• What is the lens profile of the refractive lenses delivered by RXOPTICS?
    1. All lenses have a biconcave form, due to a refractive index of x-rays in matter that is (slightly) smaller than 1.
    2. Each lens surface of 2D lenses has the form of a paraboloid of revolution.
    3. Each lens surface of 1D lenses has the form of a parabolic cylinder.
    4. In contrast to spherical lenses, imaging with ideal parabolic x-ray lenses is practically free from aberrations.
    5. For the geometrically exact focusing of an x-ray plane wave the lens surfaces must be a succession of (pairwise distinct) Cartesian ovals. However, due to the small numerical aperture and the biconcave form of our refractive x-ray lenses, the wavefront errors caused by the parabolic shape are negligible for all lens types delivered by RXOPTICS.
  • Can one stack lenses with different radii of curvature R behind one another?
    Yes, you can do that for 2D lenses as well as for 1D lenses. The frames are the same, independent of R, but of course, different for 1D and for 2D lenses.
  • Which type of lens material is appropriate for which energy range?
      1. We recommend to employ beryllium for 2 to 40 keV, aluminium for 40 to 80 keV, and nickel for energies above 80 keV (on request). The ranges given are only very rough values.
      2. Plastics as lens material are destroyed more or less rapidly in the intense beam of synchrotron radiation sources. PMMA is especially sensitive to radiation damage. The resist SU-8 is less sensitive, but beryllium and aluminium still are more favourable in the entire energy range.
    1. Diamond can be useful in the front end of the beam line due to its thermal stability. Apart from that, however, beryllium as lens material is superior to diamond for most applications.
  • What is the relevant parameter for the transmission of refractive x-ray lenses?
    1. Absorption reduces the effective aperture Deff of a stack of refractive lenses and that with increasing number N of lenses in the stack and with increasing absorption coefficient. For details see the Lengeler et al. (1999): Imaging by parabolic refractive lenses in the hard x-ray range, in particular Eq. 46, and also Lengeler (2012): Refractive x-ray lenses: New developments, pp. 34, 35.
    2. The relevant parameter for the transmissivity of refractive lenses is the effective aperture Deff and not the transmission (Eq. 48). When the geometric aperture 2R0 increases, the transmission decreases, although the number of photons transmitted by the lenses increases.
  • What are the main differences between refractive x-ray lenses and other x-ray optics, like mirrors or Fresnel zone plates?
    1. The handling of our refractive x-ray lenses is very simple. This makes them excellent working horses in a very large number of beamlines in synchrotron radiation centers worldwide. Refractive x-ray lenses are robust and compact as well as quickly and easily installed, aligned, and removed. There is no need for sophisticated benders or order sorting apertures. The focus stays on axis so that there is no need for rearranging the optical components downstream.
    2. Lenses are much more insensitive to misorientations and vibrations than mirrors.
    3. Refractive lenses are about a factor 1000 less sensitive to surface roughness and contamination than mirrors. See also Lengeler (2012): Refractive x-ray lenses: New developments, pp. 40, 41.
    4. Our refractive x-ray lenses have been successfully used in both the white beam of an undulator and the intense beam of an x-ray free-electron laser.
    5. Refractive x-ray lenses cover an energy range from about 2 keV to well above 150 keV. This range is much larger than for any other optic.
    6. In contrast to mirrors refractive x-ray lenses show strong chromatic aberration. This problem has been overcome by the development of transfocators, first developed at ESRF in Grenoble, which allow to change the number of lenses in the stack without breaking the vacuum. See also Vaughan et al. (2011): X-ray transfocators: focusing devices based on compound refractive lenses.