Correction to Horizontal Source Size due to Mirror

Since the Mirror at 8-ID is  only 10 cm long  and intercepts the beam at grazing incidence (0.15 degrees) it can act as an aperture to restrict the horizontal source size.  In order to determine whether the projected mirror size, or the horizontal source size is the limiting horizontal aperture, one needs to compare the angular size subtended from each of these apertures as viewed from the defining slits near the sample.

The slit to source distance is 67 m and the horizontal source size has a Gaussian sigma of 270 microns.  To translate this into an effective aperture size, one should convert to a full-width at half max, by multiplication by 2.35.  The projected size of the mirror is 262 microns, and the slit-mirror distance is 38 m. Thus the respective angular sizes are:

Thus the mirror is actually the defining aperture, not the horizontal source size.  In order to calculate the coherence length of the beam at the position of the defining slits, the easiest thing to do is to construct an “effective” horizontal source size given by:

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