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SPACE TELESCOPE SCIENCE INSTITUTE

STScI CTE Workshop 31 January-1 February 2000 Methods of Measuring CTE Michael R. Jones


Methods of Measuring CTE
Statement of the problem
·

For an initial signal So in a given pixel, the amount of charge Q missing after nv vertical transfers and nh horizontal transfers is
Q = So (CTEv )nv (CTEh )
n
h

·

The per pixel quantities we want to measure are the horizontal and vertical charge transfer efficiencies CTEh and CTEv
ü

Charge transfer inefficiency CTI=1 -CTE

Approximations and assumptions
·

It is typically the case for low frame rate scientific applicati ons that CTEh is better than CTEv
Q So (CTEv )
n
v

·

It can often be assumed that Q< CTE 1- Q nSo

·

... but not always
ü ü

Large format imager (many megapixels) Radiation damage


Methods of Measuring CTE
CCD charge transfer is a complex multi -parameter process
·

Clock timing
ü ü ü

Horizontal pixel transfer time and vertical row transfer time Clock overlap and edge shaping Horizontal and vertical voltage rails

·

Temperature
ü

Transfer time versus trap emission time constant
E e T -2 exp kT

·

Background charge
ü ü ü ü ü ü

Dark current Spurious charge Internal scattered light External background light Deferred charge from sources in FOV Intentional fat zero

·

Capture time constant
ü

May be important for fast frame rate applications -1 c T 2 c << e for low T operation


Methods of Measuring CTE
The measured value of CTE depends on the method
· ·

Two different measured values of CTE for same So may both be valid Applicability of measured CTE for a given scene or mode of operation depends on measurement method Interpretation of data requires a clear understanding of the str engths and weaknesses of each method

·

Laboratory methods
·

X-ray stimulation
ü ü

Fe55, 5.89 keV K, So 1620 electrons Cd109 , 22.1 keV K, So 6050 electrons

·

Electronic
ü

Charge injection

·

Optical ü Extended pixel edge response (EPER) ü First pixel response (FPR) ü Spot


Methods of Measuring CTE
On-orbit methods
·

Stellar photometry Slitless stellar spectroscopy Galactic cosmic rays (GCRs)/SAA trapped protons X-ray stimulation
ü

·

·

·

Astronomical x -ray emitting sources (ASCA, Cas A)

·

Electronic
ü

Charge injection (?)

·

Optical
ü ü ü

Extended pixel edge response (EPER) First pixel response (FPR) (?) Slit image centroid shift


Methods of Measuring CTE
Example parallel Fe55 stacking plots

CRAF/Cassini CCD (Loral-Fairchild) 1024 row x 1024 column format Left panel-unirradiated Right panel-proton irradiated Pre-rad per pixel parallel CTE=0.99999 Reference: The Effects of Proton Damage on Charge-Coupled Devices, James Janesick, George Soli, Tom Elliott, Stewart Collins, SPIE Electronic Imaging Science and Technology Conference, Volume 1147, 1991.


Methods of Measuring CTE
Advantages of x-ray stimulation technique
· ·

Simple test to perform Provides a measurement of system gain
G= So electrons/DN Io = K y - intercept in DN Io

· ·

Repeatable Widely cited in the literature

Disadvantages of x -ray stimulation technique
· · ·

Measures CTE at only one signal level Overexposure can bias results Fe55 CTE is difficult to measure post -rad
ü

Cd109 is more robust for post -rad CTE

·

Half-life limits utility for on-orbit measurement of CTE
ü ü

Fe55 2.73 years Cd109 461 days

·

Health and safety issues


Methods of Measuring CTE
Example EPER tail

Proton irradiated ACS WFC Lot 5 CCD -2.5E+09 protons/cm2 @ 63 MeV So=27501 electrons Q=1910 electrons nv=2048 nh=4096 + 100 leading and trailing serial overscan pixels Parallel transfer time=1408 µsec/row excluding serial line read Serial transfer time=22 µsec/pixel Per pixel parallel CTE=0.9999649


Methods of Measuring CTE
Example EPER parallel CTI versus signal curve
7086MDR15-01 Post-Rad
1.E-02

Parallel CTI (1-CTE)

1.E-03

1.E-04

1.E-05 1.E+02 1.E+03 Signal Level (electrons) EPER G=8 Fe55 G=4+G=8 EPER G=4 1.E+04 1.E+05

Proton irradiated ACS WFC Lot 5 CCD -2.5E+09 protons/cm2 @ 63 MeV 2048 row x 4096 column format Serial line read includes 100 leading and trailing serial overscan pixels Parallel transfer time=1408 µsec/row excluding serial line read Serial transfer time=22 µsec/pixel


Methods of Measuring CTE
Advantages of EPER technique
· · · ·

Simple test to perform Signal resolved Robust post-rad measurement Well suited for on-orbit measurement of CTE
ü

Can be obtained by overclocking calibration flats

·

Trap emission time constant can, in principle, be measured

Disadvantages of EPER technique
· · · ·

So can sometimes be difficult to establish Q is difficult to measure when CTE is very good Does not always correlate well with Fe 55 Not as widely cited in the literature as Fe 55


Methods of Measuring CTE
Flowchart of FPR clocking sequence

Reference: Justification and Requirements for On-Board ACS FPR/EPER CTE Calibration, Michael R. Jones, Mark Clampin, Gerhardt Meurer and Ronald Schrein, ACS Technical Instrument Report 99 -03, November 1999.


Methods of Measuring CTE
Example generic clock pattern for parallel FPR half-frame flush

ACS HRC CCD detector Flush of upper half of image area through AB serial register Parallel clock phases 1 (P1U/P1L) and 2 (P2U/P2L) are split on the chip Parallel clock phase 3 (P3U/P3L) is common for upper and lower p arallel halves Serial clock phases 1 (S1A/S1B) and 3 (S3A/S3B) are split on the chip Serial clock phase 2 (S2AB) is common for left and right serial halves Simplified functional diagram of HRC detector can be found in ACS Technical Information Report 99-03


Methods of Measuring CTE
Example of FPR leading edge charge loss

Proton irradiated STIS spare CCD from flight lot 2.27E+09 protons/cm2 @ 63 MeV So=4434 electrons Q=172 electrons nv=512 nh=1024 + 95 leading and trailing serial overscan pixels Parallel transfer time=640 µsec/row excluding serial line read Serial transfer time=22 µsec/pixel Per pixel parallel CTE=0.9999227


Methods of Measuring CTE
Example FPR parallel CTI versus signal curve
1794BU11-04 Parallel CTI @ -83 C 1.E-03

Parallel CTI (=1-CTE)

1.E-04

1.E-05

1.E-06

1.E-07 1.E+02

1.E+03 Signal Level (electrons) Pre-rad FPR Pre-rad Fe55

1.E+04

1.E+05

Post-rad FPR

Post-rad Fe55

Proton irradiated STIS spare CCD from flight lot 2.27E+09 protons/cm2 @ 63 MeV 1024 row x 1024 column format Serial line read includes 95 leading and trailing serial overscan pixels Parallel transfer time=640 µsec/row excluding serial line read Serial transfer time=22 µsec/pixel


Methods of Measuring CTE
Advantages of FPR technique
· · · ·

Simple measurement to make Generally correlates well with Fe 55 Robust post-rad measurement Test procedure can be modified for non-zero background
ü ü

Wait state between flush and read out to simulate accumulated dark current Post-flush, pre -read out flat exposure to simulate fat zero

·

Might be suitable for on-orbit measurement of CTE
ü

Gearing up for ACS on-orbit implementation

Disadvantages of FPR technique
·

CCD architecture must support half -frame flush
ü

Electronic knife edge could possibly be created by charge injection

· · ·

Half-frame flush timing patterns not published Half the number of transfers as EPER, smaller Q Not widely cited in the literature