Metrology of a mirror at the Advanced Photon Source : comparison between optical and x-ray measurements. Page: 4 of 14
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This paper describes a metrological investigation of a particular mirror used to vertically focus the
x-ray beam on the bending magnet (BM) beamline in sector-1 of the APS. Comparison was made between
slope error profiles obtained with the long trace profiler and x-rays tests conducted during the
commissioning of this beamline. The mirror, which is a permanent component of the beamline, is made of
a long flat Zerodur substrate that is 1020 mm long, 65 mm wide, and 60 mm high, and is bendable with a
radius from infinity (flat shape) to about 3 km (a cylinder). The reflecting surface is coated with a 750-A-
thick layer of palladium (with a -50 A chromium buffer layer), and was specified to have a 3 A rms
roughness and <5 grad rms slope error. The beam image from the mirror recorded on a CDD camera (with
the mirror fully illuminated) was found to exhibit a series of parallel horizontal stripes. This paper
summarizes both x-ray and optical studies of the mirror to determine the source of the stripes.
2. OPTICAL MEASUREMENTS OF SURFACE PROFILE
The optical measurements were carried out using the long trace profiler (LTP) at the metrology
laboratory of the APS. This laboratory provides characterization of optical components primarily on
beamlines at the APS. (A detailed description of the this laboratory and its instrumentation can be found in
2.1 Description of Long Trace Profiler
The LTP is a non-contact optical profiling instrument used for measurement of surface slope and
figure errors on large grazing incidence flat or aspheric mirrors such as those used at synchrotron radiation
sources? The LTP at the APS was designed to handle mirrors up to two meters in length. It can
characterize surface height profiles over spatial periods ranging from 2 m (the maximum scan length) to 2
mm (the Nyquist period for 1-mm sampling). It complements the other instruments available at the APS
metrology laboratory. Modification to the original LTP design has considerably improved the accuracy and
repeatability of the instrument3 A standard deviation of 0.3 prad with a corresponding error bar height of
4.6 nm was obtained from an average of 10 profiles, measured of a 500 mm long mirror with a 5 km radius
2.2 Slope Error Measurement with the LTP
The mirror measurements consisted of a series of 10 slope error scans taken along the mirror
centerline. Each scan was 990 mm in length containing 545 points at a 2-mm sampling rate. The mirror
was scanned unmounted (i.e., without the bending mechanism). It was put face up and was simply
supported at the bottom face near the ends. Therefore, the raw surface profile contains deflection due to
gravity. The residual profile was obtained by subtracting the tilt and the best fit cylinder from the raw
profile. The resulting slope error profile (see next section) led to a 4.9 grad rms slope error and 7 km radius.
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Assoufid, L. Metrology of a mirror at the Advanced Photon Source : comparison between optical and x-ray measurements., article, October 14, 1998; Illinois. (digital.library.unt.edu/ark:/67531/metadc620175/m1/4/: accessed April 22, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.