The first mirror is a cylindrical 1.2 m long x 140 mm wide Platinum-coated silicon mirror with the optical surface facing up. It is bent in the meridional direction to a value of 27.4 Km to collimate the X-ray beam, in order to optimize its energy resolution which would be determined only by the Darwin width of the first crystal of the monochromator. The main function of this mirror is to reduce the heat load of the first monochromator crystal, additionally to reject the higher order harmonics. The mirror is coated with Pt and in normal operation the glancing angle is 2.136 mrad, which corresponds to a cut-off energy of 39.5 keV. The mirror is water cooled. 

 Table IV displays some relevant parameters for the first mirror (M1).

 

First mirror angle of incidence (mrad) p (m) q (m) mv cylinder radius R (km)
M1 2.136 29.3   27.4

 Table IV: focusing parameters for the first mirror. The demagnification m is given by the ratio of the distance between the source and optical element (p) and the distance between the image point and the optical element (q).

The mirror figures of merit includes a slope error of 0.3 mrad and a mean roughness of 0.2 nm. Due to the high power loads of the ESRF beams (~ 460 Watts) cooling of the mirror is mandatory.