CCD testing

Enver Alagoz

12 April 2010

CCD testing goals

• CCD testing is to learn how to– do dark noise characterization– do gain measurements– determine quantum efficiency

• These experiences will be useful for the LSST wavefront and guider sensor studies at Purdue

CCD testing setup

Apogee CCD camera

• 12 V power• USB control• Upgradeable firmware• 16 bit digitization• TEC cooling (dT = 50 C)• Fans for heat-sink• RTD under chip and on heat-sink• CCD sensor sealed in a inner chamber filled with gas Argon • 25 mm opening window• Mechanical shutter

powerUSB

shutter

• Kodak KAF-0402ME• 768x512 pixels (9x9 μm2)• Saturation: 102 K electrons• At TCCD = -31 °C

• Gain [e-/ADU] = 1.6• Readnoise 10.3 e-’s• Bias: 2633 counts• Dark: 0.03 e-/pixel/sec

• Microlenses to enhance QE at λ < 600 nm

Kodak CCD QE

TCCD = 25 °C

QE measurements of Kodak before Apogee camera assembly

Micro lenses enhancesQE at λ < 600 nmMicro lenses enhancesQE at λ < 600 nm

poly-Si gate

Camera window reflectance• Camera window has 2x fuse silica coated with Broad Band Anti-Reflection (BBAR)

Bias level

Bias and dark measurements

- Bias- Dark+Bias

ADU = Analog digital unit

Dark measurements

Readout noise

Gaussian fit

• Two bias frames with zero time exposure

• Subtract from each other and get the readout noise

• RC = σ/sqrt(2)

• RC = 6.3 ADU

= 10.1 electrons

Light measurements• Tungsten lamp• 100 W power• λ = 580 nm

GainNC (NE) = signal noise [ADU] ([e-])

g = gain [e-/ADU]

SC (SE) = signal [ADU] ([e-])

σE = photon noise

σo,C(σo,E) = flat field noise [ADU] ([e-])

RC(RE) = readout noise [ADU] ([e-])

Slope of SC as a function of N2C is g

Using photon statistics

Variance

• Illumination images A and B • Mean signals: SA and SB

• Ratio: R = SA/SB

• Multiplication: R x image (B)• Subtraction: image (A) – image (B)• Take σ2/2.0 of residual histogram = variance• Plot S versus variance• Repeat for all time exposures

Variance

Gain: variance vs mean signal• Dark frames are subtracted• 100x100 pixels window• Gain = 1/slope

Gain

TCCD = -20 °C

Optical power measurements

Optical power measurements

QE Calculation

I is mean image for a given wavelength [ADU]D is dark count [ADU]G is gain per wavelength [e-/ADU]W is CCD window transmission per wavelength [%]PDpower is the optical power measured by PIN diode [W]Pixarea is the single CCD pixel area [m2]PDarea is the PIN diode area [m2]Texp is the time exposure [sec]Eγ is the single photon energy per wavelength [Joule]

Photons on CCD• Measure power from PIN diode• Normalize power to CCD pixel area• Calculate single photon energy Eγ = ħν where ħ = 6.63x10-34 [J.s] and

ν = c[m/s]/λ[nm] Eγ = 1.989x10-18 / λ [J]

• Calculate number of photons on a CCD pixel Etotal [J] = Pin power [W] x time exposure [s] Etotal/Eγ = # of γ’s per CCD pixel area

QE (per pixel/sec)

Expected measurements from Kodak (TCCD = 25 °C)

Error Source Effect Error in %

Wavelength(λ)

- Monochromator: 1.wavelength settings2.wavelength calibration- Bandwidth is out of range of PIN diode responsivity

Wrong QE<= 0.1

Bias Bias and image data with different clocks Wrong QE

Pixel window Badly chosen pixel window Wrong QE

I(λ) 1. Bad image (e.g. too low signal)2. Contaminated CCD3. Non linear CCD behavior4. CCD is distance from integrating sphere exit

port

Wrong QE

Texp 1. Software latency (15 ms)2. PC clock latency (?)3. Shutter latency&integration time (20 ms)

Wrong QE

PDpower 1. Current measurement error2. Calibration error

Wrong QE5

Window 1. Window transmission error Wrong QE <= 0.6

G Wrong gain Wrong QE

Error budget

Error budget

Next• QE measurements at +25 °C to validate results• Dark and bias measurements -30 °C• Optical light system error budget• Write up measurements and methods for a

LSST document• Plan to use optical setup with WFS testbed• …

BACKUP SLIDES

QEEach blue line represents QE results for a time exposure (0.5-4.5 sec with 0.5 sec steps)

Integrating sphere

Light exit port 4”

Detector port 0.5”

Light input port 1” Integrated sphere 12”

PIN & detector port adaptor

5 PM

1 PM 11 AM

8 AM

Photodiodes

• Two calibrated photodiodes (# 381 and # 384) • 11.28 mm diameter• 1 cm2 active area• Calibration accuracy ?

Wavelength[nm]

Responsivity 381[A/W]

Responsivity 384[A/W]

Difference[%]

550 0.344656562 0.343138049 0.152

560 0.35546587 0.354062382 0.140

570 0.366023475 0.364680454 0.134

580 0.376249638 0.37565275 0.060

Photodiode calibration• Diodes were calibrated before shipped to Purdue (Ref. Newport)

PIN 384

PIN 381

Light power measurements

Dark box

PIN

Sphere exit port

PIN displacements

Light power measurements

Power measured• At 50.8 mm from sphere exit port with: PIN 381 is 394.1 nW PIN 384 is 381.1 nW • At CCD position 508 mm with: PIN 381 is 11.8 nW PIN 384 is 11.5 nW

PIN 381 measured 156.3 nW at sphere detector port (0 mm)

PIN 384 measured 155.4 nW at sphere detector port (0 mm)

Wavelength scan