SPACE
TELESCOPE
SCIENCE
INSTITUTE
Operated for NASA by AURA
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 )nh
•
The per pixel quantities we want to measure are the horizontal and vertical charge transfer
efficiencies CTEh and CTEv
Ø
Charge transfer inefficiency CTI=1-CTE
Approximations and assumptions
•
It is typically the case for low frame rate scientific applications that CTEh is better than CTEv
Q ≈ So (CTEv )nv
•
It can often be assumed that ∆Q<<So
CTE ≈ 1−
•
∆Q
nSo
… but not always
Ø
Ø
Large format imager (many megapixels)
Radiation damage
Methods of Measuring CTE
CCD charge transfer is a complex multi-parameter process
•
Clock timing
Ø
Ø
Ø
•
Horizontal pixel transfer time and vertical row transfer time
Clock overlap and edge shaping
Horizontal and vertical voltage rails
Temperature
Ø
Transfer time versus trap emission time constant
E
τe ∝ T −2 exp 
 kT 
•
Background charge
Ø
Ø
Ø
Ø
Ø
Ø
•
Dark current
Spurious charge
Internal scattered light
External background light
Deferred charge from sources in FOV
Intentional fat zero
Capture time constant
Ø
May be important for fast frame rate applications
−1
τc ∝ T 2 τc << τe 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 strengths and weaknesses
of each method
Laboratory methods
•
X-ray stimulation
Ø
Ø
•
Electronic
Ø
•
Fe55, 5.89 keV Kα, So ∼ 1620 electrons
Cd109 , 22.1 keV Kα, So ∼ 6050 electrons
Charge injection
Optical
Ø Extended pixel edge response (EPER)
Ø First pixel response (FPR)
Ø Spot
Methods of Measuring CTE
On-orbit methods
•
Stellar photometry
•
Slitless stellar spectroscopy
•
Galactic cosmic rays (GCRs)/SAA trapped protons
•
X-ray stimulation
Ø
•
Electronic
Ø
•
Astronomical x-ray emitting sources (ASCA, Cas A)
Charge injection (?)
Optical
Ø
Ø
Ø
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 = Kα 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
Ø
•
Half-life limits utility for on-orbit measurement of CTE
Ø
Ø
•
Cd109 is more robust for post-rad CTE
Fe55 τ ≈ 2.73 years
Cd109 τ ≈ 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
Parallel CTI (1-CTE)
1.E-02
1.E-03
1.E-04
1.E-05
1.E+02
1.E+03
1.E+04
Signal Level (electrons)
EPER G=8
Fe55 G=4+G=8
EPER G=4
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
1.E+05
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
Ø
•
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 parallel 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
Parallel CTI (=1-CTE)
1.E-03
1.E-04
1.E-05
1.E-06
1.E-07
1.E+02
1.E+03
1.E+04
Signal Level (electrons)
Pre-rad FPR
Pre-rad Fe55
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
1.E+05
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
Ø
Ø
•
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
Ø
Gearing up for ACS on-orbit implementation
Disadvantages of FPR technique
•
CCD architecture must support half-frame flush
Ø
•
•
•
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