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COROT WEAEKMeudon 13-18 May 2002.

The CCD performances

by Miss Pernelle Bernardi

Mr Tristan Buey

and the CCD team on stage

Régis Schmidt, Bertrand le Ruyet, Jêrome Parrisot, Didier Tiphène, Alain Docclo, JP Pineau, JP Michel, Alain

Piacentino, Francois Rigaud, Gérard Epstein

Funding by CNES.

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Knowledge of the CCD performances by :

• Measurements on all the flight model by Marconi.• Measurements on flight “like” CCD (next month on the flight model!!).

• Sensibility measurements.• Photometric measurements.• Behaviour after proton irradiation.• Numerical models : coupling with the

characteristics of the instrument.

and also deep brain storming, meeting, chatting and relaxation on the beach… bench, sorry!

How we proceed ?

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Format : • 2148*2052 pixels, image and memory zone.• 13.5 m square pixel.

Electro-optical :• Back-thinned and backside illuminated. • “Visible” AR coating.• AIMO mode.• Dump Drain Register.

Timing :• 100kpixel/s nominal, up to 1MHz in windowing mode.• 150s for transfer line.

Typical numbers : • Dark current at -40°C : < 0.5e-/s.• Quantum efficiency : 90% at 650nm.• CTE : 0.99999 (I’m not sure about the last nine!).• Full Well Capacity : 100ke-.• Readout noise : few e- at 100kpixel/s.• PRNU : expressed in % over the bandwidth.• Gain : 3.8e-/V.• Few cosmetic and exotic pixel.

What’s that CCD?

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Measurements over 10 chips.

Most important parameters.Mean Dark current : < 0.5 e-/s at -40°C, goes up to 5.6e-/s!Gain : 3.8+-0.2 V/e-.Readout noise : 4e- (rms) at 100kHz.Full Well Capacity : 85 to 111ke-.Cosmetic : • In dark : about 0 !• Some traps (0 to 6).• Photo-Response : some tens of pixels and 0 to 5

columns.

Parameters with large uncertainties.Quantum efficiency : from 12.5% to 95%. Global PRNU (p-p) : from 22% to 4%.Local PRNU (p-p) : from 8.5% to 3.2%.

The requirements are all reached.Some care about the FWC and quantum efficiency.

They didn’t deliver the best CCD for Corot : PRNU=0, no cosmetic, no noise and also no dark. The best known DEAD CCD!!

Marconi measurements.

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Measurements on FM ”like” and engineering CCD :• Three 4210 CCDs before and after irradiation. • Two 4280.

Different kind of measurements :• Behaviour after proton irradiation : end of life performances.• Temperature sensibility : quantum efficiency, gain, PRNU, Quantum Efficiency.• PRNU and QE as a function of wavelength.• Photometric (optimisation).

Few words for better understanding :• The CCD is divided on 64*64 windows of 32*32 pixels, calculation are performed on theses windows.• Global characteristics are related to the average value of the pixel inside a given windows (mean and standard-deviation of all the windows). • Local characteristics are related to the dispersion of the pixel response inside a given window (mean value of all the windows).

These different parameters are estimated for 80% of the windows.

Meudon (best) measurements.

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Not so easy to measured!The CCD works in AIMO mode but only during integration. During the readout (as a line phase is at high level) and inside the register the CCD works in non-AIMO mode (or normal mode!!).So, long integration time gives lower value than typical Corot readout. For seismology/windowing readout the AIMO mode is present for only 50%…Also at -40°C the dark is so low that the uncertainty can be more than 1000% for an individual measurement on one pixel!So we used :• Large number of image.• Binning factor and long exposure time. • Dark lab and no stray light.

Results.Before irradiation After irradiation

Global < 0.3 e-/s in a few days… < 0.1%

Local < 50%

Dark current.

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Dark current behaviour in space-time!

Synthesis on the dark current .

• It is well known in temperature, both for common pixel and cosmetic.• It’s flat over the CCD surface.• It remains under the specifications (TBC).• Some cosmetics will appear during the life : < 0.1%.

Dark current.

A flat dark current and some spikes randomly distributed.Number of spikes increase with irradiation.

Protons could generate crazy pixels, fortunately not in large number…

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Pixel Random Non Uniformity.

Global PRNU shows spatial uniformity on large scale.

Local PRNU shows spatial un-uniformity on small scale.Is stable with the temperature.

Histogram of the mean pixel value of each windows.

Here 80% of the windows have aPRNU lower than 0.39%

And what about the all 100% windows??

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Pixel Response Non Uniformity.

PRNU depends on the wavelength : some pattern in the blue, fringing in the red and flat response elsewhere.

PRNU could change after irradiation :• Few percent of augmentation on all the CCDs.• Some windows have pixels with lower sensitivity. Less than 1% of the windows.

450nm 700nm

950nm

Ratio of PRNU before andafter irradiation.

Section showing a pixel with a lower sensitivity.

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Quantum efficiency.

Measured over the optical bandwidth.

Not so bad!

Changes with the temperature, depends on the wavelength.

Measured only on two CCDs : 4290 and 4720 (with same technology)…Have to be confirmed on the 4280…

Quantum Efficiency

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20

30

40

50

60

70

80

90

100

300 350 400 500 650 800 900

Wavelength (nm)

% Série2

The CCD gain represents the ratio V/e- and the output stage and is a function of the temperature.

Gain and Full Well Capacity

The FWC gives the maximal number of electrons what can be stored and transferred. Measurements give different values with flat or spot illumination : 15% lower. Oups! Explanation??

Saturation measurement.

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The question is : what’s happened during saturation?

The electrons spread over the columns during saturation.

No anti-blooming.

But when summing the charges over one saturated column we collected all the electrons generated…

Next step : is photometry possible with saturated spot???

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It gives the optimal values for both polarisations and bias clock levels.

We see no change after irradiation and so the CCD will work all their life at the same working point.

Working point.

We prefer to work in the flat part of the curve. And you?

Have to be coupled with electronic performance.

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With the different measurements some numerical models have to be done in order to determine the actual performances of the CCD for the two programs and to be able to compare them for selection :• How many windows are usable for photometry measurement? Effect of PRNU and cosmetic.• What is the sensitivity of photometry measurement versus instrumental fluctuations? • What parameters can be compare directly? Gain, QE…

Performances determination.

Some tools are developed to compare the CCDs.

Left : effect of jitter at each pixel position.Right : PSF convolution with PRNU.

Good correlation!

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Web site : http://pccorot15.obspm.fr/COROT-CAL

See you next time…

Photometry