APOD: Roots on a Rotating Planet (2022 Jul 08)

[APOD Robot]

Roots on a Rotating Planet

Explanation: With roots on a rotating planet, an old tree is centered in this sequence of 137 exposures each 20 seconds long, recorded one night from northern Sicily. Digital camera and fisheye lens were fixed to a tripod to capture the dramatic timelapse, so the stars trailed through the region's dark sky. Of course that makes it easy to spot the planet's north celestial pole. The extension of Earth's axis of rotation into space is toward the upper left, at the center of the concentric star trail arcs. The Milky Way is there too. The plane of our galaxy stretches across the wide field of view from north to east (left to right) creating a broader luminous band of diffuse starlight.

<< Previous APOD

This Day in APOD

Next APOD >>

[orin stepanek]

The roots, trunk, and a few scraggly branches seem to be all there
is left of this tree! Looks like it tangled wit a twister!

The star trails make it a nice centerpiece though!

[Bellerophon]

Old tree? This tree is dead!

It has kicked the bucket, shuffled off its mortal coil, and joined the choir invisible.
It is an ex-tree.

[raschumacher]

Does anyone else get an impression that the star field rotates slightly between the still image and the trailed image? I get a strong impression of rotation near the pole, but no such impression far from the pole. My GF gets no such impression in any part of the image. Weird.

[Ann]

Nice, Marcella. I like the fact that we get one "non-trail" image where we can see the stars clearly, and then another one with a dizzying amount of trails.

Can't resist posting two Orion trail images:

Orion rising behind plough. Image: Alan Dyer.

Orion's Belt and the Orion Nebula trails. Image: Seve Baker.

I love these two Orion trail images. As for Steve Baker's image, the most obvious triple parallel lines (above center, to the right) are Orion's Belt, of course. The slightly thicker, slightly violet-blue line is the Orion Nebula.

Ann

[Marcella Giulia Pace]

very beautiful, thank you for showing them

[Marcella Giulia Pace]

Nice, Marcella. I like the fact that we get one "non-trail2 image where we can see the stars clearly, and then another one with a dizzying amount of trails.

Can't resist posting two Orion trail images:

Orion rising behind plough Alan Dyer.png

Orion rising behind plough. Image: Alan Dyer.

Orions Belt and Orion Nebula trails Steve Baker.pngOrion's Belt and the Orion Nebula trails. Image: Seve Baker.

I love these two Orion trail images. As for Steve Baker's image, the most obvious triple parallel lines (above center, to the right) are Orion's Belt, of course. The slightly thicker, slightly violet-blue line is the Orion Nebula.

Ann

Very beautiful. Thanks for showing them
Marcella

[johnnydeep]

Does anyone else get an impression that the star field rotates slightly between the still image and the trailed image? I get a strong impression of rotation near the pole, but no such impression far from the pole. My GF gets no such impression in any part of the image. Weird.

Yes, presumably because the still image was taken slightly before the 20 second exposures started.

[johnnydeep]

Alright, what's the reason for taking 137 20 second exposures as opposed to just one 137 * 20 = 2740 second exposure? The result would look much the same, no?

[Chris Peterson]

Alright, what's the reason for taking 137 20 second exposures as opposed to just one 137 * 20 = 2740 second exposure? The result would look much the same, no?

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.

[johnnydeep]

Alright, what's the reason for taking 137 20 second exposures as opposed to just one 137 * 20 = 2740 second exposure? The result would look much the same, no?

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 )

[Chris Peterson]

Alright, what's the reason for taking 137 20 second exposures as opposed to just one 137 * 20 = 2740 second exposure? The result would look much the same, no?

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.

[alter-ego]

Does anyone else get an impression that the star field rotates slightly between the still image and the trailed image? I get a strong impression of rotation near the pole, but no such impression far from the pole. My GF gets no such impression in any part of the image. Weird.

Yes, presumably because the still image was taken slightly before the 20 second exposures started.

The sense of rotation is clearly visible to me, but it's an illusion. The tighter the circle radius (higher declination), the more prevalent the illusion. It appears to be related to a larger gradient in star-trail length over a smaller field of view of our eyes. Seeing Polaris almost motionless compared to star trails only 10° away enhances this sense of rotation. However, the effect hardly visible (if at all) when viewing stars within 10° of the equator.
I generated a similar image sequence in hover sequence below (137 images x 20 seconds). The no-trail image is the first, so no delays. I think for some people, focusing on the star trails while going back and forth might be bothersome to them.

NOTE: Increase your browser magnification until the image(s) about fill the screen. I increase the magnification to 3x (300%).
 

Click to view full size image 1 or image 2

[johnnydeep]

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 :

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.

[johnnydeep]

Does anyone else get an impression that the star field rotates slightly between the still image and the trailed image? I get a strong impression of rotation near the pole, but no such impression far from the pole. My GF gets no such impression in any part of the image. Weird.

Yes, presumably because the still image was taken slightly before the 20 second exposures started.

The sense of rotation is clearly visible to me, but it's an illusion. The tighter the circle radius (higher declination), the more prevalent the illusion. It appears to be related to a larger gradient in star-trail length over a smaller field of view of our eyes. Seeing Polaris almost motionless compared to star trails only 10° away enhances this sense of rotation. However, the effect hardly visible (if at all) when viewing stars within 10° of the equator.
I generated a similar image sequence in hover sequence below (137 images x 20 seconds). The no-trail image is the first, so no delays. I think for some people, focusing on the star trails while going back and forth might be bothersome to them.

NOTE: Increase your browser magnification until the image(s) about fill the screen. I increase the magnification to 3x (300%).
 

Click to view full size image 1 or image 2

Dang it - I am unable to view those images, which is what had been happening to some of the images posted by Ann a while back. But she seems to have remedied that somehow. These "googleusercontent" links seem problematic, at least to me and my browser.

Oh, and I do see that it's an illusion now: looking closely at one of the small inter arcs, you can clearly see that the initial star point source remains fixed, and it's only the arc it makes in the long exposure that is added. So, no real rotation: the initial short exposure is still there.