When doing X-ray measurements of a surface, the sample is held in Ultra-High Vacuum and the X-rays pass into and out of the UHV chamber through Beryllium windows. There are 2 approaches to chamber and diffractometer design that are in use. In the first method, the sample is fixed relative to the vacuum chamber, which is kept as small and light as possible and mounted on the diffractometer. In the second method, the sample is rotated within the chamber by bellows coupled to the outside. This approach avoids putting a large mechanical load on the diffractometer goniometer, making it easier to maintain fine angular resolution. One drawback of many configurations is that the sample must be moved in order to use other surface analysis methods such as LEED or AES, and after moving the sample back into the X-ray diffraction position, it must be realigned. In some setups, the sample chamber can be detached from the diffractometer without breaking vacuum, allowing for other users to have access. For examples of X-ray CTR diffractometer apparatus, see refs 15–17 in