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Ok, so I created this test image. The camera was behind the window, I just tested long exposure photography on the single star I saw. It was about 20 minutes of expo, star was out of focus (I really just tested the principles of this kind of photography)

As you can see there is a darker spiral line on the bright line of the star. I would like to know what caused that spiral line?

The white line is the star itself

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The spiral line is caused by the pattern on curtain fabric that reflects from the window glass. You can see the darker and lighter lines continue past the start trail (crank up brightness/contrast if it's not visible otherwise). Where white fabric reflects, it adds to the intensity of the star line.

For better results, take your photo outside, so there will be no reflecting glass between your lens and the star.

  • Oh thank you. Now I see! I will make normal photos in the future in summer nights outside somewhere without light pollution. This was only a test with no expectation about the quality :) – Ákos Nikházy Jan 15 '12 at 11:38
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    I think I'm with @Olin on this one — the pattern doesn't seem to line up with the reflected pattern very well. But you both come to the same real conclusion — time to go outside. – mattdm Jan 16 '12 at 0:07
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I disagree with Imre. That "spiral" is not caused by reflections. The spiral seems to be occluding the star. Reflections can only add to the transmitted light.

That window has a screen installed, right? A screen completely explains the pattern since the wire in the screen does occlude the light coming from behind it. The screen was probably not visible in the viewfinder due to being out of focus, which is also why it's not obvious in the rest of the picture. However, nighttime starts are point light sources, so will reveal something like a screen that would otherwise be out of focus.

Taking pictures out windows is difficult at best, but you really don't want to do it thru a screen or if there is much light in the room. The room needs to be considerably daker than whatever you are photographing outside. At night that means the room needs be completely dark in practise. Take the picture outside already.

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