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Detector II:
IR arrays, bolometers
Ay122a: Astronomical Measurements and Instrumentation, fall term 2015-2016
D. Mawet, Week 6, November 13, 2015
Radiation
Discovery of infrared radiation
The existence
infrared wasby
f infrared
was of
discovered
by William
el indiscovered
1800. Herschel
Herschel in 1800. Herschel
emperature
of
sunlight
of
measured the temperature of
passing
a colors
prism and
sunlightthrough
of different
passing through
a prism and
temperature
increased
just
thatthe
the temperature
red found
end of
visible range.
increased just outside of the
d that
these “calorific rays”
red end of the visible range. He
ted,later
refracted
andthese
absorbed
showed that
light.
“calorific rays” could be
reflected, refracted and
absorbed just like visible light.
ixels of the detector array are attached to the pixels of the ROIC via indium
l. Since the ROIC multiplexes the signals from each pixel to the off-chip
ed to as a “multiplexer”, although multiplexing (signal transfer) is just one of
Summary of digital imaging steps
e (left) and the 6 steps of digital imaging. Drawing on left is courtesy of Laser Focus
Anti-reflective coatings
R1=(n0-n1)2/(n0+n1)2
R2=(n1-ns)2/(n1+ns)2
Conditions for destructive interference
Phase: Φ1-Φ2 = π
Amplitude: R1 = R2
=> e = λ/4n
=> n1 = sqrt(n0 . ns)
Rayleigh condition
Charge generation via photoelectric effect
hν
An incoming photon excites an electron
from the the valence band
to the conduction band: hν > Eg
conduction band
e- Eg
Eg = energy gap of material
Critical wavelength: λc (μm) = 1.238 / Eg (eV)
valence band
Material name
Symbol
Eg (eV)
λc (μm)
Op. Temp. (*)
Silicon
Si
1.12
1.1
163-300
Indium Gallium
Arsenide
InGaAs
0.7
1.7
77-200
Mer-Cad-Tel
HgCdTe
1.00-0.09
1.24-14
20-80
Indium Antimonide
InSb
0.23
5.5
30
Arsenic doped
Silicon
Si:As
0.05
25
4
(*) to keep dark current low (thermal electrons)
E
Introductory notes on photovoltaic IR Detectors
•
Single pixel IR detectors have long used the photovoltaic effect
•
Diode is formed at the junction between a p- and n- doped
semiconductor
•
This pn junction generates an internal electric field to separate
the photon generated electron-hole pairs
•
Migration of holes and electrons changes the electric field,
hence there is a voltage change across the junction which can
be measured
Intrinsic vs extrinsic IR detector material
•
Intrinsic photoconductivity = natural photoconductivity of pure
semiconductor material
•
Extrinsic photoconductivity = synthesized photoconductivity of doped
semiconductor material
•
The gap energy Eg is replace by the impurity energy Ei :
•
•
e- gets elevated from the valence band to an impurity level to create a hole
•
Alternatively, e- may be freed by elevating it from an impurity level to the
conduction band
Either the hole or the electron can then move through the material in response
to the electric field in the detector.
Examples of extrinsic silicon detector spectral response
A. Rogalski / Infrared Physics & Technolo
Fig. 12. Examples of extrinsic silicon detector spectral response. Shown are Si:In, Si:Ga, and Si:As bulk detectors and a
in A
detec
have
IR b
lm p
are a
plica
(Si:S
FPA
have
low
devic
basis
array
Pros and cons of extrinsic detectors
•
Pros:
•
•
Can operate at much longer wavelength, because it takes
less energy to free a charge carrier from an impurity atom
than from an atom of the semiconductor crystal material
Cons:
•
Extrinsic photoconductivity is far less efficient because of
limits in the amount of impurity that can be introduced into
the semiconductor without altering the nature of the
impurity states.
ong-wave IR (8-10 μm) and very long-wave IR (up to 18 μm). The hybrid CMOS architecture enables pixels
fill factor and high QE. The quality of the HgCdTe material continues to improve and the JWST specificati
current of less than 0.01 electrons per pixel per second (at 37 K operating temperature) is being achieved fo
μm cutoff H2RG arrays.
Tuning the bandgap/cutoff of Hg Cd Te detectors
1-x of the
x cadmium fraction, x. This p
presents the bandgap and cutoff wavelength of Hg1-xCdxTe as a function
ed from the equation presented by Hansen et al4, where x is the cadmium fraction and T is the temperatu
es Kelvin.
E g = −0.302 + 1.93x − 0.81 x + 0.832 x + 5.35 × 10 T (1 − 2 x )
2
3
−4
Fig. 3: Bandgap and cutoff wavelength of Hg1-xCdxTe as a function of the cadmium fraction, x.
o the temperature dependence of the bandgap, it is important to define the temperature of operation as we
the signal to the edge of the array. The analog-to-digital conversion
associated focal plane electronics. The pixels of the detector array are a
interconnects, one indium bump per pixel. Since the ROIC multiplexe
electronics, the ROIC is sometimes referred to as a “multiplexer”, althou
the functions provided by the ROIC.
Most IR detectors are hybrid CMOS
ROIC = readout
integrated circuit, converts the charge to
Fig. 2: Hybrid CMOS image array architecture (left) and the 6 steps of digital im
World.
complimentary
metal-oxide-semiconductor.
voltage with an amplifier
in CMOS
eachdenotes
pixel,
and transfers
the signal to
the edge
of high
theperformance
array IR arrays involves three key tec
The fabrication
of large,
1.
Growth and processing of the HgCdTe detector layer
Note about CMOS
•
Complementary metal–oxide–
semiconductor (CMOS)
•
Refers to a manufacturing
technique
•
“metal–oxide–semiconductor” is a
reference to the physical structure
of certain field-effect transistors,
having a metal gate electrode
placed on top of an oxide
insulator, which in turn is on top
of a semiconductor material.
CMOS
Field Effect Transistor
FET
The readout circuit (to be described below) requires just three CMOS transistors per pixel
(and associated electrical traces), which occupy only a fraction of the area of a typical
18µm wide pixel. This leaves space for the relatively large electrical contact pad that
provides the interconnect path to the diode array, lying in the plane above it.
Indium bump interconnects
The technology for making the vertical connection between the dissimilar materials in the light sensing and signal
processing layers is the key to IR detector manufacture. The contact is constructed by depositing a thick layer of Indium
on each pad, one per pixel, of the readout IC (through an etched photo-resistive mask). Matching “Indium bumps” are
Figure 2
deposited on the underside of the photodiode array. The tops of the Indium bumps must be accurately coplanar and
very The
cleantechnology
so that whenfor
themaking
bumps on
detector
layer and the
silicon layer
are preciselymaterials
aligned then
squeezed
thethe
vertical
connection
between
the dissimilar
in the
together, a cold weld is formed making a permanent electrical and mechanical connection – one per pixel. Currently it is
light sensing and signal processing layers is the key to IR detector manufacture. The
possible to connect 4 million pixels with only a few hundred failures. A low viscosity epoxy is then wicked into the
contact
is constructed
by depositing
a thick
layer
of Indium
each
pad, one
per pixel,
<10um
wide spaces
between the
Indium columns
and the
detector
layer ison
then
polished
and etched
until of
it is only
thethick.
readout IC (through an etched photo-resistive mask). Matching “Indium bumps” are
~10um
deposited on the underside of the photodiode array. The tops of the Indium bumps must
This be
is aaccurately
very complexcoplanar
and delicate
process
yield
at every
step, so the
devices
carry
price tags
and
very with
clean
soproblems
that when
the bumps
on top
thequality
detector
layer
and
in the $250-500K range, making IR detectors five to ten times as expensive as CCDs.
the silicon layer are precisely aligned then squeezed together, a cold weld is formed
making a permanent electrical and mechanical connection – one per pixel. Currently it
Note about multiplexer (MUX)
Since the ROIC multiplexes the signals from each pixel to
the off-chip electronics, the ROIC is sometimes referred to
as a “multiplexer”, although multiplexing (signal transfer) is
just one of the functions provided by the ROIC (see below).
lexing is done in the detector material itself than
an external readout circuit. The basic element of
monolithic array is a metal–insulator–semiconctor (MIS) structure as shown in Fig. 2(c). A
IS capacitor detects and integrates the IR-gented photocurrent. Although efforts have been
de to develop monolithic FPAs using narrowp semiconductors, silicon-based FPA technol-
technology, which has matured to a level
practical use.
Hybrid FPAs detectors and multiplexers a
fabricated on different substrates and mated wi
each other by the flip-chip bonding (Fig. 3)
loophole interconnection. In this case we can o
timise the detector material and multiplexer ind
pendently. Other advantages of the hybrid FPA
Monolithic vs hybrid focal plane arrays (FPA)
Monolithic FPA
Fig. 2. Monolithic IR FPAs: (a) all-silicon; (b) heteroepitaxy-on-silicon; (c) non-silicon (e.g., HgCdTe CCD) (after Ref. [14]).
Monolithic vs hybrid focal plane arrays (FPA)
Hybrid FPA
Fig. 2. Monolithic IR FPAs: (a) all-silicon; (b) heteroepitaxy-on-silicon; (c) non-silicon (e.g., HgCdTe CCD) (after Ref. [14]).
Fig. 3. Hybrid IR FPA with independently optimised signal detection and readout: (a) indium bump technique, (b) loophole technique.
Fabrication of IR arrays
The fabrication of large, high performance IR arrays
involves three key technologies:
1.
Growth and processing of the detector layer (e.g. HgCdTe)
2. Design and fabrication of the CMOS ROIC 3. Hybridization of the detector layer to the CMOS ROIC Molecular beam epitaxy (MBE)
3” MBE at Teledyne
10” MBE at Teledyne
Fig. 4: Two of the molecular beam epitaxy (MBE) machines that Teledyne uses for growing high quality HgCdTe detectors. The
machine on the left can hold 3-inch wafers, the machine on the right can hold 10-inch wafers.
Since HgCdTe is a direct bandgap semiconductor, it is a very efficient absorber of light. The absorption depth of the
photons in HgCdTe, i.e. the distance over which 1-e-1 (63%) of the light is absorbed, is shown in Fig. 5. For high
quantum efficiency, the thickness of the HgCdTe detector layer should be at least 3 absorption depths, so that at least
tion. In addition, this multiplexer can be programmed to a
Unit Cellof(UC)
forforDirect
Readout
(DRO)enabling very sh
subsection
pixels
continuous
readout,
Reset
Voltage
Read Enable
Clock
Reset Enable
Clock
To Output FET and
Current Supply
UNIT
CELL
Integrating
Node
Unit Cell
Source Follower FET
with Inverter
Detector
Diode
Detector
Substrate
Voltage
Unit Cell
Source Follower FET
Drain
FET = field effect transistor
Detailed operation
•
Reset switch is closed to set the
photodiode in reverse biased
by Vr ~ -100 mV
mode
•
Reverse bias => depletion region widens => diode is
high-resistance insulator
•
Thermal leakage is negligible at low temperature for both
the photodiode and MOSFET
•
Change in voltage dominated by electron-pair generation,
which drops the voltage by ΔV
Correlated Double Sampling (CDS)
The exposure
time
is the time
between
samples
and in
notathe
time
since reset.
Figure 4:
Sample
timing
for the
last pixel
CDS
frame.
Signal arriving prior to the first sample is ignored (subtracted from the final
sample), so the exposure duty cycle is not 100% but approaches it when the
times are
asonableexposure
to ask why
twolong.
samples are necessary. The most obvious effect
of th
ample is to remove the DC offsets which are intrinsic to the readout circuit and
Fowler N sampling
(Al Fowler is a NOAO engineer)
Fowler sampling
Figure 9: Comparison of CDS timing and Fowler Sampling,
Fowler sampling is a simple
variant for improving the read noise. Since the signal is read
non-destructively, multiple samples at the beginning and end of the exposure can be
averaged to reduce the effective read noise. (The detector readout software will do this
Classical
Sampling”way
involves
nothing
signal
calculation
in “Correlated
real time). Double
An alternative
to think
of more
this than
is assampling
a set the
of partially
after reset
andpairs,
subtracting
the final
value.
Anyexposure
signal accumulated
outside
overlapping
CDS
which this
are from
averaged.
The
effective
time is the
difference
these
two
samples
is invisible,
exceptand
for its
linearityfirst
andand
dynamic
range.
between
the
Nth
sample
in each group
noteffect
time on
between
last sample.
2D Multiplexor
Only one row-enable
is active
at amultiplexor
time: forevery
pixel
the selected row is
Figure 5:line
Schematic
layout of a readout
an infrared
detectorin
array.
connected to a different column bus. At the edge of the array, each column bus is
pixels areto
accessed
via a 2D multiplexor.
Each
pixel output
has a single
MOSFET
connected via The
a switch
the output
buffer. To
raster
through
the
pixels one enables
which is driven hard on or off, to act as a switch. Outputs of all switches in the same
a row then sequentially
selects
then
thein each
next row.
column are connected
to a bus. columns,
The control lines
for therepeats
pixel-select for
switches
row are ganged together. Only one row-enable line is active at a time: every pixel in the
selected row is connected to a different column bus. At the edge of the array, each
Readout time
From 3 to 10μs are typically needed to access each
pixel. To reduce the time needed to raster through all
pixels the columns are subdivided into groups each
served by a separate output buffer/channel. A typical
2048x2048 H2RG has 32 outputs each serving 64
consecutive columns.
Note that since the exposure time is defined by the times
at which the pixels are read, then the exposure for the
last pixel is displaced from the first by the time it takes to
scan through whole array. (~10μs*2048*2048/32 = 1.3s)
Pros of on-pixel integration
•
Electronic shuttering: Exposure time is the time
between initial and final reads, not time since reset.
Charge accumulated between reset and first sample has
no effect on noise but can consume dynamic range and
affect linearity.
•
Since readout is non-destructive, noise reduction is
possible by combining multiple samples. (Fowler
Sampling, Sample Up the Ramp). However the
improvement isn’t as good as √N due to temporal
correlations: there is significant noise power or systematic
drift on frame-to-frame timescales.
Cons of on-pixel integration
•
CDS occurs across the exposure time. Consequently
IR detectors must be DC coupled and are at least 1000
times more sensitive than CCDs to electronic drifts and
temperature changes.
•
Because each pixel has a different signal path, there
is no “overscan” (as in a CCD) to provide an
accurate zero-point reference. The next best thing is to
use dark pixels in the image area, or unconnected pixels
around the edges to mimic the zero- point drift of the
image pixels. These “reference pixels” don’t tell us where
zero is but do tell us how much it has changed.
Cons of on-pixel integration cont’d
•
Read noise increases with exposure time. With good
electronics, the read noise is reduced to the 1/f noise in the pixel
buffer transistor and detector material.
•
The charge to voltage conversion is non-linear. The signal is
accumulated on the detector diode capacitance. The reverse bias
applied by the reset is discharged by the photocurrent causing the
width of the depletion region to be reduced, so the diode
capacitance increases: the voltage change of for a given charge
increment drops.
•
When observing bright sources or in high background, substantial
charge can be accumulated between the reset and first sample,
eating into the apparent dynamic range.
Notes on IR arrays (vs CCDs)
•
Many parameters vary considerably from pixel to pixel.
(e.g. dark current, QE, noise, temperature sensitivity).
•
Dark current is higher and is more steeply dependent on
temperature. In the best IR detectors this is just due to the
lower bandgap, but it is often the case that imperfect surface
passivation during manufacture degrades the dark current
and causes large dark current variations from pixel to pixel.
•
Dark current takes several hours to fully stabilize (!!!
major problem at observatories) after a perturbation such
as a temperature or bias voltage change (eg cycling power).
Example of IR arrays
IR Arrays
interfaces directly with the H1RG and H2RG and provides all of the functionality required from focal plane electronics
(FPE).
(System for Image
Enhancement,Orbiter
Control has
and Retrieval)
shownand
in Fig.
11,
Additionally, The
the SIDECAR
CRISM spectrometer
in theDigitization,
Mars Reconnaissance
gone intoASIC,
operation,
is performing
9
provides
significant
in the size,
weight
power
of the
.
very well.
These
arraysreduction
are 640×480
pixels,
andand
both
IR (λ
= FPE
2.5 µm)
and visible silicon PIN arrays are operating in
co
the CRISM instrument. Also, the 1024×1024 pixel (λco = 4.8 µm) IR array in the Deep Impact mission continues to
The SIDECAR contains a programmable microprocessor, bias generators, clock generators, amplifiers and analog-toperform
wellconverters
since launch
in 2005,
and36the
spacecraft
added
another cometinrendezvous
in choice
2010 as
mission.
digital
(ADCs).
Up to
analog
inputs can
be accommodated
parallel, with
of the
500EPOXI
kHz, 16-bit
IR arrays from Teledyne
ADC or 10 MHz, 12-bit ADC (36 ADCs operate in parallel). The SIDECAR presents a digital interface to instrument
In thiselectronics,
paper, weand
provide
a summary
of thedifferential
technologies,
and ongoing
developments
for IRseveral
sensors
at TIS. A
with LVDS
(low voltage
signal)products
communication,
the SIDECAR
can be placed
meters
companion
paper,
entitled
“Teledyne
advanced
silicon
CMOS sensors
for x-ray
to near-IR”,
from the
instrument
electronics.
AllImaging
operationSensors:
of the SIDECAR
is fully
programmable
via LVDS
communication
lines.presented
3
in the visible sensors section of this conference, reports on silicon-based imaging sensors of TIS .
The SIDECAR ASIC has been selected for use in 3 of the 4 instruments of the JWST. Two features of the SIDECAR
were important factors in its selection for JWST:
1. Low power operation: For JWST operation - 4 ports continuously read at 100 kHz pixel rate and 16 bit
digitization - the SIDECAR uses 11 mW power at 37K. The low power operation enables JWST to place the
SIDECAR within the very cold (37K) instrument module which is located 4 meters cable length from the
electronics located in the warm section of the observatory.
2. Low noise performance: The SIDECAR noise is negligible when compared to the H2RG readout amplifier, so
that the total noise of the H2RG-SIDECAR system is set by the low noise H2RG operation.
The SIDECAR ASIC was also selected for the repair of the Advanced Camera for Surveys (ACS) instrument in the
Hubble Space Telescope. In this system, the SIDECAR will be used to operate two 4K×2K CCDs. The ACS Repair
will take place during HST Servicing Mission 4, which is scheduled for October 2008. For the ACS Repair, a new
hermetically sealed
wasinfrared
developed
for the
SIDECAR
SIDECAR
Fig. 1:spaceflight
Examples package
of Teledyne
imaging
sensors
(left toASIC.
right): The
WFC3
1K×1K ASIC
(HST),packaging is
shown in
Fig.
11.
CRISM 640×480 (Mars Reconnaissance Orbiter), H2RG-18 2K×2K (JWST and ground-based astronomy)
Copyright 2008 Society of Photo-Optical Instrumentation Engineers
This paper was published in the Proceedings of the SPIE Conference on Astronomical Instrumentation (2008, Marseille, France) and is made available
as an electronic reprint with permission of SPIE. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction,
distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or
modification of the content of the paper are prohibited.
page 2 of 14
Fig. 11: SIDECAR ASIC and its packages. The ground-based astronomy package (left), the Hubble Space Telescope hermetically
sealed package (center), and the James Webb Space Telescope cryogenic package (right).
Both the H2RG and the SIDECAR have undergone environmental testing in spaceflight packages and have been
Typical raw image
So, here’s what we hav
Raw K-band image of field shows stars,
but also substantial sky signal
•  Ra
sh
su
•  Sk
fie
Sky signal intensity varies over field:
– Large-scale variations:
Illumination
Quantum efficiency variations
– Small-scale variations:
Pixel-to-pixel variations
–
–
Array defects • 
• 
High dark current pixels • 
These can be corrected by appropriate
calibration images:
Dark frames (bias) Flatfield images
Ar
Hi
(m
Th
ap
–
–
19 July 2010
NOAO Gemini Data Wo
Let’s try this CCD-like recipe
When we try this (CCD style)…
Obtain science images
•  Obtain science images
Obtain
calibration
images images
•  Obtain
calibration
–  Dark
at same integration
Dark frames
atframes
same integration
time time
–  Flatfield images of uniform target
Flatfield
imagesdark
of uniform
•  Subtract
frame target
from science
images
Subtract dark frame from science images
•  Divide dark-subtracted images by
Divideflatfield
dark-subtracted images by flatfield
•   Image of science field with uniform
=> Image
of science field with uniform sky
sky level
level
•  Subtract (constant) sky level from
image
Subtract
(constant) sky level from image
•  But, here is what we get…..
But, here
whatbut
westill
get.....
–  is
Better,
see substantial sky
variations
– Better, but
still see substantial sky
variations
Small flatfield errors on sky still larger than faint science targets
Small flatfield errors on sky still larger than faint science targets
19 July 2010
NOAO Gemini Data Workshop
15
The sky background
is the
problem
Since the sky
is the
problem…
•  Subtract out the sky (or as much
as possible)
the flatfield
Subtract
out the skybefore
(or as much
as
correction
possible)
before the flatfield correction
•  Obtain
two images
of field, move
Obtain
two images
of field, move
telescope
between
telescope
between
•  Subtract
two images
Subtract
two images
–  Eliminate almost all sky signal
– Eliminate
almostout
all dark
sky signal
–  Subtracts
current, maverick
pixels
– Subtracts out dark current, maverick
•  Divide by flatfield image
pixels
•  Result has almost no sky structure
Divide by flatfield image
Result has almost no sky structure
Subtracting
sky
minimizes
Subtracting
sky
minimizeseffects
effectsofofflatfield
flatfielderrors
errors
(but noise increased by 1.4)
(but noise increased by 1.4)
19 July 2010
NOAO Gemini Data Workshop
16
Typical sequence for IR imaging
•
•
Multiple observations of science field with small telescope motions in between (dithering)
•
Sky background limits integration time
•
Moving sources samples sky on all pixels
•
Moving sources avoids effects of bad/noisy pixels
Combine observations using median filtering algorithm
•
Effectively removes stars from result => sky image
•
Averaging reduces noise in sky image
•
Subtract sky frame from each science frame => sky subtracted images
•
Divide sky subtracted images by flatfield image
•
•
Dome flat using lights on – lights off to subtract background
•
Sky flat using sky image – dark image using same integration time
•
Twilight flats – short time interval in IR
Shift and combine flatfielded images
•
Rejection algorithm (or median) can be used to eliminate bad pixels from final image
Illustration
Here’s what it looks like….
Sky frame
Median
Subtract sky,
divide each by
Flatfield
19 July 2010
NOAO Gemini Data Workshop
18
Shift and add
Shift and combine images
NGC 7790, Ks filter
3 x 3 grid
50 arcsec dither offset
•  NGC 77
•  3 x 3 gr
•  50 arcs
Bad pixe
From com
Bad pixels eliminated
From combined image
Higher n
than in c
combine
Higher noise in corners
than in center (fewer
combined images)
19 July 2010
NOAO Gemini Data Workshop
Summary: data reduction strategy for IR detectors
SOURCE OBSERVATIONS
(DITHERED)
SKY OBSERVATIONS
(DITHERED)
[MEDIAN]
[MEDIAN]
SKY
[―]
DOME FLATS ON
SKY SUBTRACTED
IMAGES
DARKS
[―]
DOME FLATS OFF
[―]
FLAT
[/]
SKY SUBTRACTED,
FLATTENED IMAGES
SHIFT, ALIGN SOURCES
[MEDIAN]
AVERAGED IMAGE
Mid-infrared strategy (nodding & chopping)
•
•
•
Sky background at 10 μm is 1,000-10,000 greater than in K band
•
Detector wells saturate in very short time (< 50 ms)
•
Very small temporal variations in sky >> astronomical source intensities
Mid-infrared
strategy
Read array out very rapidly (20
ms), coadd images
•  Sky background at 10 µm is 103 – 104 greater than in K band
Sample sky at–  high
rate (~ 3 Hz) by chopping secondary mirror (15 arcsec)
Detector wells saturate in very short time (< 50 ms)
•
•
•
–  Very
temporal variations
in sky
>> astronomical
source
intensities
Synchronize
withsmall
detector
readout,
build
up “target”
and
“sky” images
But
•  Read array out very rapidly (20 ms), coadd images
tilting
of secondary
its secondary
own offset
signal
•  Sample
sky at highmirror
rate (~ 3introduces
Hz) by chopping
mirror
(15 arcsec)
Synchronize with detector readout, build up “target” and “sky” images
Remove offset–  by
nodding telescope (30 s) by amplitude of chop motion
–  But tilting of secondary mirror introduces its own offset signal
•
Removeof
offset
by nodding
telescope
(30 s) with
by amplitude
of chop
motioncycle
Relative•  phase
target
changed
by 180°
respect
to chop
–  Relative phase of target changed by 180° with respect to chop cycle
phase
of offset
unchanged
–  Relative
phase ofsignal
offset signal
unchanged
–  Subtraction adds signal from target, subtracts offset
•
Relative
•
Subtraction
adds signal from target, subtracts offset
•  http://www.gemini.edu/sciops/instruments/t-recs/imaging
chop
19 July 2010
……
nod
NOAO Gemini Data Workshop
22
Bolometers
•  Measure the energy from a radiation field, usually by measuring a
change in resistance of some device as it is heated by the radiation
•  Mainly used in FIR/sub-mm/microwave regime
•  Sensitivity is measured through the Noise Equivalent Power
• Measure the energy(NEP):
from atheradiation
field, which produces S/N=1 at the output
power absorbed
usually by measuring(units
a change
W/Hz0.5)in
Bolometers
resistance of some device as it is heated
by the radiation
•
•  Typically use a semiconductor
Mainly used in FIR/sub-mm/microwave
resistance thermometer, and a
regime
metal coated dielectric as the
• Sensitivity is measured
absorber
through the
Noise Equivalent Power (NEP): the
power absorbed which produces S/N=1
0.5
at the output (units W/Hz )
•
Typically use a semiconductor resistance
thermometer, and a metal coated
dielectric as the absorber Examples of bolometers
Semiconductor bolometers
from SCUBA
“Spiderweb” bolometer
Components of a Bolometer
Components of a bolometer
•
•
•
•
•  Absorber with heat capacity C
•  Heat
sink heldC
at fixed temperature
Absorber with heat
capacity
T0
•  Small thermal conductance G
Heat sink held at fixed
temperature
T0sink
between
absorber and heat
•  Load resistor RL
•  ThermometerG
w. resistance R
Small thermal conductance
•  Constant
current
supply generating
between absorber
and heat
sink
bias current I
•  Device to measure voltage changes
Load resistor RL
•
Thermometer w. resistance R
•
Constant current supply generating
bias current I
•
Device to measure voltage changes
Schematic of a
bolometer
Sources
•
http://spiff.rit.edu/classes/phys445/lectures/ccd1/ccd1.html
•
Observational astrophysics, 2nd edition, P. Lena
•
S. G. Djorgovski (Caltech, Ay122a, 2012)
•
R. Smith (Caltech Ay 105 notes)
•
J.W. Beletic notes (optics in astrophysics, R. & F.C. Foy editor, NATO Science Series)
•
George Rieke 2007, Ann. Rev. Astr. Ap. 45, 77.
•
An introduction to IR detectors (D. Joyce, NOAO)