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 When the beam offset and the take off angle are given, the scattering angle of an x-ray mirror is easily calculated.

• The take off angle without x-ray mirror is usually 6 degrees.
• Therefore the vertex of the triangle at the tube focus is 6 - a, with a being the take off angle WITH x-ray mirror
• The vertex of the triangle near the heart of the goniometer is the beam offset (b)
• The scattering angle of the x-ray mirror is 180 minus the vertex of the triangle near the x-ray mirror
• Therefore the scattering angle of the x-ray mirror is b+6-a
• The polarisation factor due to the x-ray mirror follows from this as: