Why does the ISS leave a dotted trail in a long exposure?
Asked 7/31/2015
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2 answers
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I tried photographing the International Space Station near the Moon and got a non-uniform trail instead of a smooth blur. My setup was a Canon EOS M with the EF-M 55-200mm at 200mm, f/6.3, ISO 400, using Av mode, which resulted in a 2-second exposure at -1 EV. In the image, the ISS trail appears intermittent, almost like it was flashing around 10–15 times per second. Is this likely caused by the camera sensor/processor, or is it more likely something about the ISS itself? I focused manually using the Moon as an approximation for infinity and turned image stabilization off.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
1
The cause is almost certainly with the object you photographed.
If it were with your sensor, then why isn't there some kinds of distortion of the clouds or any other aspect of the frame? You might reason that the sensor behaving erratically or intermittently might produce an effect like this, only visible on moving objects, but then why should it co-incidentally produce a circle each time, brighter along the bottom edge? That looks to me like light being reflected off the object and the object is spinning consistently, or a (bright) light is coming on and off (or the object is being shaded, then lit, by something moving in and out of the way between the light source and the object), highlighting the object periodically.
Unfortunately, most online images of ISS trails are wide angle landscape shots. Kudos for capturing it at 200mm! They also seem more brightly exposed, rendering the ISS a solid white line. The nearest I could find for comparison is at link below. You can click on portions of the image to load high resolution snippets. It shows periodicity in the colouration at edges of the trail but I am not certain that reflects the same thing you've documented.
http://fineartamerica.com/featured/iss-light-trail-detlev-van-ravenswaay.html
Originally by user34203. Source · Licensed CC BY-SA 4.0
user34203
9y ago
0
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It’s very unlikely to be a sensor or processor issue. If the camera were causing periodic artifacts, you’d expect to see some effect elsewhere in the frame, not only on the moving ISS trail.
The more likely explanation is that the ISS brightness changed during the exposure. As it moves, its orientation relative to the Sun and your viewpoint can make different surfaces reflect more or less light, producing a dotted or pulsing-looking trail. In other words, the object’s reflected light changed over time, not the sensor response.
Your focus may also have been a bit off, which can make each bright segment look slightly enlarged or soft, but focusing on the Moon is generally close enough to infinity that it wouldn’t create this kind of periodic pattern by itself.
So the intermittent trail is most plausibly caused by changing reflections from the ISS during the 2-second exposure, not by a cyclic camera process.
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AI11y ago
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