Astro Tutorial #1.17: Stacking - Exposure Time & SNR
Summary
TLDRIn diesem Video erklärt Chris das Konzept des Stackings in der Astrophotografie. Er führt das Publikum durch die Grundlagen, zeigt, wie längere Belichtungen mehr Informationen liefern und wie das Stacken von Kurzbelichtungen das Rauschen verringert und die Bildqualität verbessert. Es wird auch auf die Signal-Rausch-Quotient (SNR) eingegangen, der die Verhältnismäßigkeit von Signal und Rauschen angibt. Chris betont, dass man beim Aufnahmen von Bildern stets an die Gesamtbelichtungszeit denken sollte, anstatt nur an die Anzahl der Bilder.
Takeaways
- 📸 **Astrofotografie und Stacking**: Stacking ist ein Schlüsselbegriff in der Astrofotografie, der sowohl für planetare als auch für Tiefraum-Bilder verwendet wird.
- 🕒 **Langzeit-Belichtungen**: In Astrofotografie geht man normalerweise von mehreren Stunden Belichtungszeit aus, um genügend Information über das Objekt zu sammeln.
- 🔍 **Einfluss der Belichtungszeit**: Je länger die Belichtungszeit, desto mehr Details und Information können aufgezeichnet werden, aber auch desto mehr Rauschen entsteht.
- 🌌 **Rauschen und Signal-zu-Rauschen-Verhältnis (SNR)**: Rauschen entsteht bei kurzen Belichtungen, da die Verteilung der Photonen zufällig ist. Ein hohes SNR bedeutet, dass das Signal stärker ist als das Rauschen.
- 🎲 **Gesetz der großen Zahlen**: Durch das Aufnehmen mehrerer Kurzzeit-Aufnahmen und das Hinzufügen der Daten können wir das wahre Muster der Verteilung erkennen, was das Rauschen verringert.
- 📈 **Stacking-Prozess**: Das Stacking ist ein Prozess, bei dem mehrere Kurzzeit-Aufnahmen addiert und durchschnittlich ausgeglichen werden, um ein klares Bild zu erzeugen.
- 📊 **Statistische Verteilung**: In Astrofotografie wird oft auf statistische Verteilungen zurückgegriffen, um das Rauschen zu reduzieren und die Bildqualität zu verbessern.
- 📉 **Einfluss der Kamerasensoren**: Jeder Pixel eines Kamerasensors kann nur eine bestimmte Menge an Daten aufnehmen, bevor es übersättigt wird. Stacking hilft, diese Grenzen zu überwinden.
- 🌟 **Optimale Belichtungszeit**: Es gibt keine feste Regel für die ideale Belichtungszeit, aber im Allgemeinen sollte sie in der Größenordnung von Stunden liegen, um ein klares Bild zu erzeugen.
- 🔧 **Technische Herausforderungen**: Es gibt verschiedene technische Herausforderungen bei der Aufnahme von Astrofotos, wie zum Beispiel das Rauschen durch den Lesevorgang des Sensors.
Q & A
Was ist das Prinzip hinter dem Stacking in der Astrophotografie?
-Stacking ist ein Verfahren, bei dem mehrere kurze Belichtungsbilder zu einem langen Belichtungsbild zusammengefügt werden, um mehr Details und weniger Rauschen im Ergebnisbild zu erreichen.
Warum ist das Sammeln von Photonen wichtig in der Astrophotografie?
-Photonen sind die Grundeinheiten des Lichts, die von Himmelskörpern reflektiert oder emittiert werden. Indem man sie sammelt, erhält man Informationen über diese Objekte, und je länger die Belichtung, desto mehr Informationen kann man sammeln.
Was ist der Unterschied zwischen kurzen und langen Belichtungen in der Astrophotografie?
-Kurze Belichtungen sind noisier und können nicht so viele Details wie lange Belichtungen aufnehmen, da jedes Pixel nur eine begrenzte Menge an Daten aufnehmen kann, bevor es übersättigt ist. Lange Belichtungen sammeln mehr Daten und sind daher weniger rau.
Wie erklärt der Law of Large Numbers das Stacking in der Astrophotografie?
-Der Law of Large Numbers besagt, dass mit zunehmender Anzahl von Messungen die Variabilität der Ergebnisse abnimmt und die Wahrscheinlichkeitsverteilung näher am Erwartungswert liegt. In der Astrophotografie bedeutet dies, dass mehr Bilder (Mehrwürfelwürfe) das wahre Muster der Verteilung aufdecken und somit Rauschen reduzieren.
Was ist der Hauptgrund, warum kurze Belichtungen rau sind?
-Kurze Belichtungen sind rau, weil die Photonenverteilung auf den Sensoren zufällig ist und nicht genug Photonen sammeln, um ein klares Bild der Objekte zu erhalten. Dies führt zu einer unebenen Verteilung, die als Rauschen wahrgenommen wird.
Wie wird das Rauschen in Astrophotografiebildern reduziert?
-Rauschen wird reduziert, indem man mehrere Bilder miteinander stackt. Je mehr Bilder man addiert, desto glatter wird die Verteilung der Pixelwerte, was zu einem rauschenfreieren Bild führt.
Was ist der SNR und warum ist er wichtig in der Astrophotografie?
-Der SNR (Signal-to-Noise Ratio) ist das Verhältnis zwischen dem Signal eines Himmelskörpers und dem Rauschen im Hintergrund. Ein hoher SNR bedeutet, dass das Signal stärker ist als das Rauschen, was zu einer klareren Darstellung des Objekts führt.
Wie viele Bilder sollte man normalerweise stacken, um ein gutes Astrophotografie-Bild zu erhalten?
-Es gibt keine feste Anzahl, da es von der gewünschten Gesamtbelichtungszeit, der Kamera und dem Objekt abhängt. Allerdings sollte man versuchen, eine Gesamtbelichtungszeit in Stunden zu erreichen, um ein klares Bild mit geringerem Rauschen zu erhalten.
Welche Faktoren beeinflussen die Länge der einzelnen Lightframes in der Astrophotografie?
-Die Länge der Lightframes wird beeinflusst von der Kameraspezifikation, der Sensorempfindlichkeit, der Belichtungszeit und dem gewünschten Detailgrad des Objekts. Technische Faktoren wie Readout-Rauschen oder die Schwelle für die Photonenerfassung können ebenfalls eine Rolle spielen.
Was ist der Hauptunterschied zwischen einer langen Belichtungszeit und vielen kurzen Belichtungen?
-Obwohl mathematisch gesehen eine lange Belichtungszeit und viele kurze Belichtungen das gleiche Ergebnis liefern sollten, wenn die Gesamtbelichtungszeit gleich ist, kann die Praxis aufgrund von technischen Rauschen und anderen Faktoren zeigen, dass längere Lightframes oft bessere Ergebnisse liefern.
Outlines
📸 Grundlagen des Stackings in der Astrophotografie
Dieser Absatz behandelt das Konzept des Stackings in der Astrophotografie, das häufig in der Aufnahme von Planeten- und Tiefraumbildern verwendet wird. Chris erklärt, dass das Sammeln von Lichtbildern von Himmelskörpern das Erfassen von Informationen über Photonen durch den Kamerasensor ist. Je länger die Belichtung, desto mehr Informationen werden gesammelt. Da keinerlei Tracking über Stunden ohne Fehler möglich ist und langwierige Belichtungen das Bild übersättigen würden, werden mehrere kürzerer Belichtungen genommen, die als 'Light Frames' bezeichnet werden. Diese haben jedoch den Nachteil, dass sie weniger Daten enthalten und rauere Bilder erzeugen. Chris führt das Gesetz der großen Zahlen ein, um zu erklären, warum kurze Belichtungen rauer sind und wie durch das Hinzufügen von Daten die Verteilung im Bild geglättet werden kann.
🔍 Stacking als Lösung für Rauschen und Datenmangel
Chris führt das Stacking als Methode ein, um das Rauschen in Astrophotografie-Bildern zu reduzieren und mehr Daten zu sammeln, indem er mehrere kurze Belichtungen addiert. Er verwendet ein Python-Programm, um ein Beispiel zu demonstrieren, wie durch das Hinzufügen von Daten die Verteilung im Bild geglättet wird. Er erklärt, dass durch das Addieren der Werte der Pixel in den einzelnen Bildern und durch das Dividieren durch die Anzahl der Bilder die Helligkeit konstant gehalten wird. Chris zeigt, wie das Stacking die Rauschen reduziert und die Signalverteilung im Bild klarer wird, ähnlich wie bei einer langen Belichtung. Er betont, dass das Signal-Rausch-Verhältnis (SNR) ein wichtiger Faktor ist, um die Qualität von Astrophotografie-Bildern zu verbessern.
🌌 Praxistipps für das Stacking in Astrophotografie
In diesem Absatz diskutiert Chris die praktischen Aspekte des Stackings, einschließlich der Frage, wie viele Bilder man aufnehmen sollte, um ein flaches Bild zu erhalten. Er betont, dass die Gesamtbelichtungszeit und nicht die Anzahl der Bilder entscheidend ist. Er erklärt, dass die Kamera-Sensor-Öffnung und -Empfindlichkeit die Menge der gesammelten Photonen bestimmen und somit die Bildqualität beeinflussen. Chris gibt Tipps für die optimale Belichtungsdauer pro Bild und warnt davor, zu kurze Belichtungen zu verwenden, da sie zu technischen Störungen führen können, die das Signal überlagern. Er empfiehlt, so lange wie mögliche Belichtungen zu nehmen, um die Aufnahmezeit und den Download zu reduzieren.
🌟 Zusammenfassung und Ratschläge für Astrophotografie
Chris fasst die wichtigsten Punkte des Videos zusammen und gibt Ratschläge für Astrophotografen. Er betont, dass Bilder Daten benötigen, um die physischen Objekte am Himmel darzustellen, und dass Stacking das Hinzufügen von Daten zur Verbesserung der Bildqualität ist. Er erinnert daran, dass das Signal-Rausch-Verhältnis durch das Gesetz der großen Zahlen beeinflusst wird und dass mehr Daten zu weniger Rauschen führen. Chris empfiehlt, die Öffnung und Empfindlichkeit des Teleskops und der Sensor zu berücksichtigen und die Gesamtbelichtungszeit für ein Projekt in mehreren Stunden zu messen. Er schließt mit einem Aufruf, die Telescope während der Belichtung zu genießen und die Freude der Astrophotografie zu erleben.
Mindmap
Keywords
💡Stacking
💡Photonen
💡Belichtungszeit
💡Signal-zu-Rausch-Verhältnis (SNR)
💡Rauschen
💡Bildsensor
💡Law of Large Numbers
💡Astrophotografie
💡Lichtrahmen
💡Gesamteinsatzzeit
Highlights
Astrophotography involves gathering information about celestial objects by capturing photons using a camera sensor.
Longer exposures gather more information, but tracking errors and sensor limitations prevent hours-long error-free tracking.
Each pixel can only hold a certain amount of data before reaching a maximum brightness level, known as 'white'.
Short exposures are noisy due to the random distribution of captured photons, which is subject to the law of large numbers.
Long exposures smooth out the distribution of photons, resulting in a cleaner image compared to short exposures.
Stacking involves taking multiple short exposures and combining them to simulate a longer exposure, reducing noise and enhancing detail.
Stacking averages the values of multiple images to smooth out the distribution and enhance the signal-to-noise ratio (SNR).
The signal-to-noise ratio is crucial in astrophotography as it indicates the clarity of the desired signal against the background noise.
Brighter objects in astrophotography are less noisy because more photons are captured, leading to a smoother distribution.
Deep-sky imaging aims to reduce background noise as much as possible to increase the SNR and reveal faint details.
The number of images to stack depends on the total integrated exposure time rather than just the number of frames.
There is no one-size-fits-all answer for the best number of frames to stack; it depends on the specific imaging project.
Technical noise sources, such as read noise, can affect the quality of stacked images, especially with short exposures.
The ideal length for individual light frames should be as long as possible to minimize the number of frames and post-processing time.
Stacking can still produce impressive results with hundreds of short exposures, although longer subs are generally preferred.
The final stacked image's quality highly depends on the quality of the input light frames and the camera's sensitivity.
Astrophotographers should consider the law of large numbers when planning their imaging sessions to optimize data collection.
Practical advice for astrophotographers includes considering the camera's aperture and sensor sensitivity when planning exposure times.
The integrated exposure time for a project is typically measured in hours, emphasizing the importance of patience in astrophotography.
Transcripts
hey folks it's Chris welcome back so
today's episode will be about the term
stacking you will hear this term a lot
in astrophotography both planetary and
deep-space imaging and I thought you
should know at least something about
this term so we're gonna dive into the
fundamental theory of stacking do a
little bit of math to understand what
all this is about ready let's go so if
we break it down to the most basic level
taking images of objects in the night
sky means we gather information about
that certain object and we do that by
catching photons hence the name of that
certain object using our camera sensor
the longer the exposures the more
information we gather about this object
in astrophotography we normally talk
about hours of exposure time but
unfortunately no track amount can
support hours of error-free tracking and
in addition to that if we expose hours
of data on to your camera sensor
everything will be just white that's the
case because each pixel can only hold a
certain amount of data before calling
this white with 8-bit images that's 256
star points in each of the RGB channels
and then that's white and adding 10
additional data points won't change
anything because there's no whiter than
white
so whatever and because of this problem
instead of taking one hour long exposure
we take only say exposures of a few
minutes those single
so-called light frames suffer from two
main problems a they obviously can't
contain as much data as the long
exposures will have no kidding and B the
image will be quite noisy so and both
problems are naturally heavily related
so what's the deal
why is a short exposure noisy and a long
exposure is not when taking images of
objects within the night sky we struggle
for every bit of information there's a
great book by Steve Richards making
every photon count and that's exactly it
so while waiting for incoming photons we
are effectively rolling the dice we
can predict how many photons from a
certain object will hit the sensor
during a given period of time within a
given interval of certainty we call
those incoming photons from the target
signal and we can predict that there
will be hopefully far less photons
coming from the glow of the surrounding
cities going but we can't predict where
the next photon will hit that's totally
random so we are rolling the dice and
therefore submitted ourselves to the law
of large numbers and here we need to do
some basic statistics I'm sorry guys
this is a Python program I wrote that
throws the dice for us imagine each of
the numbers representing a pixel on the
camera sensor now if you start the game
and wait say what is that twenty rounds
after those rounds there is no certain
pattern the distribution seems to be
random is pixel 5 a signals stronger
than the others or are four and six
extra dark background pixels this uneven
distribution of these short exposures is
what we later call noise but if you
continue the game and wait for no we
have a few thousand throws then the more
throws you get the more say the true
character of the distribution will
unveil itself and so this is the
representation of a long exposure and
this pattern is nearly evenly spread
over all the background pixels but
indicates a significant signal within
the pixel 1 and for sure there's our
target in this final distribution the
solitary pixels are no longer all over
the place but follow more or less sort
of the true nature of what they
represent and thereby we get a smooth
image from the long exposure comparing
to the noisy image from the short first
exposure and so that's the law of large
numbers and we have to obey this law and
astir imaging all the time taking an
image of a target and the night sky
means unveiling the true distribution of
it and to do that many dices need to be
thrown in other words we need a long
exposure time otherwise the result will
be unevenly spread over our pixels and
the image is noisy poof so hey we wanted
to talk about stacking didn't we so
let's get back to it let's see some
action
okay stays from Astro states did a
fantastic video about stacking she used
a hunt drawn grid to illustrate a camera
sensor I really liked the video and the
idea and stuck to it definitely check
out her video I took the idea one step
further and wrote a Python script to
generate a totally fictional nine by
nine pixel camera sensor with simulated
photons sitting in totally arbitrary
time intervals the underlying
distribution represents something like a
doughnut shape maybe a planetary nebula
I don't know it was quite a fun as
mentioned we can't take hours of
exposure time with just one single frame
that's just not possible but we do need
the information of those hours so what
we can do is take multiple short
exposures and simply add all the data
add all the information together simple
as that and in the most basic form this
is stacking by adding all those images
together we simply enlarge the numbers
we threw the dice doublet triplet etc
etc and thereby we smooth the
distribution of the image when doing
basic stacking we do one more thing you
see here that we add everything up and
then we divide the whole set by the
numbers of light frames we take we do so
in order to keep the brightness level
constant otherwise the image would get
brighter and brighter with every image
that is added to the stack it's a little
trick but it doesn't change any math
behind it the throwing the dice program
did it to see the length of the bars and
this diagram never exceeded the fixed
height the program factor thumb down but
the distribution got evenly spread
either way to make the point again to
even out the image in other words to
smooth it we need to add additional
information so that's the plus that
gains us access to the law of large
numbers and I know is free image the
division by N only keeps the brightness
level constant nothing else now let's
see some stacking and simulated action
here we have two single images from the
simulated center the number Z are
totally arbitrary let's just pretend
that these some of the incoming photons
gave this little pixel here a value of 1
three one the same pixel on the second
light frame has a value of 1.1 so 1.3 1
plus 1 dot 1 equals 2 dot 4 1 & 2
average means we divide 2.4 1 by 2 and
that equals 1 dot 2 0 5 so the value of
the pixel after stacking the first two
images is 1.2 0 5 somewhere between the
first and the second sub but the magic
is not done with two images though this
first stack image does look a lot
cleaner than the first two subs that's
for sure look at this this is the
process going fast through a series of
hundred light frames you see that the
stacking process produces a very smooth
image in the end equally smooth as the
single but a hundred times longer
exposure and this method of adding the
data to smooth the image is still valid
even if the image is so noisy that you
can't recognize the signal distribution
in one single frame I mean look at those
very short light frames they appear to
be a messy mess everything is just all
over the place but beneath all that
random noise there is a slightly higher
probability for a photon to hit the
pixels representing the imaged object so
all we have to do is add more data more
information so that over time we get a
grip on the underlying distribution and
for sure there it is it's still a bit
noisy due to the short exposure time I
would have needed 10 times more data to
look as smooth as the final image from
the last taking process or look at that
this say chart with a take an ultrashort
thus noisy exposures and this is a chart
listing the stacking process you can see
the noise decrease and the signal
building up over time that brings us to
the next and for practical usage most
important subtopic of stacking the snr
the snr is the so-called signal-to-noise
ratio you therefore divide a signal
value by the corresponding noise value
and get a number the ratio indicates the
higher the snr the bigger is the signal
value compared to the noise value and in
astrophotography this always means to
bring down the noise and we are
especially concerned about the noise of
the dark background because the bright
objects we want to capture are not that
noisy but why is that so it does trace
back itself toodle-oo
of large numbers in our initial game of
dice remember the more often you throw
the dice the smoother the distribution
of the results will be the first ten
rounds will look like a messy mess but
the longer you throw the dice the
smoother the distribution will be so
that's the reason with brighter objects
as well more photons from the objects
means we played the game of dice more
often in this area and so naturally the
distribution will be much smoother
compared to the dark background area
with only a few thrown dices aka lonely
photons arrived yet imaging deep sky
objects thereby always means to get
enough information to reduce the
background noise as much as possible
thereby you raise the SNR and you will
be able to stretch the dodging your
final processing steps much harder but
more on that later but don't get me
wrong hire as an r also means a smoother
signal in the brighter target regions as
well it's not all about the background
look at that image both images show the
same region of the great Orion Nebula
the left dimensions worth one hour of
exposure time where's the right
dimensions worth three hours both images
are no cracker but you can see the
difference between them left is noisy
the right image is much smoother same
camera same processing just different
exposure times and having this extra
information will allow you to bring out
fine details of the nebula without
having to deal with noise too much so
all in all bringing the SNR up by adding
more data mainly means dumping the
background noise but it also softens the
color gradient within your target so now
how many images should you capture in
order to stack them together to get a
nice and flat image when should you stop
taking images at night this question is
a very urgent one for astro beginners as
well as for so-called pros 20 images or
30 images but unfortunately there's no
general answer because the question
kinda goes in the wrong direction first
step would be not to ask for the best
number of frames but for the total
integrated exposure time so the added
exposure lengths for all our light
frames hanging obviously say 20
10-second frames won't do the same job
as 20 10 minute frames I think that's
clear
so this integrated exposure
should be in the area of hours for each
project the image quality improvement
thereby will be gradually one hour and
you can see the object another and you
will see fainter details another and the
oval noisiness will get low another nd
background starts to equalize this is
again down to the law of large numbers
the aperture and the sensitivity of your
camera sensor defines how many photons
we capture and process and the more
information we collect in a given area
the smoother the area will look like so
the question is not how many light
frames but what's the integrated
exposure time for which telescope in
which camera for which deep sky object
everything depends now the second big
question is how long shall the
individual light frames be and so there
are two answers right here and quite
opposite ones technically I mean from a
purely mathematical standpoint there
should be no difference between a few
long and many short subs as long as the
total integrated exposure time is the
same so for regular background noise and
image smoothness only without any other
influences ten one-minute subs should be
and I say should be the same as one ten
minute light frame the amount of photons
hitting the sensor will be the same the
amount of information to smooth the
distribution of our image will be the
same so the noise level of that
distribution should be the same one
individuals up of course will look a lot
more noisy but on the other hand for the
same total integrated exposure time you
would simply take much more subs and I
mean much more subs with our example
you'd need to take ten times the number
of light frames ten times more memory
usage downloading time stacking time etc
etc avoiding this is the first but a
minor reason against short 10-second
subs but purely mathematically speaking
no matter how noisy the image seems as
long as there's on average one photon
more showing of inside your target area
than in the surrounding background this
signal will sooner or later show up
given enough subs to stack why shouldn't
it look at that live frames they are
nearly entirely made out of noise but on
every
every 20 image or so some pixels
contains slightly more signal than the
average noise level and so it's just a
matter of adding information and
information information up and up and up
and up and finally you see in the end
the slightly more present information
will show up but sad thing told in real
life this doesn't work it just doesn't
it does tell us though that we can stack
images out of short exposures and if you
fail to produce longer subs out of
technical reasons like me here
I fail to poll a line and my old man was
a kinda over challenged so I could only
produce 20 second subs
I ended taking hundreds of them each
with nearly no signal and in the end I
at least got some fine structure inside
the Crab Nebula m-1 not bad for the
ancient desire was working with but in
the end where there is no signal
there will be no signal and the final
stacked image highly depends on your
inputs ups that's for sure and the main
reason there are other influences
despite the random background noise our
camera has several different other noise
sources mostly technical based one of
them is Reed noise a noise or rather a
distortion of the information
distribution a slight uncertainty
created by accessing the individual
pixels during readout those errors will
raise the number of sub Staton even
though we can fight against those more
later and if those noise patterns
overlay your signal well that's bad
another factor though I'm not quite sure
about this might be some kind of
threshold within our sensor say a sensor
delivers the first bit of information
only if more than X photons hit then in
every sub too short to significantly
gather more than this X photons your
signal would effectively be deleted out
of these subs or am i doing a mistake
here leave your comments below but all
in all thumb rule for Astro imaging take
as long subs as you can then you only
need to take a few of them to get to the
desired integrated exposure time less
downloading less taking etc etc some
targets though have
right course so you might get a bunch of
shorter exposures too just to not let
the core burn out more on that later
when we dive into the programs you can
use to stack just keep the histogram
one-third on the left side that way you
play safe as a first guessing lots of
folks tend to use two or three minute
exposures some go deep with 10-15
minutes but others produce sweet images
with shorter ones go and play huge yeah
so that was taking and stuff so
takeaways our images need data to show
the physical objects in the night sky
stacking simply means adding more data
to the pool averaging the values not too
overexposed everything the ratio between
signal and noise depends on the law of
large numbers throw the dice often
enough and the distribution will be even
smoother more data less noise software
details on faint nebula structures think
in exposure time and not in substation
keep the aperture or fastness of your
scope and the sensitivity of the sensor
in mind normally the all and all stacked
and integrated exposure time for a
project is measured in multiple hours so
don't do target hopping during one night
take your time subs may be as long as
possible but either way it is possible
to produce stunning things using
hundreds of short subs normal sub
lengths are measured in minutes and
that's it boy was the day right you
stayed with me be proud of you but
therefore I hope you really learn
something about the underlying
principles of stacking and data
collection during an Astra imaging
session and while your telescope tracks
reliably the sky while your camera takes
light frame after light frame sit back
take a pair of binoculars and just gaze
it's so refreshing and that's it thanks
for watching as always hit like and
subscribe if you liked it or do you know
any newcomers to this hobby lead them
here maybe they can find some useful
information here and as always I say
please guys everyone until next time
here on catching photons
[Music]
you
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