On Thursday we installed the InFOCuS X-ray mirror in the X-Calibur telescope. The mirror with a focal length of 8m focuses X-rays onto the detector by deflecting them at a very small angle off 255 nested conical aluminum shells. Each shell has about 200 alternating thin layers of platinum and carbon. We have written about the X-ray mirror and the alignment procedure in the past, so here I will just summarize it and focus on something new we introduced this time.
In order for X-Calibur to work properly the mirror, which basically has the same effect as a focusing lens, must focus X-rays onto the center of a Beryllium cylinder with a diameter of 12mm. The closer to the center the better: if the focal spot remains within 1mm of the center no corrections are necessary, if it is within 3mm we can correct for that offset without loss of precision. This close alignment is achieved through a procedure consisting of multiple steps based on the fact that the X-ray mirror focuses visible light in the same way as it focuses X-rays. X-Calibur team member Takashi Okajima from NASA’s Goddard Space Flight Center attached two cameras to the center of the X-ray mirror. One views the sky as seen from the mirror (let’s call that the “forward-looking camera”), and the other the detector (let’s call that the “back-looking camera”). In his lab he then used the mirror with those cameras attached to focus a parallel beam of light onto a plane 8m from the mirror. He then recorded the spot of light from the parallel beam in the forward-looking camera, and the focused spot in the back-looking camera. After installation of the mirror we have to align it using shims such that the detector is in the same spot as the focused light in the back-looking camera in the lab. This is essentially the procedure we used last time.
This year, our graduate student Quin Abarr developed a method to verify this alignment in the field. The main challenge is to create a parallel light beam with the limited equipment we can ship to the field and in the limited amount of space available in the payload building (the building is big, but so is X-Calibur). We do so by mounting an off-the-shelf 14″ optical telescope in front of the X-ray mirror. A laser connected to an optical fiber forms an approximately point-like light source at the other end of the fiber, which we place in the focal point of the telescope. Usually, a telescope focuses a parallel beam of light onto this focal spot. We use it inversely: turning a point-like light source into a parallel beam. The telescope is mounted to a tilt and rotation stage that allows us to adjust its orientation until the parallel beam appears in the same location in the forward-looking camera as the light beam in Takashi’s lab. At this point, all we had to do was to verify that the focused light beam hits the detector in the correct location. We can do that using the back-looking camera.