3 ms·

I do kinda struggle to understand the geometry here though. Especially in the picture with the black-out fabric, how is the camera rotating to track stars?

by zokier 2y ago

4gotunameagain 2y ago
In the image you referenced, the camera is tilting (or pitching, if you think in aircraft terms). Since the axis of rotation (the axle coming out of the tracker) is not super offset from the camera, the arc that the camera traces is not that wide. It seems like the surrounding blanket is mounted loosely enough to allow quite a bit of movement. It also seems like it does not need to be capable of very long exposures, as also betrayed by the traces of the bright terrestrial lights in the demo photograph at the end.
- dylan604 2y ago
  For one of the images it says 15 second exposures. That’s a long time for something moving at orbital velocities. How is that betrayed in the image? Did you mean portrayed or displayed?
  - 4gotunameagain 2y ago
    The orbital period of the ISS is 93 minutes, and it is under Earth-pointing control. That means that facing prograde (direction of travel), there will be 360° of rotation in one orbital period. To find the angle that the camera must rotate in 15 seconds: 15sec/93min * 360° ≈ 1°
    - dylan604 2y ago
      That's a lot of great information that doesn't come close to actually answering the question. How does that image betray anything you just said. In the example image, the background is clear while the earth is a motion blur. This is obvious of a long exposure image while showing the camera is moving to track the stars with the earth is in motion below it.
      - 4gotunameagain 2y ago
        Okay, I will explain like you're 5. If you have seen any videos from LEO satellites, including the ISS, you would notice that any particular point on the ground moves very fast from the camera's point of view. In the photo presented on the post, you can see the tracks of man made light pollution - the bright light emanating from cities being smeared across the image plane. But you can see the start and the end of those tracks, and they are small. Which means you can gauge the relative position of a single point on the ground at the beginning and at the end of the exposure. And it hasn't moved much. Ergo, you can estimate that the exposure time is quite small. Let me know if that wasn't clear enough, I am not used to explaining things to this level.

I do kinda struggle to understand the geometry here though. Especially in the picture with the black-out fabric, how is the camera rotating to track stars?

by zokier 2y ago

I do kinda struggle to understand the geometry here though. Especially in the picture with the black-out fabric, how is the camera rotating to track stars?

4gotunameagain 2y ago

In the image you referenced, the camera is tilting (or pitching, if you think in aircraft terms). Since the axis of rotation (the axle coming out of the tracker) is not super offset from the camera, the arc that the camera traces is not that wide. It seems like the surrounding blanket is mounted loosely enough to allow quite a bit of movement. It also seems like it does not need to be capable of very long exposures, as also betrayed by the traces of the bright terrestrial lights in the demo photograph at the end.

dylan604 2y ago
For one of the images it says 15 second exposures. That’s a long time for something moving at orbital velocities. How is that betrayed in the image? Did you mean portrayed or displayed?
- 4gotunameagain 2y ago
  The orbital period of the ISS is 93 minutes, and it is under Earth-pointing control. That means that facing prograde (direction of travel), there will be 360° of rotation in one orbital period. To find the angle that the camera must rotate in 15 seconds: 15sec/93min * 360° ≈ 1°
  - dylan604 2y ago
    That's a lot of great information that doesn't come close to actually answering the question. How does that image betray anything you just said. In the example image, the background is clear while the earth is a motion blur. This is obvious of a long exposure image while showing the camera is moving to track the stars with the earth is in motion below it.
    - 4gotunameagain 2y ago
      Okay, I will explain like you're 5. If you have seen any videos from LEO satellites, including the ISS, you would notice that any particular point on the ground moves very fast from the camera's point of view. In the photo presented on the post, you can see the tracks of man made light pollution - the bright light emanating from cities being smeared across the image plane. But you can see the start and the end of those tracks, and they are small. Which means you can gauge the relative position of a single point on the ground at the beginning and at the end of the exposure. And it hasn't moved much. Ergo, you can estimate that the exposure time is quite small. Let me know if that wasn't clear enough, I am not used to explaining things to this level.