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I have just made my first attempt to photograph the International Space Station, and I clearly need to improve my technique. Here are the details:

  • Body: Canon EOS-M (1)
  • Lens: EF-M 55-200mm at 200m, f/6.3
  • ISO: 400
  • Exposure bias: -1EV

With AV priority, this gave:

  • Exposure time: 2 sec

During the two-second exposure, the ISS obviously moved along quite a bit, but I was surprised that the blurred image was not uniform, but intermittent, as if something had been flashing at around 10-15 hertz.

Here's an 800x800 crop of the (3456x5184) image; the ISS moves right to left:

enter image description here

Clearly this is the sort o photo I wouldn't hope to take again, but I'm curious; can anyone tell me what might be going on here? Is this some kind of cyclic process in the sensor or processor?

BTW, I'm reasonably certain this is the ISS and not something else, as it appeared at the right time, positioned correctly relative to the moon, and with the trajectory I expected.


A note on focus:

In his comment, @MichaelClark rightly draws attention to my focussing -- another aspect that left a lot to be desired.

The photo was taken with manual focus, and with image stabilisation disabled. As I knew the ISS would be moving fast, I tried to pre-establish focus by using the Moon. The ISS was about 1350km from me, and I assumed (perhaps incorrectly?) the Moon's infinity to be a good enough approximation.

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    The pulsing looks like what you get with a tumbling rocket body, so my guess is that the ISS may have had the solar array moving relative to your position. It also appears you weren't quite focused precisely at the distance of the ISS (unless this is a very tight crop of your photo). With astronomical photography stars, planets, the moon, satellites in high orbits, and satellites in low orbits all require very slightly different focus distances to be sharp. Auto focus generally isn't precise enough for such subjects. – Michael C Jul 31 '15 at 22:45
  • @MichaelClark : Thank you. I've added a note to clarify focussing. Tumbling looks like a good reason; a fresh calculation suggests about 6.7 rotations per second, which seems a bit high? – Brent.Longborough Aug 1 '15 at 8:27
  • This looks like a "Handheld Night Scene" mode, where the camera takes multiple short exposure photos to avoid blur, but the Canon EOS M manual says it only takes 4 frames, not 20-30. – szulat Aug 1 '15 at 11:40
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    ISS obviously can't rotate 6 times a second, but perhaps its solar panels can vibrate sometimes and in the appropriate configuration the reflected sunlight might be modulated this way... who knows... – szulat Aug 1 '15 at 11:42
  • @szulat : Nice idea, but I don't think this is my case -- I was in plain AV mode, taking RAW only, single shot. – Brent.Longborough Aug 1 '15 at 14:38
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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

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Try to take a picture with the same conditions in a complete dark room. The only reason for a non-uniform shot should be the sensor-sensitivity. If you get a noisy image try reducing the ISO, the lower the ISO the less noise you should get in an image. In addition try, cleaning the lens itself. Some dust on the lens could cause additional noise..

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    Or the cap lens on. – Rafael Nov 7 '15 at 17:55
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This is more like a physics mechanics question. When we capture a photo, its actually a bundle of light particles projected on our camera sensors bounced from the subject we are taking picture of. So more each capture, the light has to traverse a straight line from the from bouncing on the subject to the sensor body. Lets say, this time is x. Now, if the sensor is open for 2 seconds and the subject is moving comparatively faster than x, then there will be an intermittent loss of light rays in between traversing from the subject to the sensor. This is whats happening here.

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