Download Folie 1

Astronomical Detectors
The First Detector in Astronomy
Retina
• Light sensitive, chemical process
Rods
• 100 million
• 2mm wide
Eye: complete system of telescope (variable
aperture, ~1cm max), detector, and data
reduction processor
• can detect low light levels
• averted vision
Cones
• color information
• 7 million
• cluster near axis
• 1000 per square mm
Reusable
• integration times 100 ms
Quantum efficiency ~1%
Resolution 80 arcsecs
Discoveries made with the Eye
Naked Eyes
•
Nomenclature: Constellations, star names
•
Astrometry: Stellar positions
•
Magnitude scale
•
Planets in solar system, planet orbits (Tycho, Kepler)
•
Comets
•
Supernovae explosions, variable stars
•
Sunspots (by Chinese 800 B.C.)
•
Solar corona (eclipse)
•
Lunar Phases
•
Star Clusters (e.g. Pleiades)
Discoveries made with the Eye
With telescopes
•
Sunspot cycle
•
Saturn‘s rings, Jupiter‘s Red Spot
•
Nebulae, Spiral Nebulae
•
Parallax, astrometric measurements
•
Asteroids, moons of most planets
•
Craters on Moon
Observations with the eye can be unreliable
•
No record of observations so as to check results
•
Precise measurements cannot be made (position, magnitude, etc)
because observations are not recorded or digitized
•
Integration time of eye is short so faint objects cannot be seen without
using larger and larger telescopes
Example of unreliability of eye: Canals on Mars
Eta Carinae Images over
the past 100 years
Ground-based CTIO 4m
Hubble Space Telescope
A Revolution in Detectors: Photographic Plates
• 1840 J.W. Draper makes a photograph of the moon. Followed by
photographs of the Sun by Foucault and Fizeau
• Sunspots photographed in 1858 by W. De La Rue
• Jansen and Lockyer in the 1870s photographed the solar
spectrum and discovered the spectral lines of Helium.
• Ainsee Common photographed Orion Nebula and these revealed
stars and details you could not see in a telescope
• Photographs by Hubble in the early 1900‘s established that some
nebula where „island universes“ (i.e. galaxies). His spectral
observations of galaxies (exposures of more than one night) led to
the discovery of the expansion of the Universe.
• For 100 years photographic plates/film dominated the field of
astronomical detectors.
Single Channel Detectors: Photomultiplier Tubes
• Photomultiplers are photon counting devices that consist of vacuum
tube with a photocathode and focusing electrodes for amplification.
• Photocathode: light striking the photocathode produces an electron
through the photoelectric effect
• Focusing electrodes focus the electron toward electron multiplier or
dynode. Each dynode is held at a more positive voltage as the
previous one
• Electrons are accelerated as they approach the first dynode arriving
with greater energy.
• More low energy electrons are released and acclerated by the
second dynode.
•A cascade amplifies the number of electrons (1 photon can produce
up to 108 electrons)
• At anode an electric current is produced
In 1907 Joel Stebbins pioneered the use of photoelectric
devices in Astronomy
Spectral response of some commonly used
photocathode materials
Photomultiplier tubes: pile up errors
Each detected photon produces a pulse of finite duration, t0,
which causes a dead time in the detector. The number of
pulses (exposure time) is reduced by the amount of overlapping
deadtimes.
N = n/(1–t0n)
N is the true rate, and n the apparent rate
Pile-up
errors
System blocks completely at
high light levels
Disadvantages of Photomultiplier tubes:
• Must be operated at high voltages
• High light levels can destroy the tube
• Not very efficient in optical regions: 3500 to 10.000 Ang.
• Single channel devices (more on this later)
Multi-Channel Detectors (Arrays): Photographic Plates
Photographic plates were the first two-dimensional detectors
Advantages of photographic plates:
• Wide field of view. TLS Schmidt plates are 3 deg x 3 deg.
• Resolution, more so than a CCD of the same area
So why do we not still use plates?
Disadvantages of photographic plates:
• Low quantum efficiency. The best plates have a QE of about 3%
• Long exposure times, inefficient use of time
• Reciprocity failure. It becomes less effective as exposure time increases
• Non-linear color sensitivity. Plates are more sensitive to blue light
• Hypersensitising and Developing. Hypersensitising involves baking plates to
increase efficiency (up to 10%). You cannot see results until after developing usually
many hours later.
• Storage. They are fragile and take up space. They also decay with age.
• Digitisation. Must be scanned to put the data in digital form
• Cost and availability. In 1996 a single 30 cm x 30 cm plate costs $100 USD. Kodak
no longer makes plates.
Reciprocity Failure of Photograhic Plates
Cross section of a photographic plate
CCD: The Digital Revolution
Nomenclature:
• CCD = Charge Coupled Device
- A photon detecting device that exploits the photoelectric effect and the
semiconducting properties of silicon
- The voltage generated is coupled to the intensity of the incident light
• Pixel = picture element. Each pixel is an independent photon detector
• DN = Data Number (ADU = Analog to Digital Unit)
- The output signal from a CCD. Value and range depend on the nature of
the voltage digitization. E.g. an 8-bit CCD produces DNs from 0 to 28 (0 to
255). Most CCDs are 16 bit (maximum = 65535) or higher
CCD Properties and Definitions
lmax = 12400 Å/Eg
Material
Eg (eV)
lmax (Å)
Pure Si
1.1
11300
GaAs
1.43
8670
InSb
0.36
34400
n-type: a material with more than 4 valence electrons is added (As, from
group Va). The extra electrons cannot be accommodated in the valence
band and so occupy the conduction band. They represent a persistent set
of negative carriers
p-type: a material with fewer than 4 valence electrons is added (e.g. B,
from group IIIa). This has one fewer electron than normal and creates a
small "vacuum" in the electron sea of the valence band. This is called a
"hole." As valence electrons shift to fill it, the hole propagates like a positive
charge in the opposite direction. The holes represent a persistent set of
positive carriers.
From http://www.astro.virginia.edu/class/oconnell/astr511/lec11-f03.html
In the depletion region a photon produces an electron. This
migrates into the conduction band
The Basic Unit of a CCD
The basic element of a CCD consists of a Metal Oxide Semiconductor. The
bulk material is p-silicon on which an insulating layer of silicon oxide has been
grown as well as thin conducting electrodes of transparent polysilicon. The
central electrode is set to a positive bias while the two flanking electrodes are
negative. This creates a "depletion" region under the central electrode
containing no holes but a deep potential well to trap electrons. The region
shown is about 10µ thick. During exposure light enters through the "front-side"
electrodes. Photoelectrons generated under the central electrode will be
attracted toward the electrode and held below it. The corresponding holes will
be swept away into the bulk silicon.
From http://www.astro.virginia.edu/class/oconnell/astr511/lec11-f03.html
Reading out a CCD
A „3-phase CCD“
Parallel registers shift the charge
along columns
There is one serial register at the
end which reads the charge
along the final row and records it
to a computer
Columns
For last row, shift is
done along the row
Figure from O‘Connell‘s lecture notes on detectors
The CCD is first clocked along the parallel register to
shift the charge down a column
The CCD is then clocked along a serial register to
readout the last row of the CCD
The process continues until the CCD is fully read out.
How much charge is lost in the this charge transfer process?
Typical Charge Transfer Efficiency of a CCD is >99.999 %
Suppose you have a 4096 x 4096 CCD and detect 40.000 photons
(electrons). Signal to Noise ratio = √N = 200
Charge recorded = 40.000 x 0.999994096 = 38.394
1605 electrons „lost“
S/N decreased to 195
Typical CCD readout times are 90 – 240 secs, depending on the size of the
CCD. This is for single amplifier CCDs. To reduce the readout time some
devices can have 4 channels (amplifiers) for readout:
Serial register with
one amplfier
Normal readout
4 Serial registers
with 4 amplfiers
4 Channel CCD
4 channel CCD cuts readout time by a factor of 4. Problem: each
quadrant usually behaves differently, with its own bias, flat field
response, etc. In the data reduction 4 channel CCDs have to be
reduced as if they were 4 independent frames.
Quantum Efficiency of some science grade CCDs from ESO:
The real power of CCDs is their high quantum efficiency
Quantum Efficiency of CCDs compared to other devices
Two basic types of CCDs:
Front illuminated CCD: A CCD whose gate structure is located in front of the
potential wells. In other words the light has to pass through the gates
Back illuminated CCD: A CCD whose thickness is reduced to 10 mm so that it
can be focused on the front where there are no gates. A back illuminated
CCD is just a front illuminated CCD flipped and thinned. It is more efficient,
particularly in the blue.
Risk: There is a risk that in the thinning process the CCD will be destroyed
Thick Front-illuminated CCD
Thin Back-illuminated CCD
Front side (thick) versus Back side (thin) CCDs
For wavelengths shorter than
4000 Å electrons are
generated close to the
surface. Thus backside CCDs
have a much higher potential
for ultraviolet sensitivity than
front side thick devices
Front side (thick) versus Back side (thin) CCDs
Thin CCDs have a much higher quantum
efficiency, particularly in the blue. Most CCDs in
use are thinned as the thinning process is fairly
reliable and it is rare when a CCDs is destroyed
in the thinning process.
CCD Parameters Important for Observations
• Gain: Converts ADU to number of photons detected. Important for
Signal-to-Noise estimate. Typically 0.5–10 e–1/ADU
• Linearity: Detected counts should be proportional to the exposure
time. If a CCD has a non-linear regime these level of counts should be
avoided
• Readout Noise: Noise introduced by CCD readout electronics.
negligible for High Signal-to Noise observations
• Dark: Thermal noise. Neglible for High Signal-to-Noise Observations
Most science grades CCDs are kept at –120 C or cooler.
• Bias level: Constant level added to the data by the electronics to
ensure that there are no negative numbers
McDonald CCD for coude spectrograph:
Gain = 0.56 ± 0.015 e–1/ADU
Readout Noise = 3.06 electrons
Bias level = 1024
Noise Tests for CCD: Linearity
Mean Intensity
Take a series of frames of a low intensity lamp and plot the mean
counts as a function of exposure time
1.5 x 105
If the curve followed the red line at the high count rate end (and some CCDs do!) then you would know
to keep your exposure to under 150.000
Noise Tests for CCD: Gain
For Photon statistics the variance, s = √Photons. Therefore s2
should be a measure of the number of detected photons
• Take a series of frames at with a constant light level
• Compute s for frames
• Change the exposure time and take another series of frames calculating
a new s
• Plot the observed mean intensity versus the variance squared (s2)
• The slope is a measure of the gain
Mean Intensity
1.5 x 105
Problems and Pitfalls of CCD Usage
Saturation
If too many electrons are produced (too high intensity level)
then the full well of the CCD is reached and the maximum
count level will be obtained. Additional detected photons
will not increase the measured intensity level:
65535
16-bit AD
converter
ADU
Exposure time
Problems and Pitfalls of CCDs
Blooming:
If the full well is exceeded then charge starts to spill over in
the readout direction, i.e. columns. This can destroy data
far away from the saturated pixels.
Blooming
columns
Saturated
stars
Anti-blooming CCD can eliminate this effect:
Blooming
No blooming
One solution: Anti-blooming CCDs
Anti-blooming CCDs have
additional gates to bleed off the
overflow due to saturation
The problem is these
gates cover 30% of the
pixel. This results in
reduced sensitivity,
smaller well depth, and
lower resolution (gaps
between pixels has
increased)
Residual Images
If the intensity is too high this will leave a residual image. Left is a
normal CCD image. Right is a bias frame showing residual charge in
the CCD. This can effect photometry
Solution: several dark frames readout or shift image between
successive exposures
Fringing
CCDs especially back illuminated ones are bonded to a glass plate
SiO2
10 mm
Glue 1 mm
Glass
When the glass is illuminated by monochromatic light it creates a fringe
pattern. Fringing can also occur without a glass plate due to the
thickness of the CCD
l (Å)
6600
6760
6920
7080
7280
7460
7650
7850
8100
8400
Depending on the CCD fringing becomes important for wavelengths
greater than about 6500 Å
Signal-to-Noise Ratio
Readout Noise
0
1
3
10
Readout noise in electrons
Intensity
High readout noise CCDs (older ones) could seriously affect
your Signal-to-Noise ratios of observations
Basic CCD reductions
• Subtract the Bias level. The bias level is an artificial constant
added in the electronics to ensure that there are no negative pixels
• Divide by a Flat lamp to ensure that there are no pixel to pixel
variations
• Optional: Removal of cosmic rays. These are high energy particles
from space that create „hot pixels“ on your detector. Also can be
caused by natural radiactive decay on the earth.
Bias
Overscan region
Pixel
Most CCDs have an overscan region, a portion of the chip that is not
exposed so as to record the bias level. The prefered way is to record a
separate bias (a dark with 0 sec exposure) frame and fit a surface to this.
This is then subtracted from every frame as the first step in the reduction.
If the bias changes with time then it is better to use the overscan region
Flat Field Division
Raw Frame
Flat Field
Raw divided by Flat
Every CCD has different pixel-to-pixel sensitivity, defects, dust particles, etc that
not only make the image look bad, but if the sensitivity of pixels change with time
can influence your results. Every observation must be divided by a flat field after
bias subtraction. The flat field is an observation of a white lamp. For imaging one
must take either sky flats, or dome flats (an illuminated white screen or dome
observed with the telescope). For spectral observations „internal“ lamps (i.e. ones
that illuminate the spectrograph, but not observed through the telescope are
taken. Often even for spectroscopy „dome flats“ produce better results,
particularly if you want to minimize fringing.
CCD versus Photographic Plate
CCD
1/9 coverage
Cost: 120.000 Euros, ~1.000.000 Euros for
same FOV
Plate
3 x 3 deg (less since image is trimmed) in one
exposure
Cost 100 Euros
Requires 20 m telescope to detect same
number of photons as CCD
Cost per exposure over lifetime: < 1 Euro
CCD versus CMOS
CCD
CCD versus CMOS
CMOS (digital cameras)
• In CMOS detectors the photon is converted to a voltage at the pixel
level.
• Each pixel can be read independently at any time.
• No destruction of the charge and pixels can be read repeatedly to
reduce the readout noise
• CMOS detectors have use in infrared arrays. Less experience with
materials in the IR means that one cannot use a charge transfer
method to readout.
• No problems with blooming in CMOS CCDs. Just read out the charge
before it saturates. Can thus have a very large dynamic range.
Ultraviolet Detectors
Multi-anode Microchannel Plate Array (MAMA) Detectors
MAMA detectors are essential photomultipliers. They operate a smaller
voltages and are small so that they can be put into a two dimensional array.
MAMAs are photon counting devices and each photon has a time tag.
Quantum Efficiency of the Hubble Space
Telescope STIS instrument
And yes there is a PAPA detector…
Precision Analog Photon Address (PAPA) Detectors
Also for use in the UV.
See Papalios & Mertz, 1982, SPIE, Instrumentation in Astronomy IV,
331, p360
Ultraviolet Detectors
Another method uses a CCD as a hybrid. The photocathode
is used to produce an electron from the UV photon. These
are accelerated to produce a large number that strike a
phosphor that generates optical photons. Thse are coupled
to a CCD with fibre optics
X-ray CCD Detectors
CCDs can also be used for X-ray observations with some
differences to optical CCDs:
• Optical CCDs produce one electron per photon. Many photons
have to be detected before a measurable signal can be produced. A
single X-ray photon can produce 100-1000 electrons (1 electron per
3.6 eV of energy). With low amplifier noise this can be measured.
Thus X-ray CCDs are photon counting devices
• Requires one photon incident per pixel which means integration
times of ~secs for most astronomical sources
• Lower CTE with losses of about 1%
The detector of the future: a photon counter that measures each
photon, its time of arrival, and its energy. This will be a detector and
spectrograph in one.