This image illustrates the measuring mechanism of the Wide-field X-ray Telescope of Einstein Probe. X-rays coming from the Universe enter the telescope and are passed down square tubes towards a CMOS light detector where they create a ‘plus’ shaped image.
Some X-rays don’t interact with the walls of the square tubes and can end up anywhere on the detector. However, most X-rays will be reflected on the walls. X-rays that only are reflected by a top or bottom wall will cause a horizontal bar on the detector. Similarly, X-rays that only reflect off a left or right wall will cause the vertical bar. Then finally there are X-rays that reflect of both walls, these double reflected rays end up in the middle of the image causing an intense signal.