Kepler Mission – Mr. Danvk, you have an answer.
In the aftermath of Kepler’s first light images a question made by danvk, a BtC reader, arrived our comment box:
In the full image there are lots of white lines that are perfectly horizontal or vertical. What are these?
Natalie Batalha, Kepler Co-Investigator, gives us a solution for the enigma:
The white streaks are CCD artifacts associated with the saturation that occurs with the very brightest stars in the field. CCDs are constructed by putting very tiny electronic circuitry on top of a wafer of silicon. When light strikes the silicon surface, the photons knock electrons loose. These (negatively charged) electrons are attracted to tiny electrodes in the circuitry because they have a positive voltage applied to them. The electrodes themselves define individual pixels. A very bright star will liberate so many electrons that they pile up and literally spill over to the adjacent pixel (electrode). They spill in the direction of least resistance and that happens to be in the direction that the electrodes are chained together (up and down the columns in our case). When spillover occurs, we call this “saturation.” In the image, you see that some of the saturation bleeds are vertical while others are horizontal. The individual ccds (the rectangles) were mosaic’d so that we could rotate the spacecraft 90 degrees each quarter (to keep the solar panels pointing at the Sun) and still have the image look the same (rotational symmetry). If you train your eye on the gaps between the rectangles, you can see that they form a bit of a spiral pattern. That’s the rotational symmetry pattern due to the orientation of the individual CCDs.