Starship Asterisk*

In today's APOD, we see the arc of Anthe, but the background has numerous star trails which seem to trace different trajectories, one is even curved.

Why is this?

Taken from here

The general brightness of the image (along with the faint horizontal banding pattern) results from the long exposure time of 32 seconds required to capture the extremely faint ring arc and the processing needed to enhance its visibility (which also enhances the digital background noise in the image). The image was digitally processed to remove most of the background noise. The long exposure also produced star trails in the background. This view looks toward the un-illuminated side of the rings from about 3 degrees above the ringplane.

Hope that helps.

http://apod.nasa.gov/apod/ap080910.html

I was wondering if some of the imaging experts in the fourm might be able to explain why some of the stars in the background show streaking from exposure times while others do not?

Bill Carney

My apologies. I see this question was already answered further down the list.

http://apod.nasa.gov/apod/ap080910.html
I like the was Anthe is lit up. If it were at the head of the arc it would look like a comet.
Orin

i understand the long duration (32 sec) exposure causing the star streaks, but that doesn't explain the streaks going in different directions. the streaks should be either all going in the same direction or no streak at all if the camera is moving toward those stars... tom

tom2688 wrote:i understand the long duration (32 sec) exposure causing the star streaks, but that doesn't explain the streaks going in different directions. the streaks should be either all going in the same direction or no streak at all if the camera is moving toward those stars... tom

Out of curiosity, have you clicked any of the links found in the 'Explanation'? There's a ton of information contained in them, and your question may be answered. 8)

Plus, I believe the link I first quoted from (see above) explains the different "streaks": some being star trails, some being background noise from the length of the exposures.

Some may be other arcs and it looks like some may be scratches in the film. I'm referring to the dark up and down streaks that are just barely noticeable.

Orin

DerekSmith wrote:In today's APOD, we see the arc of Anthe, but the background has numerous star trails which seem to trace different trajectories, one is even curved. Why is this?

FranksHobbies asked a similar question. I was a little puzzled by the quote that Indigo_Sunrise included, especially:

The long exposure also produced star trails in the background.

Suppose the camera is tracking Anthe whilst Anthe is moving with respect to the stars. Then one can expect stellar trails which are parallel. The reason: the stars are much further away than Anthe. Possibly lense distortion may cause some deviations near the edges of the image, but i reckon that ESA/NASA did not use a low quality lense of 10 Euro, but a high quality product.

When one enlarges the image and when it is dark in your room, basically ione can see three kinds of objects:

1. Anthe and her trail.
2. Point like objects
3. Short tracks

The point like object are several pixels in size. That will exclude random noise in just one of the elements of the light sensitive transducer. Some of the short tracks are more or less parallel to Anthes trail and perpendicular to the predominant direction of the main part of the short tracks.

So i will bring forth a suggestion, a working hypothesis, maybe just a hunch, which explains the different directions of the short tracks.

• The point like objects are background stars. Many lightyears away.
  The short tracks are near objects, (a few thousands of km) random orbiting debris of the ring system.

The fact that the random debris can be found at rather short distance of the spacecraft, explains the different directions of some short tracks.

orin stepanek wrote:http://apod.nasa.gov/apod/ap080910.html
I like the was Anthe is lit up. If it were at the head of the arc it would look like a comet. Orin

http://en.wikipedia.org/wiki/Alkyoneus
http://en.wikipedia.org/wiki/Alkyonides

<<Alcyoneus or Alkyoneus (Gr. Ἀλκυονεύς) was in Greek mythology the eldest of the Thracian Gigantes. He was born (like all other Gigantes) in full armor with a spear in his hand. He was the most prominent of the Gigantes who led a major rebellion against the Olympian Gods, and was said to be immortal in his homeland, Pallene.
.
Alcyoneus was in possession of the Isthmus of Corinth at the time when Heracles drove away the oxen of Geryon. The giant attacked him, crushed twelve waggons and twenty-four of the men of Heracles with a huge block of stone. Heracles himself warded off the stone with his club and slew Alcyoneus. The block with which the giant had attacked Heracles was shown on the isthmus down to a very late period.

His seven daughters are the Alkyonides. When their father was slain by Heracles, they threw themselves into the sea, and were transformed into halcyons (kingfishers) by Amphitrite. They were Alkippe, Anthe, Asteria, Drimo, Methone, Pallene and Phthonia.>>
---------------------------------
Mimas
Mean orbit radius 185,520 km
Orbital period 0.9424218 d
Mean diameter 396.6 km
---------------------------------
Methone
Semimajor axis 194,440 ± 20 km
Orbital period 1.009573975 d [14:15 resonance with Mimas]
Mean diameter ≈ 3 km
.............................................
Methone (pronounced mi-THOE-nee, or as Greek Μεθωνη) is a very small natural satellite of Saturn lying between the orbits of Mimas and Enceladus. Methone is visibly affected by a perturbing mean longitude resonance with the much larger Mimas. This causes its osculating orbital elements to vary with an amplitude of about 20 km in semi-major axis, and 5° in longitude on a timescale of about 450 days.
---------------------------------
Anthe
Semi-major axis 197,700 km
Orbital period 1.03650 d [10:11 resonance with Mimas]
Mean diameter ≈ 2 km
.............................................
Anthe (pronounced AN-thee, or as Greek Άνθη) is a very small natural satellite of Saturn lying between the orbits of Mimas and Enceladus. Anthe is visibly affected by a perturbing mean longitude resonance with the much larger Mimas. This causes its osculating orbital elements to vary with an amplitude of about 20 km in semi-major axis on a timescale of about 2 Earth years. The close proximity to the orbits of Pallene and Methone suggests that these moons may form a dynamical family.
---------------------------------
Pallene:
Semimajor axis 212,280 km
Orbital period 1.153745829 d [19:16 resonance with Enceladus]
Mean diameter ≈ 4 km
.............................................
<<Pallene (pronounced pə-LEE-nee, or as Greek Παλλήνη) is a very small natural satellite of Saturn lying between the orbits of Mimas and Enceladus. Pallene is visibly affected by a perturbing mean longitude resonance with the much larger Enceladus, although this effect is not as large as the Mimas perturbations on Methone. The perturbations cause Pallene's osculating orbital elements to vary with an amplitude of about 4 km in semi-major axis.

The Pallene Ring, a faint dust ring, also discovered by the Cassini Imaging Team, shares Pallene's orbit, as revealed by images taken in forward-scattered light by the Cassini spacecraft in 2006. The ring has a radial extent of about 2,500 km. Its source is particles blasted off Pallene's surface by meteoroid impacts, which then form a diffuse ring around its orbital path.>>
---------------------------------
Enceladus
Semi-major axis 237,948 km
Orbital period 1.370218 days
Mean radius 504.2 km

orin stepanek wrote:Some may be other arcs and it looks like some may be scratches in the film. I'm referring to the dark up and down streaks that are just barely noticeable.

Nothing about this image has even remotely involved film. And data doesn't scratch <g>.

There seem to be three types of background objects. The point sources are hot pixels- defects related to the CCD. The bulk of the short streaks that are the same length and orientation are stars, motion streaked by the long exposure. The handful of streaks that don't have a common length or orientation appear to be cosmic ray hits on the detector- to be expected for a 32-second exposure using a CCD in space.

The horizontal structure is probably bias noise- a sensor defect that would normally be invisible except for situations like this where the contrast has been stretched to an extreme, given that the object of interest is barely above the noise floor of the detector.

Chris Peterson wrote:The point sources are hot pixels- defects related to the CCD.

Chris, what precisely are hot pixel defects? I heared about pixel defects, single pixels, which do not work. Do hot poixel defects occur in groups of about 10 adjacent circular or square elements?

henk21cm wrote:Chris, what precisely are hot pixel defects? I heared about pixel defects, single pixels, which do not work. Do hot poixel defects occur in groups of about 10 adjacent circular or square elements?

Every pixel on a CCD has a slightly different dark current rate. So if you were simply to make an exposure with the shutter closed, you would see a slight variation in the signal over the entire image. A few pixels may have a significantly higher dark current rate. With a long enough exposure, these pixels will saturate. Even with a short exposure, they may be bright enough that their individual noise component swamps the true signal, even after the image is calibrated with a dark frame. Generally, these pixels are responsive, however. Completely dead pixels do happen, but not on the quality of sensors selected for space missions (although pixels might fail later, as a result of cosmic ray damage, for instance).

Sometimes hot pixels are isolated, and sometimes they are part of a small cluster of pixels with similar characteristics.

It's hard to know for certain what's going on in this image, because the processing steps haven't been described. If you examine the best quality image available, the full resolution TIFF from the NASA site, it appears that a low pass filter was used, probably to soften the background noise. But that also serves to spread any point defects across several pixels.

Hot pixels normally calibrate out, and aren't visible in an image. But most images are of brighter objects, where the total S/N is much higher. This image has a low S/N, and the processing that boosted the signal enough to make it clear also pushed the noise up pretty far, so we tend to see every little artifact.

Great point about the cosmic rays Chris. I was starting to get genuinely mind-boggled by what they might be. I would suggest that some of the "point sources" might also be cosmic rays coming nearly straight in rather than hot pixels.

Chris Peterson wrote:A few pixels may have a significantly higher dark current rate. With a long enough exposure, these pixels will saturate. <snip> Sometimes hot pixels are isolated, and sometimes they are part of a small cluster of pixels with similar characteristics.

Chris, is this due to localized chemical impurities, which lead to a slightly lower energy gap and thus higher "sensitivity"?

If you examine the best quality image available, the full resolution TIFF from the NASA site, it appears that a low pass filter was used, probably to soften the background noise. But that also serves to spread any point defects across several pixels.

The softening is the explanation i rejected during my analysis of the image. Indeed, background noise can be somewhat dimished, e.g. using a 5x5 Gaussian blur. I would have preferred first to get rid of the hot isolated pixel defects using morphological methods: erode-dilate.

Hot pixels normally calibrate out, and aren't visible in an image. But most images are of brighter objects, where the total S/N is much higher.

Agreed, this is an extra ordinary image, in which some usually hidden effects of CCD's are visible. During the weekend i will try to run an analysis (distribution of direction and length) of the short streaked objects in the image.

About cosmic rays: intuitively i would have expected long scratches, of the order of 10% of the image size. There is a movie of the SOHO satelite, during a solar storm. The streaks are rather long. Do you have experience or knowledge about the signature of cosmic rays on a chip?