Transcript Optimisation of an EMCCD - Reduction of parallel clock induced charge (CIC). - Investigation of dark current and the effect of “Dither”. -

Optimisation of an EMCCD
- Reduction of parallel clock induced charge (CIC).
- Investigation of dark current and the effect of “Dither”.
- Measurement and reduction of serial register CIC.
- Some astronomical results.
-(Application of Dither to a large format CCD).
SDW Munich 2009
www.qucam.com
Cuts through EMCCD bias frames
Optimisation process
Clock induced charge the dominant noise source.
SDW Munich 2009
www.qucam.com
EMCCD primer
FH3
FH2HV
FH2DC
FH1
FH3
FH2HV
FH2DC
EM part of register
FH1
FH3
FH2
FH1
FH3
FH2
FH1
Conventional part of register
t1
t2
t3
SDW Munich 2009
www.qucam.com
EMCCD primer
1056 columns
CIC produced in all
sections of the CCD.
Image Area.
Store Area.
16 elements
604 elements
EM Amp.
EM register
16 elements
468 elements
Link section
Serial register
Conventional Amp.
E2V CCD201
SDW Munich 2009
www.qucam.com
Multiplication noise
Output of EM register in response to inputs between 1 and 5 e-.
Note that an output signal of 400 ecould result from an input of either
4 or 5 e- with almost equal probability
-> “Multiplication noise”
SDW Munich 2009
www.qucam.com
Multiplication noise: Monte Carlo model
Passage of 10 seperate photo-electrons
are followed through the EM register.
Overall EM gain=1000
SDW Munich 2009
www.qucam.com
Inverted Mode
SDW Munich 2009
www.qucam.com
Inverted mode operation reduces dark current
E2V CCD201, T=293K
Holes are attracted from the channel stops.
These then populate the surface of the
CCD mopping up surface dark current.
SDW Munich 2009
www.qucam.com
Inverted mode operation increases CIC
-8V
Electrode
n
P+
P+
Channel
stop
Pixel charge packet
p
+4V
n
P+
P+
p
Electron produced
by impact ionisation
SDW Munich 2009
SiO2
During integration
surface populated
with holes that
suppress surface
dark current.
During charge
transfer when
the pixel comes
out of inversion
the holes produce
clock induced
charge.
www.qucam.com
Measuring parallel CIC in an FT CCD
Integration
Transfer
Readout
Image
Store
1037 rows
1032 rows
CCD201
Next frame
integrating
First row of parallel
overscan will contain
only 1038 rows of CIC
Last row of image
will contain 2069
rows of CIC.
So the parallel CIC should show a step in vertical cuts through bias frames
that include a parallel overscan area.
SDW Munich 2009
www.qucam.com
Non-inverted mode reduces parallel CIC
Inverted operation
Non-inverted operation
CIC
from 1032
row transfers
Cuts through bias images that contain a parallel overscan.
SDW Munich 2009
www.qucam.com
Summary
Inverted Mode
Non Inverted Mode
Low Dark current
High Dark current
Huge CIC
Low CIC
SDW Munich 2009
www.qucam.com
But…..dark current non-linear with time!
CCD201 data
Non-inverted dark current suddenly
drops by a factor of almost 100!
CCD201 data
SDW Munich 2009
www.qucam.com
Non-inverted dark current versus exposure time
CCD201 data
SDW Munich 2009
www.qucam.com
Non-Inverted mode conclusions
Non inverted mode required for low parallel CIC.
For short exposures the corresponding increase in dark current is not seen.
Non-inverted mode operation preferred for EMCCDs
-8V
Electrode
n
p
P+
P+
Pixel charge packet
SiO2
Channel
stop
The suppression of dark current could be explained by `Dither` (Jorden et al.
`Secrets of E2V Technologies CCDs` SDW 2004). However, this explanation
requires the presence of holes at the surface. The low CIC seems to indicate
the very opposite (??).
SDW Munich 2009
www.qucam.com
Measurement of serial-clock generated CIC
The CCD201 contains a dump gate (DG)
structure to assist in rapid clearing. It can
also be used to measure serial generated
CIC.
Removed by DG operation
CIC left behind by previous line readout
CIC from current line readout
Sum of the two
SDW Munich 2009
www.qucam.com
Measurement
Reduction ofofserial
serial-clock
clock generated
generatedCIC
CIC
New image dimensions
for purposes of test.
Image Area.
Store Area.
EM Amp.
EM register
Link section
Serial register
DG
Complete readout of pipeline for every row of image.
SDW Munich 2009
www.qucam.com
Reduction of serial-register CIC
Reducing the serial high clock voltage from 10 to 8.5V reduced serial CIC .
Lower voltages gave poor CTE.
SDW Munich 2009
www.qucam.com
Final CIC levels
Model used to find relative proportions of pre-EM-register
and in-EM-register generated CIC events.
Pre-reg = 0.02e- per pixel , In-reg = 0.011e- per pixel. Total 0.013e- per pixel.
SDW Munich 2009
www.qucam.com
Dual-EMCCD spectroscopy system
on William Herschel Telescope La Palma
Red arm of ISIS spectrograph: CCD201
SDW Munich 2009
Blue arm : additional CCD201
www.qucam.com
CCD201 cryogenic EMCCD camera
SDW Munich 2009
www.qucam.com
EMCCD spectroscopy: astronomical results
Cataclysmic Variable: white dwarf and less massive donor
orbiting around their common centre of gravity. Orbital
periods from 5 minutes to > 12 hours. Most of the light is
emitted from an accretion disc surrounding the
white dwarf.
SDSSJ1433
Artists impression, Mark Garlick
Spectrographic observations show the
double emission lines produced by the high
velocity material orbiting within the
accretion disc.
SDW Munich 2009
www.qucam.com
Appearance of an EMCCD spectrum
0.22A per pixel dispersion.
Mean intensity of continuum=0.08e-/s per wavelength step
Exposure time=30s
Target
g´=18.5
Reference
SDSSJ1433
SDW Munich 2009
www.qucam.com
EMCCD spectroscopy: astronomical results
With an EMCCD we can use short exposures to obtain time
resolved spectra of the accretion disc. It is then possible to
measure the to-and-fro motion of the white dwarf and
constrain the mass of the secondary star.
Radial velocity of the white dwarf.
SDW Munich 2009
Series of time resolved spectra
Tulloch, Rodriguez-Gil, Dhillon, MNRAS 397, L82-86, 2009
www.qucam.com
EMCCD spectroscopy: astronomical results
This type of time-resolved high-dispersion spectroscopy would have been
impossible with a conventional detector.
Model spectrum: 3e- read noise
SDW Munich 2009
Actual EMCCD spectrum
www.qucam.com
Aside: Dither clocking in a large format CCD
2k x 6k pixel
frame-transfer
CCD42C0.
Conventional inverted
mode operation
Intended for Eddington.
Now destined for Mercator
telescope in La Palma
High-speed photometry,
short exposure time.
Use of Peltier cooler.
SDW Munich 2009
www.qucam.com
´Dither´ induced cosmetic defects
+3V
-9V
FV1 FV2 FV3
“Wobble” sequence
repeated at intervals
ranging from 1ms to 4s
at temperatures from
213 to 233K during exposure.
Flat field
SDW Munich 2009
www.qucam.com
Use of ´Dither´ with a CCD42CO
Profile through 6 defects after 10,000 dither clock cycles.
Charge is conserved, defect amplitude < 1e- / cycle.
T=220K
SDW Munich 2009
www.qucam.com
Use of ´Dither´ with a CCD42CO
SDW Munich 2009
www.qucam.com
Use of ´Dither´ with a CCD42CO
Approximately equal to an extra 10
degrees of cooling.
SDW Munich 2009
www.qucam.com
Optimisation of an EMCCD
End of presentation
www.qucam.com
SDW Munich 2009
www.qucam.com