by johnnydeep » Sat Jul 09, 2022 2:19 pm
Chris Peterson wrote: ↑Fri Jul 08, 2022 10:03 pm
johnnydeep wrote: ↑Fri Jul 08, 2022 9:18 pm
Chris Peterson wrote: ↑Fri Jul 08, 2022 9:10 pm
The sensor doesn't just respond to light, but also to heat. Each pixel is like a little bucket that holds electrons. In a perfect world, each electron would be produced by a single photon hitting that bucket. But in the real world, the silicon spontaneously produces electrons at a steady rate due to the fact that it isn't at absolute zero. (Cameras designed for long exposure astronomical imaging use cooled sensors; operating them at -30°C or colder isn't uncommon; professional setups operate even colder.) And each of those buckets can only hold so many electrons before it is full (saturated). Non-astronomical cameras tend to have problems with excessive
dark current (and the noise that comes with it) in exposures longer than just a few minutes.
From a practical standpoint, astronomical images usually benefit from being stacked because transient artifacts caused by planes, satellites, cosmic ray hits, and other such things are not consistent across frames, and are trivially removed in the stacking process. In a single exposure, there's no simple way to determine what is image and what is artifact.
Thanks! My naivete about all things astrophotographical strikes again. What is "dark current"? (heh - dark matter, dark energy, dark current
)
Dark current is the thermal electron flow/accumulation. So called because it accumulates... you guessed it... in the dark.
There is inherent statistical noise on any signal, which is equal to the square root of the signal. If you collect 100 photons, you only know the intensity ±10 (S/N = 10). Collect 10,000 photons and the uncertainty is ±100 (S/N = 100). That's why you want to collect as much light as possible: it improves the S/N. But the same statistics apply to dark current. It comes with its own noise, which just adds to the intensity uncertainty. The dark current itself can be subtracted off (assuming it hasn't saturated the pixel). But you can't remove noise.
Great, thanks. And of course, had I bothered to check Wikipedia first, I would have found an explanation of why it occurs (though I'm still in the dark about the fundamental physics here
:
https://en.wikipedia.org/wiki/Dark_current_(physics) wrote:Dark current (physics)
In physics and in electronic engineering, dark current is the relatively small electric current that flows through photosensitive devices such as a photomultiplier tube, photodiode, or charge-coupled device even when no photons enter the device; it consists of the charges generated in the detector when no outside radiation is entering the detector. It is referred to as reverse bias leakage current in non-optical devices and is present in all diodes. Physically, dark current is due to the random generation of electrons and holes within the depletion region of the device.
The charge generation rate is related to specific crystallographic defects within the depletion region. Dark-current spectroscopy can be used to determine the defects present by monitoring the peaks in the dark current histogram's evolution with temperature.
Dark current is one of the main sources for noise in image sensors such as charge-coupled devices. The pattern of different dark currents can result in a fixed-pattern noise; dark frame subtraction can remove an estimate of the mean fixed pattern, but there still remains a temporal noise, because the dark current itself has a shot noise. This dark current is the same that is studied in PN-Junction studies.
[quote="Chris Peterson" post_id=324028 time=1657317810 user_id=117706]
[quote=johnnydeep post_id=324027 time=1657315125 user_id=132061]
[quote="Chris Peterson" post_id=324026 time=1657314627 user_id=117706]
The sensor doesn't just respond to light, but also to heat. Each pixel is like a little bucket that holds electrons. In a perfect world, each electron would be produced by a single photon hitting that bucket. But in the real world, the silicon spontaneously produces electrons at a steady rate due to the fact that it isn't at absolute zero. (Cameras designed for long exposure astronomical imaging use cooled sensors; operating them at -30°C or colder isn't uncommon; professional setups operate even colder.) And each of those buckets can only hold so many electrons before it is full (saturated). Non-astronomical cameras tend to have problems with excessive [b][i][u]dark current[/u][/i][/b] (and the noise that comes with it) in exposures longer than just a few minutes.
From a practical standpoint, astronomical images usually benefit from being stacked because transient artifacts caused by planes, satellites, cosmic ray hits, and other such things are not consistent across frames, and are trivially removed in the stacking process. In a single exposure, there's no simple way to determine what is image and what is artifact.
[/quote]
Thanks! My naivete about all things astrophotographical strikes again. What is "dark current"? (heh - dark matter, dark energy, dark current :-))
[/quote]
Dark current is the thermal electron flow/accumulation. So called because it accumulates... you guessed it... in the dark.
There is inherent statistical noise on any signal, which is equal to the square root of the signal. If you collect 100 photons, you only know the intensity ±10 (S/N = 10). Collect 10,000 photons and the uncertainty is ±100 (S/N = 100). That's why you want to collect as much light as possible: it improves the S/N. But the same statistics apply to dark current. It comes with its own noise, which just adds to the intensity uncertainty. The dark current itself can be subtracted off (assuming it hasn't saturated the pixel). But you can't remove noise.
[/quote]
Great, thanks. And of course, had I bothered to check Wikipedia first, I would have found an explanation of why it occurs (though I'm still in the dark about the fundamental physics here :-):
[quote=https://en.wikipedia.org/wiki/Dark_current_(physics)][size=150]Dark current (physics)[/size]
In physics and in electronic engineering, dark current is the relatively small electric current that flows through photosensitive devices such as a photomultiplier tube, photodiode, or charge-coupled device even when no photons enter the device; it consists of the charges generated in the detector when no outside radiation is entering the detector. It is referred to as reverse bias leakage current in non-optical devices and is present in all diodes. Physically, dark current is due to the random generation of electrons and holes within the depletion region of the device.
The charge generation rate is related to specific crystallographic defects within the depletion region. Dark-current spectroscopy can be used to determine the defects present by monitoring the peaks in the dark current histogram's evolution with temperature.
Dark current is one of the main sources for noise in image sensors such as charge-coupled devices. The pattern of different dark currents can result in a fixed-pattern noise; dark frame subtraction can remove an estimate of the mean fixed pattern, but there still remains a temporal noise, because the dark current itself has a shot noise. This dark current is the same that is studied in PN-Junction studies.[/quote]