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Basic Detection
Techniques
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Detector
• More than ‘sensing
device’
• Measuring
– ‘Meten is weten’
• Meta information
• Counting vs analog
rms 
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Poisson
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Gaussian
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Accuracy
• Distribution: stochastic measurement
process only
• ==Precision
• Accuracy -> no systematic
– Hubble
– Target shooting
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Statistics
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Mean
Variance
Chi-squared
Median
1
mean  x  n xi
n
2
1
2
var    n xi  x (sample var)
n
chi  squared   2  n xi2

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Moments
Central moments


 k   x  x f x dx
k

0  1
1  0
2   2
3  0 ?
• skewness
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Systematic errors
• Instrument environment widest sense
– Coal
– Parallax
– Gaia
• Gal rotation
• Pressure
• Model -> none
• Outliers
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Modelling
• Solve
• L2 (least-squares)
• L1 (outliers)
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(In)direct
• Direct
– Raindrops
– Planet directly
• Indirect
– Crop size
– ‘systematic’ movement of Centre of G.
• ‘Test particle’
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Measurables
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EM waves
Neutrinos
Matter (nuclei -> meteorites & space craft)
Gravitational waves (<=c)
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Neutrinos
Weak interaction:
electron
neutrinos
n  p  e  e

p  n  e  e

Strong interaction
Tau & muon
neutrinos
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Neutrinos 2
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Long pathlength -> memory
1931: Pauli – 1959: e – 1962: new muon
Indirect
Icecube
Ocean
Moon
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Neutrino 3
• Solar problem
• 1987 SN -> 19 neutrinos (water, proton
decay)
• 50000 tons; 11000 PMT (50cm)
• Mass < 2.2eV
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Cherenkov
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GW
• 10-38 weaker than EM
force
• Transparant universe
• Tensor (cf vector and
potential)
• Helicity +-2 (+-1)
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GW 2
•
Direct resonant
–
–
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Direct non-resident
–
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Block > 1 ton Al; eigen freq. 1.5Hz
Coincident
Michelson between 2 blocks (multiple reflections)
Interferometer
LISA, in 2015 5Gm long 3.
Indirect: (but questioned again)
–
–
–
dP/dt decay in binary pulsar.
Calculated: -2.403(0.002) 10-12 ss-1
Observed
-2.4 (0.09) 10-12 ss-1
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Matter
• Cosmic Rays (later lecture)
– Pierre Auger (AR) + Northern
– LOFAR
• Meteorites -> history
• Returning spacecraft
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EM radiation
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Energy == wavelength == frequency
Flux
Time variation
Spatial dependence
Polarisation:
– Only ‘directional’ measurement (magnetic field)
• Resolution in all:
– Uncertainty
– ‘aperture’
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EM radiation
I 
Q 
   f (t , , l , m,  )
U 
 
V 
•Not all simultaneous -- choose
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Spectrum
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•
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21 cm = 1420 MHz [Hyperfine line, HI]
1 cm = 30 GHz
1 mm = 300 GHz = 1000μm
1 μm = 1000 nm
550 nm = 5.5 × 1014 Hz [V band centre]
1 eV = 1.60 × 10−12 erg = 1240 nm
13.6 eV = 91.2 nm [Lyman limit = IP of HI]
1 keV = 1.24 nm = 2.4 × 1017 Hz
1 PHz = 1015 Hz (petahertz)
mec2 = 511 keV
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Sensitivity
Faintest UVOIR point source detected:
• Naked eye: 5-6 mag
• Galileo telescope (1610): 8-9 mag
• Palomar 5-m (1948): 21-22 mag (pg),
• 25-26 mag (CCD)
• Keck 10-m (1992): 27-28 mag
• HST (2.4-m in space, 1990): 29-30 mag
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Measure
Flux is the energy incident per unit time per unit area
within a defined EM band:
f ≡ Ein band/A t
(or power per unit area)
Usually quoted at top of Earth’s atmosphere
o “Bolometric”: all frequencies
o Finite bands (typically 1-20%) defined by, e.g., filters such
as U,B,V,K
o “Monochromatic”: infinitesimal band, ν → ν + dν
Also called “spectral flux density”
Denoted: fν or fλ
Note conversion: since fνdν = fλdλ and ν = c/λ,
→ νfν = λfλ
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Flux 2
1 Jy = 10−26 W m−2 Hz−1
[= 10−23 erg s−1 cm−2 Hz−1] non SI
Monochromatic Apparent Magnitudes
o mλ ≡ −2.5 log10 fλ − 21.1,
where fλ is in units of erg s−1 cm−2 A−1
o Normalization is chosen to coincide with the zero point of the widelyused “visual” or standard “broad-band” V magnitude system:
i.e. mλ(5500 ˚A) = V
o Zero Point: fluxes at 5500 ˚A corresponding to mλ(5500˚A) = 0, are
(Bessell 1998)
f0 ν = 3630 Jy (janskys) or 3.63 × 10−20 erg s−1 cm−2 Hz−1
λ/hν = 1005 photons cm−2 s−1 A−1 is the corresponding photon rate per
unit wavelength
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Flux 3
• Absolute Magnitudes
o M ≡ m− 5 log10(D/10), where D is the distance to the source in
parsec
o M is the apparent magnitude the source would have if it were
placed at a distance of 10 pc.
o M is an intrinsic property of a source
o For the Sun, MV = 4.83
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Flux 4
• Luminosity L (W)
– Power (energy/s) radiated by source into
4π sterad
• Flux (W m-2)
– f = L/4πD2 if source isotropic, no
absorption
• Brightness I (W m-2 sr-1)
– f ~ IΔΩ
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Planck
2h 3
1
B  , T   2 h / kT
c e
1
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Planck 2
• Limiting forms:
• hν/kT << 1 → Bν(T) =
2kT /λ2 (“RayleighJeans”)
• hν/kT >> 1 → Bν(T) =
2hν3 e−hν/kT /c2
(“Wien”)

• Non-thermal
B 
– T > 1020
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Stars
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IR windows
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Atmosphere transmission
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QE
Eye
10-20%
Photographic
2-10%
CCD
70-90%
PMT
20-30%
IR (HgCdTe)
30-50%
CMOS
60-80%
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QE(2)
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Spectrum
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Detectors
Bolometers
• Most basic detector type: a simple absorber
• Temperature responds to total EM energy
deposited by all mechanisms during thermal
time-scale
• Electrical properties change with temperature
• Broad-band (unselective); slow response
• Primarily far infrared, sub-millimetre (but also
high energy thermal pulse detectors)
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Bolometer
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Detectors 2
Coherent Detectors
Multiparticle detection of electric field amplitude of incident
EM wave
• Phase information preserved
• Frequency band generally narrow but tuneable
• Heterodyne technique mixes incident wave with local
oscillator
• Response proportional to instantaneous power collected in
band
• Primarily radio, millimetre wave, but some IR systems with
laser LOs
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Detectors 3
Photon Detectors
• Respond to individual photon interaction with
electron(s)
• Phase not preserved
• Broad-band above threshold frequency
• Instantaneous response proportional to
collected photon rate (not energy deposition)
• Many devices are integrating (store
photoelectrons prior to readout stage)
•
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Detector 4
UVOIR, X-ray, Gamma-ray
o Photo excitation devices: photon absorption changes
distribution of electrons over states. E.g.: CCDs,
photography
o Photoemission devices: photon absorption causes
ejection of photoelectron. E.g.: photocathodes and
dynodes in photomultiplier tubes.
o High energy cascade devices: X- or gamma-ray
ionization, Compton scattering, pair-production
produces multiparticle pulse. E.g. gas proportional
counters, scintillators
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Detector 5
• Chemical detectors
• Eye
• Photographic plate
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Eye
• Rods (10-20%)
• Cones (1-2%) – 3 varieties
• 1ps response; 1/20s integration; 15min to
revitalise
• Flashes
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Photographic
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- non-linear
- low dynamic range
+ # pixels
Photon excites e AgCl -> +Ag- into
Ag.(defect)
• Developing == amplification
• Slow (but stroboscopic)
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PMT
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PMT-a
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PMT2
• QE 5-10%
• UV/B poor in R/IR
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MCP
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MCP2
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QE 20%
Can be staggered (chevron)
Up to million amplification
1-1000nm
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IPCS
• TV: photo electron (from Si) stored in
micro-capacitors
• Scanned/recharged 25Hz -> discharge
current
• High readout noise (snow)
• 1st intensifier 3 stage million gain
• Read out == photon counting digital
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Image intensifier
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CCD
• Charge Coupled
Device
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CCD layout
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CCD transfer
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CCD readout
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CCD
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Workhorse up to 1.1 um -> bandgap
Dynamic range: bits; 30000:1
Linearity: same
Read-out noise 2-3 eDark current (thermal) -> cool
Shot noise: random photons
Non-uniformity -> flat fielding
Charge transfer efficiency (>.99999 has to be)
Cosmic rays: pixel error
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CCD2
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Large: 10.5 * 10.5 kpixel
4 stitched -> 500 million pixels
Thinned back-illuminated: no reflection
Thinned very expensive: fragile, but
efficient
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CCD perfect?
Cosmic
rays
Hot Spots
(high dark
current,
but
sometimes
LEDs!)
Bright
Column
(charge traps)
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Columns
(charge
traps)
QE
variations
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CMOS
• Complementary Metal Oxide Silicon
• Direct readout
• But: 15-30 photomasks; rather than 10 for
CCD
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CMOS 2
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NIR (hybrid)
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NIR
• Similar to CCD
• Non-Si layer to generate photo electrons:
HgCdTe and InSb for between 0.9 and 25 um
• Hybrid Si system: well developed
• Cooled to 30-60K
• Si part: CCD or MOS capacitors: direct read-out
•Pixel cost 10* CCD (0.10-0.30 USD)
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SIS – BIB - SSPM
• Superconductor-Insulator-Superconductor
tunnel junctions
• Blocked-Impurity-Band detectors
• Solid-State-PhotoMultipliers
• Josephson junctions
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Energy resolving STJ/TES
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STJ
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Fast
Spectral resolution 1000
UV->IR
Cooled < 1K
Magnetic field + Electric field
1 meV electron pair split (1eV for CCD!)
More depending on energy
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Tip-tilt CCD wavefront
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Info
• C.R. Kitchin, Astrophysical Techniques
(0 7503 0946 6)
• http://www.ctio.noao.edu/mailman/listinfo/ccdworld
• Real life CCD:
http://imaging.e2vtechnologies.com
• Experimental Astronomy 2006
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