emccd photometry - rts2

OutlineIntroduction

Characterization

EMCCD Photometry

Alejandro FerreroSchool of Physics

University College [email protected]

13/08/2008

Alejandro Ferrero School of Physics University College Dublin [email protected] Photometry

OutlineIntroduction

Characterization

IntroductionEMCCD vs CCDEMCCD in Photometry

CharacterizationExperimental setupParameters choiceReproducibilityActual GainSaturationLinearitySpectral responsivity and external quantum efficiency


OutlineIntroduction

Characterization

EMCCD vs CCDEMCCD in Photometry

EMCCD vs CCD

◮ EMCCD allows higher temporal resolution than CCD.


OutlineIntroduction

Characterization


EMCCD vs CCD


◮ Readout noise is negligible in EMCCD.


OutlineIntroduction

Characterization


EMCCD vs CCD


◮ Readout noise is negligible in EMCCD.◮ Dynamic range depends on gain. The dynamic range is better

for slow scan. Lower dynamic range than CCD.


OutlineIntroduction

Characterization


EMCCD vs CCD



for slow scan. Lower dynamic range than CCD.◮ A noise factor (

√2) is introduced by the additional

multiplication channel. This factor does not affect readoutnoise, but photon, dark and CIC noises.


OutlineIntroduction

Characterization


EMCCD vs CCD






◮ Not cooled below −95oC , the limiting noise source is readoutdarkcurrent, which is not important in conventional CCDs.Cooled below −95oC , the limiting noise source is CIC.


OutlineIntroduction

Characterization


EMCCD vs CCD






◮ Not cooled below −95oC , the limiting noise source is readoutdarkcurrent, which is not important in conventional CCDs.Cooled below −95oC , the limiting noise source is CIC.

◮ When photon noise limited, SNR in EMCCDs is half that inCCDs, because of the noise factor.


OutlineIntroduction

Characterization


EMCCD in Photometry

◮ Temporal resolution improves by a factor 1/G , if readout timenegligible with respect to the integration time.


OutlineIntroduction

Characterization


EMCCD in Photometry


◮ The advantage disappears when limiting uncertainty is not dueto sensitivity of the detector (readout, dark, charge transfer orspurious charge noises), but to the light level of the source.


OutlineIntroduction

Characterization


EMCCD in Photometry



◮ When source is bright and shot noise higher than the detectornoise; EM unable to overcome the noise and puts anadditional noise in the measurement.


OutlineIntroduction

Characterization


EMCCD in Photometry



◮ When source is bright and shot noise higher than the detectornoise; EM unable to overcome the noise and puts anadditional noise in the measurement.

◮ When the source is faint and shot noise of the backgroundmakes the source undetectable; EM useless, because themultiplication does not distinguish between source andbackground photo-electrons.


OutlineIntroduction

Characterization


EMCCD in Photometry

Figure: Different illumination levels in a source. Some pixels are favouredby EM, whereas the SNR of other pixels may decrease due to the EM.


OutlineIntroduction

Characterization


EMCCD in Photometry

◮ In any case, the SNR of the brighter sources will be alwayshigher than the SNR of the fainter sources.


OutlineIntroduction

Characterization


EMCCD in Photometry

◮ In any case, the SNR of the brighter sources will be alwayshigher than the SNR of the fainter sources.

◮ Therefore, if a chosen gain allows to detect a faint star, itshould be never a problem the detection of brighter sources,although the relative uncertainty will increase a factor

√2 for

these sources.


OutlineIntroduction

Characterization


EMCCD in Photometry

◮ SNR is proportional to√

Gtint .


OutlineIntroduction

Characterization


EMCCD in Photometry


Gtint .

◮ The lower limit of tint depends on the photon flux, but aboveall on the CCD, because the accuracy decreases below 0.1 secof integration time. In addition, below tint << treadout

temporal resolution can not be improved.


OutlineIntroduction

Characterization


EMCCD in Photometry


Gtint .

◮ The lower limit of tint depends on the photon flux, but aboveall on the CCD, because the accuracy decreases below 0.1 secof integration time. In addition, below tint << treadout

temporal resolution can not be improved.

◮ The upper limit of G is imposed by the aging. The higher G ,the more the aging. Very high gains decrease the dynamicrange of the detector. Since the sources are distributed in awide range of irradiance levels on the EMCCD, the largestdynamic range gain should be chosen.


OutlineIntroduction

Characterization


EMCCD noise

σ2(npe) = σ2

r (npe) + npe + nd (1)

σ2(nae) = σ2

r (nae) + 2G (nae + nad) (2)

σ2(N) = σ2

r (N) + 2GKN (3)

GK =σ2(N) − σ2

r (N)

2N(4)


OutlineIntroduction

Characterization


Theoretical effect of EM on a source

Figure: Theoretical effect of EM on a source in the border of thedetection (SNR∼3).


OutlineIntroduction

Characterization


EMCCD vs CCD in Photometry

1

10

100

1000

10000

6 8 10 12 14 16 18 20 22

SN

R

Magnitude

CCDEMCCD (G=3.8)

3

Figure: EMCCD (G=3.8) SNR vs conventional mode CCD SNR.


OutlineIntroduction

Characterization

Experimental setupParameters choiceReproducibilityActual GainSaturationLinearitySpectral responsivity and external quantum efficiency

Experimental setup

Figure: Radiant source for radiometrical calibration.


OutlineIntroduction

Characterization


Andor IXON DU-897 EMCCD: Parameters choice

◮ Integration time > 0.05sec .

◮ Vertical Pixel Shift: Shift Speed 2.2 µs, Vertical ClockVoltage Normal

◮ Horizontal Pixel Shift: Readout rate 1MHz@16bits,Preamplifier Gain × 1.

◮ Temperarature −80oC .


OutlineIntroduction

Characterization


Stabilization

600

700

800

900

1000

1100

1200

1300

1400

1500

1600

0 2 4 6 8 10 12

-100

-80

-60

-40

-20

0

Res

pons

e (c

ount

s)

Tem

pera

ture

(C

elsi

us)

Time (minutes)

G=300, tint=0.3 secTemperature

Figure: The response of the EMCCD is clearly temperature dependentand it is stabilized at 0.16% after around 5 minutes.


OutlineIntroduction

Characterization


Reproducibility

1560

1580

1600

1620

1640

0 1 2 3 4 5 6 7 8

Res

pons

e (c

ount

s)

Time (minutes)

Conventional mode, tint=1 sec, 800 counts subtractedG=300, tint=0.3 sec

Figure: At G=300 and at conventional mode the stability was 0.1% and0.2%, respectively.


OutlineIntroduction

Characterization


Photon Transfer Technique

0

500

1000

1500

2000

2500

3000

3500

0 500 1000 1500 2000 2500 3000

Var

ianc

e (c

ount

s2 )

<N-Nd> (counts)

G=3G=6

G=12G=25G=50

G=100G=200G=300

Figure: Photon transfer technique result.


OutlineIntroduction

Characterization


GK product

0.1

1

10

1 10 100 1000 0.1

1

10

GK

(ph

oto-

elec

tron

s/co

unts

)

Rea

dout

noi

se (

phot

o-el

ectr

ons)

Display Gain

Figure: GK product and readout noise as a function of the softwaredisplayed gain.


OutlineIntroduction

Characterization


Actual Gain

1

10

100

1000

1 10 100 1000

Act

ual G

ain

Display Gain

Photon-transfer with more than 10 levelsPhoton-transfer with only one level

Signal ratios comparison

Figure: Actual gain as a function of the software displayed gain,assuming that they are identical at G=3.


OutlineIntroduction

Characterization


Saturation

0.6

0.8

1

1.2

1.4

100 1000 10000 100000

<N

-Nd>

/tint

(N

orm

aliz

ed)

<N-Nd> (counts)

Conventional modeG=50

G=100G=200G=300

Figure: Saturation of EMCCD at several gains.


OutlineIntroduction

Characterization


Linearity

0.97

0.98

0.99

1

1.01

1.02

0 500 1000 1500 2000 2500 3000

Line

arity

fact

or

<N-Nd> (counts)

G=300, level 1G=300, level 2G=300, level 3G=300, level 4

Conventional mode

Figure: EMCCD linearity.


OutlineIntroduction

Characterization


Spectral responsivity and external quantum efficiency

0

0.05

0.1

0.15

0.2

0.25

300 400 500 600 700 800 900 1000 1100 0

0.2

0.4

0.6

0.8

1

Res

pons

ivity

(co

unts

/pho

ton)

Ext

erna

l qua

ntum

effi

cien

cy

Wavelength (nm)

Figure: Spectral responsivity and external quantum efficiency.


OutlineIntroduction

Characterization


Alejandro Ferrero

School of PhysicsUniversity College [email protected]


emccd photometry - rts2

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