Set-up scheme for the thermal deformation profile measurements using rocking angle scans at various vertical positions of a narrow-gap exit slit. The rocking angle θ′ corresponding to the peak intensity in the rocking curve for the distorted first crystal is slightly different from the Bragg angle θB (peak position for the undistorted crystal). The distortion of the crystal in terms of angle is given by Δθ = θ′ − θB. Each slit position xs (xs = 0 for the centre of the beam) allows the rocking curve to be recorded relative to the X-ray beam impinging on the first crystal surface at the position xc = xs/sinθB. The thermal slope distribution Δθ(xc) of the crystal can be measured by varying the vertical positions of a narrow-gap slit after the monochromator.