Sunday, January 15, 2012

What causes strange spiral pattern on my star line?

Question

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

Answer

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.

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