Low Light Level CCDs (LLLCCD)
A new idea from Marconi (EEV) to reduce or eliminate CCD read-out noise.
Photomicrograph of a corner of an EEV CCD.
Bus wires
Serial Register
Read Out Amplifier
Edge of
Silicon
Image Area
Charge Collection in a CCD.
Charge packet
pixel
boundary
pixel
boundary
incoming
photons
Photons entering the CCD create electron-hole pairs. The electrons are then attracted towards
the most positive potential in the device where they create ‘charge packets’. Each packet
corresponds to one pixel.
n-type silicon
Electrode Structure
p-type silicon
SiO2 Insulating layer
Conventional Clocking 1
Insulating layer
Surface electrodes
Charge packet (photo-electrons)
Potential Energy
P-type silicon
Charge packets occupy potential minimums
N-type silicon
Potential Energy
Conventional Clocking 2
Potential Energy
Conventional Clocking 3
Potential Energy
Conventional Clocking 4
Potential Energy
Conventional Clocking 5
Potential Energy
Conventional Clocking 6
Potential Energy
Conventional Clocking 7
Potential Energy
Conventional Clocking 8
Potential Energy
Conventional Clocking 9
Conventional Clocking 10
Potential Energy
Charge packets have moved one pixel to the right
LLLCCD Gain Register Architecture
Conventional CCD
LLLCCD
Image Area
Image Area
On-Chip
Amplifier
Serial register
{
On-Chip
Amplifier
(Architecture unchanged)
Serial register
Gain register
The Gain Register can be added to any existing design
Multiplication Clocking 1
In this diagram we see a small section of the gain register
Potential Energy
Gain electrode
Multiplication Clocking 2
Gain electrode energised. Charge packets accelerated strongly into deep potential well.
Energetic electrons loose energy through creation of more charge carriers (analogous to
multiplication effects in the dynodes of a photo-multiplier) .
Potential Energy
Gain electrode
Multiplication Clocking 3
Potential Energy
Clocking continues but each time the charge packets pass through the gain electrode, further
amplification is produced. Gain per stage is low, <1.015, however the number of stages is high so the
total gain can easily exceed 10,000
Multiplication Clocking 4
Gain Sensitivity of CCD65
10000
Gain
1000
100
10
1
20
25
30
35
40
35
40
Clock High Voltage
Readout Noise of CCD65
Equivalent noise
electrons RMS
100
10
1
0.1
0.01
20
25
30
Clock High Voltage
The Multiplication Register has a gain strongly dependant on the clock voltage
Noise Equations 1.
Conventional CCD SNR Equation
-0.5
SNR = Q.I.t.[Q.t.( I +B ) +Nr2 ]
SKY
Q
I
t
BSKY
Nr
= Quantum Efficiency
= Photons per pixel per second
= Integration time in seconds
= Sky background in photons per pixel per second
= Amplifier (read-out) noise in electrons RMS
Very hard to get Nr < 3e, and then only by slowing down the readout
significantly. At TV frame rates, noise > 50e
Trade-off between readout speed and readout noise
Noise Equations 2.
LLLCCD SNR Equation
SNR = Q.I.t.Fn.[Q.t.Fn.( I +BSKY) +(Nr/G)2 ]
-0.5
G = Gain of the Gain Register
Fn = Multiplication Noise factor = 0.5
With G set sufficiently high,
this term goes to zero, even at
TV frame rates.
Unfortunately, the problem of multiplication noise is introduced
Readout speed and readout noise are decoupled
Multiplication Noise 1.
In this example, A flat field image is read out through the multiplication register.
Mean illumination is 16e/pixel. Multiplication register gain =100
Ideal Histogram, StdDev=Gain x (Mean Illumination in electrons )0.5
Actual Histogram, StdDev=Gain x (Mean Illumination in electrons )0.5 x M
Probability
Histogram broadened
by multiplication noise
M=1.4
Electrons per pixel at output of multiplication register
Multiplication Noise 2.
SNR
Multiplication noise has the same effect as a reduction of QE by a factor of two.
In high signal environments , LLLCCDs will generally perform worse than
conventional CCDs. They come into their own, however, in low signal, high-speed
regimes.
Conventional CCD
LLLCCD
Signal Level
Photon Counting 1.
Offers a way of removing multiplication noise.
Photo-electron
detection threshold
CCD Video waveform
One
No
photo-electron photo-electron
One
photo-electron
No
No
Two
photo-electron photo-electron photo-electrons
Co-incidence loss
here
Photo-electron detection pulses
Fast comparator
CCD
Approx 100ns
SNR = Q.I.t.[Q.t.( I +BSKY)]
Noiseless Detector
-0.5
Photon Counting 2.
If exposure levels are too high, multi-electron events will be counted as single-electron
events, leading to co-incidence losses . This limits the linearity and reduces the effective
QE of the system.
Non-Linearity from Photon-Counting Coincidence Losses
Photo-electron
generation rate
Non-Linearity
(electrons per pixel per frame)
%
0.02
1
0.033
1.6
0.1
5
In the case of a hypothetical 1K x 1K photon counting CCD, the maximum frame rate
would be approximately 10Hz. If we can only accept 5% non-linearity then the maximum
illumination would be approximately 1 photo-electron per pixel per second.
Summary.
The three operational regimes of LLLCCDs
1) Unity Gain Mode.
The CCD operates normally with the SNR dictated by the photon shot noise added in
quadrature with the amplifier read noise. In general a slow readout is required (300KPix/second)
to obtain low read noise (4 electrons would be typical). Higher readout speeds possible but there
will be a trade-off with the read-noise.
2) High Gain Mode.
Gain set sufficiently high to make noise in the readout amplifier of the CCD negligible.
The drawback is the introduction of Multiplication Noise that reduces the SNR
by a factor of 1.4. Read noise is de-coupled from read-out speed. Very high speed readout
possible, up to 11MPixels per second, although in practice the frame rate will probably be
limited by factors external to the CCD.
3) Photon Counting Mode.
Gain is again set high but the video waveform is passed through a comparator. Each trigger
of the comparator is then treated as a single photo-electron of equal weight. Multiplication
noise is thus eliminated. Risk of coincidence losses at higher illumination levels.
Possible Application 1.
Acquisition Cameras
Performance at CASS of WHT analysed below. The calculated SNR is for a single TV frame (40ms).
It is assumed that the seeing disc of the target star evenly illuminates 28 pixels
(0.6” seeing, 0.1”/pixel plate scale). SNR calculated for each pixel of the image.
3.5
Normal CCD
3
L3CS (LLLCCD)
2.5
SNR
theoretical limit
2
Zero-noise image tube
1.5
1
0.5
0
17
18
19
20
21
22
Mv
Assumptions: CCD QE=85%, LLLCCD QE=30%, Image Tube QE =11%
dark of moon, seeing 0.6”, 24um pixels (0.1”per pixel), 25Hz frame rate
Possible Application 3.
Photon Counting Faint Object Spectroscopy
LLLCCDs operating in photon counting mode would seem to offer some promise.
The graph below shows the time taken to reach a SNR=3 for various source intensities
Thinned LLLCCD with Gain=1000
Source intensity at the detector
(photons per pixel per second)
10
Thinned LLLCCD +Photon Counting
Conventional CCD
1
0.1
0.01
0
200
400
600
Exposure Time Seconds
QE=70%
Amplifier Noise =5e
Background =0.001 photons per pixel per second
800
1000
E2V Technologies L3 Detector Schematic
Store Area
Avalanche multiplication takes
place in Multiplication Register,
using an HV clock (40-45Volts).
1000e- signal out
Devices used :
CCD60 128 x 128
Multiplication register
1e- in
Normal Serial register
Image Area
Multiplication register
Multiplication register
Standard MOSFET
amplifier
CCD87/97 512x512
CCD201 1K x 1K
Appearance of L3 Bias Frame
Dominated by Clock Induced Charge (CIC):
0.03e per
Top right of CCD97 image
showing over-scan regions.
Noise levels typical at ING,
better performance
is possible.
Vertical scale in electrons
pixel in over-scans
0.1e per pixel
in Image area
Histogram
of conventional
CCD bias
frame.
Histogram
of L3 CCD
bias frame.
Cut along Image row
The L3 bias contains almost entirely single electron events, however, a cut
through the image shows events with a wide range of heights. This
distribution has the effect of reducing the SNR at higher illumination levels.
Multiplication Noise
Log base e of Histogram value
12
11
10
The statistics of the
multiplication process give a
range of output signals in
response to a single electron
input.
Gradient =-0.0046
Inverse Gain=215ADU/e-
9
8
7
6
5
4
3
2
0
200
400
600
800
1000
1200
Bias Subtracted signal (ADU)
Normal photon statistics
do not apply, instead the
RMS variation of the signal
= (2xmean signal).
Equivalent to an effective
halving of QE
RMS variation in signal (electrons)
200
180
Photon Transfer Graph
160
140
120
Normal CCD
(Poissonian)
100
80
60
Gain=12
40
Gain=39
Gain=283
20
0
0
20
40
60
80
100
Root of Mean Signal Level (electrons)
120
140
L3 Applications
Low flux regimes normally limited by detector read noise :
Adaptive Optics Wave-front sensing.
High time resolution imaging/spectroscopy.
‘Lucky’ Astronomy
GLAS
Ground-layer Laser Adaptive optics System
25W Rayleigh Laser beacon. Height defined by hi-speed Pockels cell shutter. On-axis laser guide
star will give full sky coverage, however, natural guide star still required for tip-tilt correction.
beacon
20 km
turbulence
laser
Comes into operation
On the WHT in
Autumn 2006
Need for a Natural Guide Star
Laser beacons are insensitive to global tip-tilt
terms because of the double passage of the
laser through the turbulent layer.
A field star close to the science object must also be
observed for tip-tilt correction. Availability of
suitable guide stars now limits sky coverage, so an L3
detector will be useful. (Laser guide star bright enough
to use a Conventional CCD).
Sky Coverage for Mv17 guide star and 1.5’ search field
NAOMI/GLAS System Schematic
Pockels shutter synched to laser pulses
Corrected
NIR science
image
Wavefront
Re-constructor
Primary
Pockels
Shack-Hartmann
cell
WFS observing Laser
Guide Star (LGS).
CCD39
Pulsed
Rayleigh
Laser
Detail of Shack Hartmann Sensor
CCD
Conjugate planes
FSM=fast steering mirror
(tip tilt wave-front error correction)
DM=deformable mirror
(to remove higher order errors)
Larger errors
Off-loaded to TCS
Wave-front
Uncorrected
image
Tip-Tilt WFS
observing Natural
Guide Star (NGS).
L3 CCD60
Image displacement
on CCD proportional
to average wave-front
gradient
Lenslet array conjugate with primary aperture
ING’s L3 Wavefront Sensor
Redundant slave camera head replaced with CCD60 L3 WFS Head in May 2005.
Initial tests as a Shack Hartmann sensor, although will finally be used for
Natural Guide Star tip-tilt sensing.
NAOMI AO system WFS Heads
New L3 CCD60 Head
Slave
Master
Beamsplitter
Expected GLAS performance
FWHM using R=17th mag tip-tilt star
R
I
J
H
Uncorrected
0.61
0.58
0.53
0.51
Corrected
0.30
0.20
0.11
0.11
Will extend current NAOMI AO system performance to almost full sky
coverage when seeing is dominated by the Ground Layer.
Uncorrected image 0.7” seeing
FWHM -> 0.2” I-Band
-> 0.3” V-band
0.37”
The CCD60 as a Shack Hartmann NGS Wavefront Sensor
prior to the commissioning of GLAS
Main focus of our work is to use the CCD60 as a tip-tilt sensor for GLAS,
in the meantime, however, we hope to use it in SH mode to overcome
the current noise limitations of the (non-laser) WFS.
Predicted gains from using L3 NGS sensor
on current NAOMI AO system
Sequence of L3 CCD60 images
taken on William Herschel Telescope
NAOMI adaptive optics system.
Closed loop operation has been
demonstrated but direct comparison
with non-L3 sensor not yet done.
0.8
0.7
0.6
H Band Strehl
0.5
0.4
0.3
Zero noise detector
0.2
CCD60 with 0.2e Mean CIC
CCD39 with 5e noise
0.1
0
11
12
13
14
15
16
Guide Star M v
Data supplied by Richard Wilson, Univ. Durham
17
Controller Performance
All work done with SDSUII controller + custom built Multiplication Clock board:
Using DSP Modulo-Addressing scheme
(circular waveform tables),
Minimum pixel timings were:
Pixel Skips : 460ns
Pixel Reads : 1280ns
For CCD60 Tip-Tilt AO modes,
an 8x8 pixel window read out at
>300Hz.
SDSUII
Just fast enough for our current applications.
Mult.Clk. PCB design
Hi Time Resolution Photometry
Example observation: Crab Nebula Pulsar using L3 CCD60
on the WHT:
CCD60 Test Camera
Sub-electron read noise at 180 frames per second
Larger Format Detectors
512 and 1K square detectors incorporated into cryogenic cameras:
‘QUCAM1’ CCD87
‘QUCAM2’ CCD201
Hi Time Resolution Spectroscopy
Example observation: IP Pegasus, eclipsing binary
Using L3 CCD87 on the WHT:
Ha
White dwarf with accretion disc in orbit around
larger primary star. Accretion disc luminosity dominates
and its light is highly Doppler shifted.
Sequence of raw spectra, taken using CCD87 L3 Camera on
ISIS spectrograph of the William Herschel telescope
-500km/s
+500km/s
Lucky Astronomy
Hi frame rates are used to ‘freeze’ atmospheric motion. Frames corrupted
by turbulence are then discarded. Remaining frames shifted and added to take
out tip-tilt motions. Near diffraction limited images result.
Select best 10%
Short exposures
HST-like resolution
Images courtesy of Craig Mackay IOA Cambridge
Future possibilities: Photon Counting
There is no longer any loss of SNR due to multiplication noise.
L3 CCD performance then approaches that of the ideal detector.
Low CIC essential, positioning of threshold critical.
Cryogenic CCD87 imaging a faint pinhole
Threshold
Raw input frames
Particles?
Thresholded and accumulated
Waves?
Mosaic Cameras 6.
This colossal mosaic of 12 CCDs is in operation at the CFHT in Hawaii. Here is an
example of what it can produce. The chips are of fairly low cosmetic quality.
Picture : Canada France Hawaii Telescope
Camera Construction Techniques 1.
The photo below shows a scientific CCD camera in use at the Isaac Newton Group. It is
approximately 50cm long, weighs about 10Kg and contains a single cryogenically cooled CCD.
The camera is general purpose detector with a universal face-plate for attachment to various
telescope ports.
Mounting clamp
Pre-amplifier Pressure Vessel
Vacuum pump port
Camera mounting
Face-plate.
Liquid Nitrogen
fill port
Camera Construction Techniques 4.
A cutaway diagram of the same camera is shown below.
Thermally
Insulating
Pillars
Electrical feed-through
Vacuum Space
Pressure vessel
Pump Port
Telescope beam
Face-plate
CCD
Focal Plane
of Telescope
Optical window
...
CCD Mounting Block Thermal coupling
Boil-off
Nitrogen can
Activated charcoal ‘Getter’
Camera Construction Techniques 5.
The camera with the face-plate removed is shown below
CCD
Retaining
clamp
Temperature servo circuit board
Aluminised Mylar
sheet
Gold plated copper
mounting block
Top of LN2
can
Platinum resistance
thermometer
Pressure
Vessel
‘Spider’.
The CCD mounting
block is stood off from
the spider using
insulating pillars.
Location points (x3)
for insulating pillars
that reference the CCD
to the camera face-plate
Signal wires to CCD
Camera Construction Techniques 6.
A ‘Radiation Shield’ is then screwed down onto the spider , covering the cold components but
not obstructing the CCD view. This shield is highly polished and cooled to an intermediate temperature
by a copper braid that connects it to the LN2 can.
Radiation Shield
Camera Construction Techniques 7.
Some CCDs cameras are embedded into optical instruments as dedicated detectors.
The CCD shown below is mounted in a spider assembly and placed at the focus of a
Schmidt camera.
CCD Signal connector (x3)
Copper rod or ‘cold finger’
used to cool the CCD. It is
connected to an LN2 can.
‘Spider’ Vane
CCD Clamp plate
Gold plated
copper CCD
mounting
block.
FOS 1 Spectrograph
CCD Package