praktická astronomie
IR a UV astronomie,
detektory obecně v optickém, IR a UV oboru
zobrazovací prvky CCD, CCD kamery
cvičení
práce se CCD kamerou
IR astronomie
◼ 1800 William Herschel – vložení
teploměrů za červený konec
slunečního spektra
◼ 1878 Edison, měření IR během
zatmění Slunce, tasimeter
◼ počátek 20. stol. Pettit, Nicholson, M,
pl., hvězdy v IR
◼ 60. léta – InSb, HgCdTe – detektory
pro NIR
◼ FIR – balóny, letadla, družice podrobnější časová osa
Coolcosmos IR galeriewikipedia
UV astronomie
◼ úvod do UV astronomie
◼ přehled
◼ UV Astronomy (wiki)
vesmír mnoha vlnových délek
typy detektorů v optické, UV a IR oblasti elmg
spektra
◼ dvě skupiny detektorů
◼ kvantové (fotonové) – reagují na přímou interakci fotonů
◼ tepelné – reagují na vzestup teploty vlivem absorpce energie
záření
◼ v obou případech jsou nekoherentní, informace o fázi je
ztracena
typy detektorů
detektor princip spektrální oblast
fotočlánek, fotonásobič interakce s elektronem uv, světlo, ir
fotografická emulze chemická reakce uv, světlo, ir
charge coupled device
(CCD) „nábojově vázaný
snímač“
el. náboj uv, světlo, ir
fotovoltaický článek,
termočlánek
el. napětí uv, světlo, ir
bolometr, fotovodivostní d. rezistance ir
Golayova buňka tlak plynu ir
lidské oko chemická reakce světlo
charakteristiky detektorů
◼ kvantová účinnost (počet detekcí/počet dopadajících fotonů)
◼ linearita detektoru
◼ dynamický rozsah
◼ závislost citlivosti na vlnové délce
◼ šum
◼ integrační schopnost
◼ rozlišovací schopnost (prostorová)
◼ digitální výstup
kvantová účinnost
◼ krystaly fotografické emulze – jen pár %
◼ elektrony uvolněné fotoelektickým jevem – 5 až 40% ve
fotonásobičích
◼ vznik páru elektron-díra - ~ 50% v CCD
linearita detektoru
◼ požadavek na lineární „odezvu“
detektoru, aby R = F·t
◼ fotografická emulze není lineárním
detektorem
◼ CCD a fotonásobiče jsou lineární ve
velkém rozsahu
závislost citlivosti na vlnové délce
šum
◼ šum způsobený kvantovou podstatou záření
(Poissonovo rozdělení)
◼ šum pozadí (oblohy), způsoben změnou průzračnosti,
seeingem a scintilací
◼ Johnsonův šum – elektrony v detektoru vlivem
tepelných pohybů, potlačen chlazením
◼ „vyčítací“ šum
◼ elektronický šum
digitalizace
oko
oko - applet
oko
fotografická emulze
fotonásobič
◼ fotonásobič
◼ fotoelektrický jev
+ velmi rychlá odezva (ns)
+ mohou měřit i velmi slabé signály
+ povaha výstupu umožňuje použít „pulzní čítače“
- poměrně malá kvantová účinnost
- omezený spektrální rozsah
fotonásobič
◼ skleněná „lampa“ – obal
◼ fotokatoda
◼ dynody
◼ anoda
charakteristiky
◼ citlivost, zesílení, drift (změny v čase)
◼ temný proud
◼ „mrtvý“ čas
jiné detektory
◼ detektory STJ (Superconducting tunnel junction detectors)
◼ fotovoltaický článek
◼ termočlánek
◼ fototranzistor
◼ CID
◼ TV trubice
◼ IR detektory
◼ fotovodivostní detektor
◼ bolometr
CCD
◼ CCD aplety
◼ CCD v astronomii
◼ AAVSO CCD manuál
◼ přednáška ing. Cagaše
obsah
◼ prezentace obsahuje:
• co je to CCD
• jak funguje CCD
• možné problémy a jejich řešení
• výhody CCD (ve srovnání s fotografií);
What is a CCD?
The acronym CCD stands for Charge Coupled Device.
CCDs were invented at Bell Laboratories in the early 1970s.
It is this application which has revolutionised astronomy and which we will
study further in these Activities.
They were originally designed as computer memory but it quickly became
apparent that there were other uses for them. Their primary use today is as a
solid-state imaging device.
Astronomical use of CCDs
In this discussion of CCDs we will only consider their astronomical use. CCDs
are in widespread use today in all manner of devices (video and still cameras,
scanners etc.) but none of these applications are as demanding as astronomy.
Most CCDs are used in places where there is plenty of light available and so
concerns of efficiency are not relevant. Exposures are brief and so noise
sources are unimportant. The subject matter is forgiving and so cosmetic
blemishes are not noticed. Alas, this is not so when trying to detect a distant
galaxy.
Advantages of CCDs over film
CCDs have a higher quantum efficiency
(QE) than film. QE is a measure of how
efficient a device is in turning input
energy (in this case light) into a
measurable signal.
Greater efficiency means that more
data can be gathered in a shorter
time, or that in the same time you
can measure a fainter signal.
Best film
Amateur CCD
Professional CCD
CCDs have a linear response to light, i.e. the measured
signal is directly proportional to the amount of light which
was received. This is not true for film.
A linear response means that if
the exposure is doubled, then the
measurable signal will double.
Also, twice the signal means the
source is twice as bright.
CCD linear response Film non-linear response
More Advantages of CCDs over film
CCDs have a wide dynamic range. Coupled with their
linearity they can measure both very faint targets
and very bright ones.
• CCDs are dimensionally stable. The sensing elements (pixels – or
picture elements) are laid out in a regular grid formed on the silicon
substrate. This makes them excellent for most forms of positional
measurement.
• CCDs are digital and so modern computers can be put to use in
processing the images. No more messing about with photographic
chemicals or working in the dark.
Advantages of CCDs
◼This all adds up to a revolution in
astronomy:The increase in QE over
film is like making your
telescope into a much
bigger one - effectively
allowing a 1-m telescope to
perform like a 4-m.
The accuracy of CCDs in
both linearity and stability
means the measurements
made are of the highest
quality, and a wider band
of the spectrum is utilised.
The digital nature of CCDs allows new techniques to be
devised, both in taking the data and extracting the most
from it.
How do CCDs work?
There are 4 basic stages to CCD operation.
• Light (photons) is converted to a charge (electrons) by
the photoelectric effect in a layer of silicon.
• The charge is accumulated in “wells” during the exposure.
• At the end of the exposure the CCD is “read out” - the
charge is shifted to the readout register.
• Finally, the charge in each pixel is measured.
How do CCDs work?
An analogy is useful to picture the mechanisms involved in
how CCDs collect, transfer and count charge.
Imagine an array of buckets
ready to catch rain.
BucketSiphon pump
Rain gauge
A single master rain gauge
will be used to measure the
amount of rain caught in
each bucket.
The buckets are connected
by siphon pumps.
How do CCDs work? – charge transfer
To measure the rain in each
bucket (after the rain has
stopped), the siphon pumps
are used to move the
accumulated rain towards
the master rain gauge.
Different buckets
hold different
amounts of rain
How do CCDs work? – charge transfer
First, the end line of
buckets are emptied into
the empty row lined up
with the master gauge.
Contents of all buckets
move to left
How do CCDs work? – charge counting
Then each bucket in turn is siphoned
into the master gauge for measuring.
After each measurement, the master
gauge must be emptied before a new
measure can be taken.
Rain gauge
is emptied
How do CCDs work? – charge counting
Then each bucket in turn is
siphoned into the master gauge
for measuring.
After each measurement, the
master gauge must be emptied
before a new measure can be
taken.
Rain gauge
is emptied
How do CCDs work? – charge counting
Rain gauge
is emptied
Then each bucket in turn is
siphoned into the master gauge for
measuring.
After each measurement, the
master gauge must be emptied
before a new measure can be
taken.
How do CCDs work?
This shift-and-measure is carried on
until all the water along the
“readout register” has been
measured.
The remaining water to be measured
is siphoned along into the now
empty buckets (the readout register)
and the process of shifting-andmeasuring
is repeated.
How do CCDs work?
In the previous analogy, the raindrops represent photons
The accumulated water represents the charge detected by the CCD.
The buckets represent pixels on the CCD (and their depth represents
the well depth, or how much charge each pixel can hold).
The siphon pumps represent the CCD shift registers.
The master gauge is the sense capacitor (and the fineness of the graduations
represents the measurement accuracy).
How do CCDs work?
Of course CCDs are not quite so simple, but the
underlying electronics does a good job at mimicking
the analogy. CCDs are actually the most complex
electronic circuits fabricated today, mainly because of
their size and need for perfection over large areas of
silicon. This makes them expensive, too!
Lets start by looking at the individual pixels.
You should have been
able to spot from the
analogy some of the
potential problems
associated with CCDs.
By examining these
areas we will get a
better understanding of
how they work.
Problems with CCDs – Pixels
The physics of turning photons into electrons is well
understood and causes of efficiency loss can be controlled –
up to a point. The practical problems associated with the
design of CCDs is a limitation, however.
CCDs are 3-dimensional circuitry fabricated on a base of silicon (which is the light
sensitive layer). It isn’t possible for 100% of the front surface of a CCD to be free for
light to enter as there is nowhere for the circuitry which connects the pixels to go.
Therefore, the light has to go through the circuitry which causes obvious losses.
This is called “front-side illumination” and is what is used for most commercial
CCDs.
An obvious solution is to turn the CCD over and let the light fall on the back side
(“Back-side illumination”), but this has its own problem – there is then nothing to
support the silicon.
The thickness of the silicon also means that the charge can’t be held in the right
position and can drift – this is called “charge diffusion”. The silicon must be thinned
to a few tens of microns to avoid this, and supported in a special way.
However, thinned and back-illuminated CCDs are the norm in
professional astronomy today as they offer significant benefits, like 90%
or greater QE.
This is the first major difference between professional and amateur CCDs.
Another improvement in not losing light is to apply an antireflection
coating to the CCD surface. Again, normal in
professional CCDs.
In front-side illuminated devices, the loss of light is most apparent in the blue
end of the spectrum. One solution found was to coat the chip with “lumigen” –
an organic substance similar to the “glow” in highlighter markers.
Lumigen works by converting any photons short of 420nm to 520nm, thus
keeping the QE constant in the blue-UV part of the spectrum. Lumigen is cheap
compared to thinning and is available for some amateur CCDs.
Problems with CCDs – Charge transfer
How efficient are the siphons in moving the water between
buckets? Will every drop be moved or will some be lost? This is
called “Charge Transfer Efficiency” (CTE).
The earliest CCDs had a CTE of only ~98 %. Today CTE is typically better
than 99,995 % in commercial devices and much higher in scientific devices
(99,9999 %).
Poor CTE means that not all of the photons which arrived on the CCD will be
counted, and the further from the readout register the worse the effect.
Problems with CCDs – saturation
What happens when the buckets fill? This is a problem of both pixels and charge
transfer. The physical size of the pixel determines how much charge it can hold.
Larger pixels can hold more charge.
When the pixels are full, they are said to be saturated. What then happens depends
very much on the electronic design of the CCD. During readout, not all the charge
can be shifted – some is left behind. This leads to streaks (blooming or bleeding)
forming behind saturated pixels.
This can be minimised somewhat by the inclusion of electronic “drains” in the CCD,
called an Anti-Blooming Gate (ABG). Unfortunately, this also drains off wanted
charge and so reduces the QE of the device.
Problems with CCDs – Accuracy
How accurately can we measure the number of rain drops?
How finely graduated is the master gauge?
A CCD has an analogue output. Photons are converted to a charge and finally
to a voltage for measurement. An on-chip amplifier boost the signal to a useful
level. Is it possible to measure exactly how many photons fell on each pixel?
So far, the answer is not exactly. There are many reasons why it isn’t possible
to count electrons (e–) – the closest that can be achieved at the moment is an
RMS error of ~2e–. That’s close enough for most applications, but not all.
Amateur CCDs manage around 20-30 e– RMS*.
*Click here to find out more about RMS
Problems with CCDs – Noise
We’ve just seen “readout noise” – how accurately the number of electrons can be
measured. Unfortunately, there are other sources of noise in a CCD.
There’s thermal noise. Astronomical exposures tend to be long – from a few
seconds to many minutes – and many thermally induced electrons appear in that
time. There is no way to distinguish these from the photo-electrons which we wish
to measure.
The solution is to cool the CCD enough so that thermal noise isn’t a problem. This is
the next major difference between amateur and professional CCDs. Professional
CCD systems are in evacuated chambers and cooled to around 170°K (-100°C);
amateur CCDs barely manage -30°C. The difference is very noticeable.
This thermal – or “dark” noise – grows linearly with time and
is a function of the temperature of the CCD. Fortunately,
because it is fairly repeatable this “dark current” can mostly
be removed by careful calibration.
“Bias structure” is another source of noise but can also be calibrated out. The
electronics as well as the physical make-up of a CCD will imprint a certain
background structure to all images.
Finally, there is the problem that each pixel responds to light slightly
differently to its neighbour. Again this is an effect which we can calibrate and
remove.
Problems with CCDs – Summary
It might seem that there is a lot more to using a CCD than taking a photograph,
but some of the problems discussed also affect film – but aren’t possible to
control. With CCDs there is greater control and so it is possible to get so much
more from them. The benefits far outweigh the problems.
There are other problems which we’ve not mentioned – such as how the amplifier in
some CCDs glows! Defects in the silicon wafer causing non-fatal cosmetic
problems etc. Even the Universe is out to get you, sending cosmic rays which zap
new stars into existence on your exposure.
CCD Calibrations
How do we turn
this raw image... into this...
NGC 2736, part of the Vela SN remnant.
Imaged with 20cm f/4·5 Newtonian and
Cookbook 245 CCD camera.
CCD Characterisation
Engineers characterise their CCDs in a laboratory before they are put on a
telescope. This allows any problems to be corrected (or bypassed!) and then
allows astronomers to use them to their full potential.
The parameters which are needed are:
• The amplifier gain – how many electrons per count.
• The linearity of the amplifier and electronics – there will
always be some slight variation from perfection.
• QE and CTE – how good is the CCD.
• Any cosmetic or electronic blemishes (“trapping sites”, etc.) – every
CCD is unique!
CCD Calibrations
Amateurs don’t usually bother with such characterisation tests, nor can they do
much about them if present. Calibrations that can (and should) be done by
everybody are BIAS, DARK and FLAT FIELD.
Let’s start with the BIAS, which is a zero-length exposure designed to show what,
if any, underlying structure there is on the CCD and electronics.
The bias actually consist of two components; a non-varying level which is the
electronic zero-point, plus any structure present. Professional systems usually
produce an overscan region to allow the zero-point for each exposure to be seen.
CCD Calibrations – bias
Here is a bias frame from a typical amateur CCD.
The image is scaled
with only 6 ADU
from black to white
The mean level
is 100·8 ADUThe most obvious
structure is this bright
stripe on the left
Little other structure
is evident; statistical
variation is only 0·4
ADU so can be
considered quite a
clean bias.
CCD Calibrations – bias
The bias structure is a constant and may simply be subtracted from each
image.
As the readout noise is a significant part of the variation in each image, it is
better to average several (say 10–20) bias frames and create a master bias.
The bias should not change in the short term and so once a master bias has
been created it can be re-used until such time as the electronics are
changed.
Removing the bias is the first stage in image processing.
CCD Calibrations - dark
To remove the thermal content of an exposure, take a DARK frame. A dark
frame is the same length as a normal exposure but with the shutter closed so no
light falls on the CCD.
It is subtracted from a normal image, provided they are of the same duration.
(After the bias has been removed, of course.)
All images, including darks, contain the bias. A shortcut often used is to not
separate out the bias but subtract the dark+bias. This works well enough.
Again, statistical variations can be minimised if you average several dark
frames together.
CCD Calibrations
Removing the dark is the second step in image processing.
Here is a 4 minute dark frame from a typical amateur CCD.
The image is scaled
with 20 ADU from
black to white
The bright stripe is now insignificant
The mean level is
102.9 ADU (little
more than the bias),
but the maximum is
now 709 ADU
The statistical
variation is now 20
ADU and the whole
CCD is covered in
bright spots
CCD Calibrations – flat field
The next stage is to remove the pixel-to-pixel variations. This is done with a flat
field – an image of a featureless, uniform source (twilight sky is a good source for
this).
What a flat field shows is not only the minor pixel variations, but all the defects in
the optical train such as vignetting and dust spots which cause sensitivity to vary
across the frame.
The de-biased, dark subtracted image is divided by the normalised (image mean
reduced to 1) flat field. This enhances areas of low sensitivity and reduces areas of
higher sensitivity, creating a field with apparent uniform response.
Dividing by a flat field is the third step in image processing.
CCD Calibrations – flat field
Here is a flat field from a typical amateur CCD.
The image is scaled
with 50 ADU from
black to white
The mean level is 1800 ADU
with only 9 ADU variation, so
is actually quite uniform
The “dust donuts” here look bad, but represent a
variation of only 1% in the most extreme case.
(Dust donuts are an inverse “pinhole camera”
image of the telescope – with a central obstruction
in this case.)
The variation in
intensity from
centre to edge
represents a
change of only
1.7%
Summary
Once the quirks of CCDs are understood, the necessary calibrations become a
simple exercise (which can be carried out automatically under some software).
This Activity has concentrated on understanding the basics of CCDs. It has
shown both the advantages and some of the problems of electronic
imaging.
In the next Activity we shall see how to put all this knowledge to use and
take some CCD images.
Appendices
Some terms used when discussing CCDs:
ABG Anti-Blooming Gate. An electronic “drain” on pixels
to try to minimise bleeding due to over-saturation.
Has unwanted side effects like lowering QE.
ADC Analogue-to-Digital Converter; it converts an
analogue voltage to a digital count.
ADU Analogue-to-Digital Unit; one “count” out of a CCD
Bias The background level of the CCD
Bias Frame A zero length exposure to show the bias
structure of the CCD
Bleed or Bloom When a pixel is over-filled the charge
has to go somewhere, usually into ugly streaks.
Calibration frame An auxiliary image taken to help
calibrate a science exposure.
CCD Charge Coupled Device
Channel Stop An electronic structure on a CCD to
stop the charge in a pixel from migrating.
Clocks, clocking etc. The charge on a CCD is moved
around by stepping (or clocking) voltages. There are various
electronic signals which control this.
CTE Charge Transfer Efficiency; how good the electronics
are at shifting the accumulated charge around.
Dark current The rate of build-up due to thermal noise
Dark Frame An exposure to measure the dark current
Exposure The time the CCD is exposed to light
Flat Field An image of a blank target designed to
show imperfections in the CCD and imaging system
Full-frame Device A CCD which has its entire area
exposed to light. It needs an external shutter to
stop it from being exposed to light during read out.
Frame Transfer Device A CCD which has only half its
area exposed to light, and the other half covered.
The exposure is transferred first to the covered
area before being read out.
Gain The number of e– per ADU
Interline Transfer Device A CCD which has adjacent active and
readout columns. Not widely used in astronomy.
Image processing The art and science of calibrating a digital image
(not necessarily CCD) to extract the most information from it.
Lumigen A fluorescent coating which can be applied to a CCD to
improve its UV/blue response.
Overscan By reading out more pixels that actually exist on the
CCD, you create an overscan strip. This gives the bias level on an
exposure.
Pixel Picture element; the resolution element of the CCD
approximately 6 to 30µm in size (not always square).
QE Quantum Efficiency; how well the device responds
to light of different wavelengths
Saturation When a pixel well is full. If it continues to
receive light it may bleed (or bloom). See ABG.
Shift Register The mechanism by which charge is
shifted around on the CCD.
Readout Noise The accuracy to which the charge in
a pixel can be measured. Usually given as e– RMS
Readout Register The place on a CCD where the
charge is measured
Trapping site A defect on a CCD which impedes the flow
of electrons
Well depth How many electrons a pixel can hold before
saturating
Return to Activity
Average value versus RMS
◼In engineering and physics, sets of data often
“alternate”: they vary from positive to negative and
spend equal amounts of time in each state.
◼If you found the average value of the data in the
normal way, you would obtain zero.
◼The average values are shown on the graphs in blue.
Because of this, the usual type of average gives very
little information when information or energy is obviously
being transmitted and work is being done (as in the use
of alternating electric current).
In situations like that, scientists and engineers tend to
use the RMS value instead.
time
signal
time
signal
time
signal
alternating square wave
sawtooth wave
sine wave
RMS
◼RMS stands for Root Mean Square.
◼To find the RMS value for a series of data,
◼ the data are all squared
◼ the mean (or average) of the squares is found
◼ the square root of this mean is taken.
◼The rms values are shown in green.
time
signal
time
signal
time
signal
alternating square wave
sawtooth wave
sine wave
The power of a signal, or its intensity, are very often
related not to the signal’s amplitude, but instead, to the
square of the signal’s amplitude.
For that reason, the RMS value usually gives a very
useful indication of the average power or intensity of a
signal, and is a standard measure used in many
branches of physics and engineering.
cvičení
práce se CCD kamerou
příklady CCD a CMOS kamer MI (ing. Cagaš)
... the end ...