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If the images are stacked, doesn't that mean that there's plenty of images with the parts that are occluded in others not occluded? I understand for any specif

If the images are stacked, doesn't that mean that there's plenty of images with the parts that are occluded in others not occluded? I understand for any specific image, there's going to be some lost background because of Starlink satellites, but that's not what this is showing, this is showing something that's not possible, right? Shifting all the satellites temporally so they appear together, arbitrarily maximizing the problem beyond what is real isn't an accurate depiction of the problem, IMO. Put another way, if you erase the Starlink satellites from the images before stacking them, you then get a fairly accurate representation of the sky without any Starlink satellites, and you still have the data behind them (from the other pictures where that portion of the sky was not occluded). You can also probably fix the intensity of anything occluded in a few of the pictures but not others through some math.

cma 6y ago
> If the images are stacked, doesn't that mean that there's plenty of images with the parts that are occluded in others not occluded? As he mentioned, flare can cover a broader area of the sensor.
- kbenson 6y ago
  There's still many other images that the flare isn't affecting that area at all though, right? The point is that there's many different images used here, and instead of combining them in a way to remove transient artifacts, which is the norm, they combined them in a way to emphasize transient artifacts, which is not something people normally would want to do. It's worse than any single image that's used to make it, so why make it at all?
  - Rusky 6y ago
    It's an interesting way to demonstrate the density of the satellites- they're not hiding the number or duration of images, so you can get a fuzzy intuition for how likely you are to catch them in a given image.
    - lutorm 6y ago
      Except you're not actually demonstrating the density. That could have been one satellite if you exposed long enough. It's like taking a picture of the one car every day driving on some empty road and then coadding them and claiming you're demonstrating density of traffic.
      - platz 6y ago
        what if the research requires long/multiple exposures.
        m_mueller 6y ago
        If I understand correctly the satellites would be much dimmer there, but otherwise you'd see the same problem - correct me if I'm wrong.
mturmon 6y ago
Not an actual astronomer, but somewhat aware of detector issues for space-based telescopes. Typically, you want to use one single integration time, if possible. Doing so can limit readout noise, for one thing. (Note that ground-based astrophotographers, with consumer cameras, often take multiple exposures to cool down the detectors. This is not a factor with the actively-cooled detectors in astronomical telescopes.) For instance, the observations leading to the recent discovery of the two-planet exosystem [1] used roughly 15-minute integration times [2, sec. 2] across an 8 meter (!) aperture. This is a longer integration time than the 17, 30-second integrations that produced OP. I think the above considerations about stacking vs single exposures are a side issue, though. The saving grace is that many astronomical observations are narrow-field-of-view, and are done away from the illuminated part of the StarLink constellation. But, surveys or wide-field images could really be affected strongly, at the 30-40% level [3]. [1] https://news.ycombinator.com/item?id=23917559 https://news.ycombinator.com/item?id=23917559 [2] https://academic.oup.com/mnras/article/492/1/431/5680498 https://academic.oup.com/mnras/article/492/1/431/5680498 [3] https://www.eso.org/public/archives/releases/sciencepapers/eso2004/eso2004a.pdf https://www.eso.org/public/archives/releases/sciencepapers/e...
- rimliu 6y ago
  There are plenty of amateurs using actively cooled specialized astrophotography cameras with total integration times reaching hours (2-3 are typical). Readout noise can be cancelled by taking so called bias frames. Shorter exposures are sometimes preferable because of the light pollution and to minimize the possibility of an exposure being rendered usles by some plane or satellite: it is less painful to throw away three minutes of exposure time than 15.
  - mturmon 6y ago
    > Readout noise can be cancelled by taking so called bias frames. That’s not what bias frames [1] are doing. Bias frames measure a systematic effect in the conversion of stored charge to a digital number in the CCD device. You can then subtract it out. The calibration offered by bias frames does not fix readout noise, which is a random, not systematic, effect that is inherent in the same conversion. You can use the bias frames to compute the strength of the readout noise (i.e., the rms magnitude of its random part) but you can’t remove the random noise itself. See sec. 1.1 of the linked reference for more. [1] https://users.astro.ufl.edu/~lee/ast325/handouts/ccd.pdf https://users.astro.ufl.edu/~lee/ast325/handouts/ccd.pdf

cma 6y ago

> If the images are stacked, doesn't that mean that there's plenty of images with the parts that are occluded in others not occluded? As he mentioned, flare can cover a broader area of the sensor.

kbenson 6y ago
There's still many other images that the flare isn't affecting that area at all though, right? The point is that there's many different images used here, and instead of combining them in a way to remove transient artifacts, which is the norm, they combined them in a way to emphasize transient artifacts, which is not something people normally would want to do. It's worse than any single image that's used to make it, so why make it at all?
- Rusky 6y ago
  It's an interesting way to demonstrate the density of the satellites- they're not hiding the number or duration of images, so you can get a fuzzy intuition for how likely you are to catch them in a given image.
  - lutorm 6y ago
    Except you're not actually demonstrating the density. That could have been one satellite if you exposed long enough. It's like taking a picture of the one car every day driving on some empty road and then coadding them and claiming you're demonstrating density of traffic.
    - platz 6y ago
      what if the research requires long/multiple exposures.
      - m_mueller 6y ago
        If I understand correctly the satellites would be much dimmer there, but otherwise you'd see the same problem - correct me if I'm wrong.

mturmon 6y ago

Not an actual astronomer, but somewhat aware of detector issues for space-based telescopes. Typically, you want to use one single integration time, if possible. Doing so can limit readout noise, for one thing. (Note that ground-based astrophotographers, with consumer cameras, often take multiple exposures to cool down the detectors. This is not a factor with the actively-cooled detectors in astronomical telescopes.) For instance, the observations leading to the recent discovery of the two-planet exosystem [1] used roughly 15-minute integration times [2, sec. 2] across an 8 meter (!) aperture. This is a longer integration time than the 17, 30-second integrations that produced OP. I think the above considerations about stacking vs single exposures are a side issue, though. The saving grace is that many astronomical observations are narrow-field-of-view, and are done away from the illuminated part of the StarLink constellation. But, surveys or wide-field images could really be affected strongly, at the 30-40% level [3]. [1] https://news.ycombinator.com/item?id=23917559 https://news.ycombinator.com/item?id=23917559 [2] https://academic.oup.com/mnras/article/492/1/431/5680498 https://academic.oup.com/mnras/article/492/1/431/5680498 [3] https://www.eso.org/public/archives/releases/sciencepapers/eso2004/eso2004a.pdf https://www.eso.org/public/archives/releases/sciencepapers/e...

rimliu 6y ago
There are plenty of amateurs using actively cooled specialized astrophotography cameras with total integration times reaching hours (2-3 are typical). Readout noise can be cancelled by taking so called bias frames. Shorter exposures are sometimes preferable because of the light pollution and to minimize the possibility of an exposure being rendered usles by some plane or satellite: it is less painful to throw away three minutes of exposure time than 15.
- mturmon 6y ago
  > Readout noise can be cancelled by taking so called bias frames. That’s not what bias frames [1] are doing. Bias frames measure a systematic effect in the conversion of stored charge to a digital number in the CCD device. You can then subtract it out. The calibration offered by bias frames does not fix readout noise, which is a random, not systematic, effect that is inherent in the same conversion. You can use the bias frames to compute the strength of the readout noise (i.e., the rms magnitude of its random part) but you can’t remove the random noise itself. See sec. 1.1 of the linked reference for more. [1] https://users.astro.ufl.edu/~lee/ast325/handouts/ccd.pdf https://users.astro.ufl.edu/~lee/ast325/handouts/ccd.pdf