non-‐destructive readout • Two main kinds of readout
• Fowler Sampling: Sample the counts a few @mes each at the beginning and end of integra@on
• Linear fisng: Least-‐squares fit to slope of integra@on ramp (up-‐the-‐ramp sampling)
• Slope = photon rate in both cases For long exposures, Fowler sampling can incur less readnoise because fewer samples are necessary.
Fowler Sampling Linear Fitting
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Chandra ACIS X-‐ray detector • Configura@on of 10 CCDs • Unlike op@cal CCDs, one X-‐ray photon releases many electrons (~1000)
• Ader every (short) exposure, on-‐board sodware catalogs x,y posi@on of photon and its energy
Image: Chandra/Harvard/SAO
But, if more than one photon hits a pixel during that exposure, can cause pileup -‐> fortunately, uncommon for X-‐ray astronomy!
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Theoretical Noise Sources • Poisson noise from your object = sqrt(N) • Poisson noise from the subtracted sky = sqrt(Nsky) • Noise from reading out your detector
• Given in electrons, usually well-‐characterized • Dark current
• Thermal noise – probability of electrons jumping the bandgap without a photon; is a func@on of @me (and generally temperature!)
• Rarely important in the op@cal, but a bigger deal in the infrared
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Expected Signal
Incoming Count Rate
(photons per
second)
Total incoming energy rate at your detector
Σ = (Fλ*Area*bandwidth*transmission)/(h ν)
Energy per photon
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Expected Noise Terms
• Poisson noise from your object
• Poisson noise from the sky
• Readnoise from reading out your detector • Can include “digi@za@on noise”
• Dark Current
Σ = photon count rate = (Fλ*Area*bandwidth*transmission)/(h ν)
Sqrt(Σobj * texp )
Sqrt(Σsky * texp )
RN * Sqrt(npix)
Sqrt(Σdark * npix * texp )
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Noise terms are independent, add in quadrature
Σobj*texp Signal to
Noise Ra&o
√ (Σobj*texp) + (Σsky*texp) + (Σdark*texp*npix) + npix*RN2
Source Noise Sky Noise Dark Current Readnoise
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Noise terms are independent, add in quadrature
Σobj*texp Signal to
Noise Ra&o
√ (Σobj*texp) + (Σsky*texp) + (Σdark*texp*npix) + npix*RN2
Source Noise Sky Noise Dark Current Readnoise
For different regimes, different noise terms dominate