Performances of the COROT CCDs for high accuracy photometry

Pernelle Bernardi

and the CCD teamFrom Meudon : Tristan Buey, Vincent Lapeyrere, Régis Schmidt, Bertrand le Ruyet,

Jêrome Parisot, Didier Tiphène

From CNES : Olivier Gilard, Guy Rolland (for irradiation tests)

2nd Eddington Worskshop, Mondello (Italy), 9-11 April 2003

2nd Eddington Workshop, Mondello, 9-11 April 2003

2

Corot CCD

• E2V 42-80

• Back-thinned, MPP, anti-reflection coating

• Operating in frame transfer mode

• Size of image area : 2048 rows, 2048 columns

• Pixel pitch: 13.5µm

• 2 outputs, dump drain register

• Readout frequency: 100kHz

• Readout timing: 100µs to transfer a row from the image area to the storage area

150µs to transfer a row in the register

• Three sides buttable

No specific technological developments

Space qualification

2nd Eddington Workshop, Mondello, 9-11 April 2003

3

Geometrical requirements and packaging

• Flatness of image section: 3µm

• Top of view:

14mm 15µm (spec)

10 µm (meas)e(Invar) = 10 mm

e(silicon) ~ 625 µm

e(thinned silicon) ~ 15 µm

3 shims

silicon

invar

thinned silicon

50µm min (spec)

50µm min (spec)

500µm max

300µm max

With this packaging: distance between image areas of 2 CCDs > 1mm

2nd Eddington Workshop, Mondello, 9-11 April 2003

4

Working point

• 3 bias voltages are optimised: VOD, VRD, VOG

• Working range: measurement of the video signal for different values of the bias voltages

5 CCDs have the same working ranges for the 3 bias voltages

1 has a different behaviour: excluded from the flight

Video signal vs Vod

20000

30000

40000

50000

60000

70000

80000

90000

100000

1800 2300 2800 3300

Vod (ADU)

Left output

Right output

• Sensitivity of the video signal to the bias voltages :

About same order at worst frequency (100kHz, readout frequency). Higher sensitivities = 4e-/mV Specification for the electronics: 1mV peak to peak ---> equivalent noise of few e-.

2nd Eddington Workshop, Mondello, 9-11 April 2003

5

CCD gain (µV/e-)

• Specification: 3.8 < GCCD < 6.0 µV/e-

• CCD gain is measured independently of Qe

• Good homogeneity of 5 CCDs

• Global gain (e-/ADU) measured at different temperatures from –45°C to 20°C

• Temperature coefficient of the gain:

~ -900ppm/K

CCD Gain at –40°CTemp coefficient of

the CCD gain (ppm/K)Left output Right output

76 3.97 4.15 -900

56 4.31 4.28 -1090

71 4.06 3.99 -780

55 4.27 4.16 -870

60 4.18 4.06 -820

CCD Gain (µV/e-)

3,80

3,85

3,90

3,95

4,00

4,05

4,10

4,15

4,20

4,25

4,30

-45 -35 -25 -15 -5 5 15 25T (°C)

2nd Eddington Workshop, Mondello, 9-11 April 2003

6

Quantum Efficiency

• E2V data: Qe max = 0.90 at 650nm

average Qe = 0.62

• Temperature coefficient of Qe:

– Measure of the flux at:

- different wavelengths from 400nm to 1000nm

- different temperatures from –45°C to –30°C

– Calculation of the temperature coefficient of the CCD response

– Correction by the temperature coefficient of the CCD gain (G ~ -900ppm/K)

Temperature coefficient of Qe ~ 2000ppm/K for the center of the bandwith

Quantum Efficiency

0

10

20

30

40

50

60

70

80

90

100

200 300 400 500 600 700 800 900 1000

Wavelength (nm)

Qe

(%)

Temperature coefficient of Quantum Efficiency

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

400 500 600 700 800 900 1000

Wavelength (nm)

DQ

e/Q

e (p

pm

/K)

2nd Eddington Workshop, Mondello, 9-11 April 2003

7

Dark current

CCD is divided into windows of 32*32 pixels

For 5 CCDs: at –40°C

– Mean dark current value < 0.5e-/px/s

– 90% of the windows < 0.5e-/px/s

Map of dark current

0.4280

0.33506

0.24531

0.15557

0.06583

Mean dark current = 0.18e-/px/s

2nd Eddington Workshop, Mondello, 9-11 April 2003

8

Pixel Response Non Uniformity (1/2)

= 420nm, D = 10nm

= 700nm, D = 10nm

= 900nm, D = 10nm

Achieved with flat illumination

Shows the uniformityof the state of surface and AR coating Surface Pattern

Shows the uniformity of the physical characteristics High homogeneity

Shows the uniformity of the thickness Fringing Pattern

= 900nm 10nm

= 900nm 100nm

2nd Eddington Workshop, Mondello, 9-11 April 2003

9

Pixel Response Non Uniformity (2/2)

• CCD is divided into windows of 32*32 pixels.

• Local PRNU = dispersion of the pixel response in a 32*32 pixel window.

• For 5 CCDs: local PRNU is 5 times lower than specification.

• Stable with temperature

Local PRNU

0,00

0,50

1,00

1,50

2,00

2,50

450 550 650 750 850Wavelength (nm )

76

56

71

55

60

2nd Eddington Workshop, Mondello, 9-11 April 2003

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Cosmetics

• Definitions:– White spot: > 100e-/px/s at –40°C– Black spot: when pixel response is less than 50% of the local mean response, at -40°C.– Column defect: more than 100 contiguous blemish elements (white and black spots)

• Specifications:– Less than 750 black and white spots– Less than 6 column defects

• Measurements: specifications are fullfilled– 5 CCDs have no white spots, 1 CCD has 2 white spots– The worst CCD has 2 columns defects (black columns)

2nd Eddington Workshop, Mondello, 9-11 April 2003

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Maps of defects

• 2 maps calculated per CCD: one for each scientific program.– Images of dark current and flat fields in three parts of the bandwith– 2 shapes of PSF (seismo and exoplanets)– Performances of the instrument for each program

Seismology channel Exoplanet channel

Same CCD

2nd Eddington Workshop, Mondello, 9-11 April 2003

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Results of the irradiation tests

• 3 chips tested (4210)

• Tested characteristics :– Working point.– Gain and Full Well Capacity.– Dark Current (mean value, defects).– Pixel Response Non Uniformity.

Irradiation induces local defects (spatial and temporal) in the dark current

Global evolution

Beginning of life: < 0.5e-/px/s

End of life: 3 to 10e-/px/s

Rapid evolution of the dark current with temperature gives strong constraint on the CCD operational temperature below -40°C.

Appart from the dark current all the other characteristics of the CCD will not be degraded enough to impact on the system performances.

2nd Eddington Workshop, Mondello, 9-11 April 2003

13

Conclusion

• Sixth CCD currently under tests

• Calibration of 10 CCDs FM will be finished before summer

• Then: sort the 10 CCDs

select 4 CCDs for the flight using software tools

• If necessary, some specific measurements can be done on selected CCDs