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SRI 2007: CLS Optical Metrology Facility- Optical Tip/Tilt Stage, Software Development. Brian Yates 1 , Dylan Maxwell 1 , Siyue Chen 1 , Bruce Truax 2 Canadian Light Source Inc., 101 Perimeter Road, Saskatoon, SK S7N 0X4, Canada
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SRI 2007: CLS Optical Metrology Facility- Optical Tip/Tilt Stage, Software Development
Canadian Light Source Metrology Facility – Software Development:
These metrology instruments are each provided with their own control and analysis software. In general this software provides adequate functionality for standard metrology applications. However for the measurement of high quality precision synchrotron optics, increased or improved functionality is often required. Furthermore, it is impossible to determine, verify, or modify the algorithm that has been implemented in a closed‑source commercial software package. The process of generating a detailed report for each optical component can also be very tedious and time consuming. The data from each instrument is analyzed using the corresponding commercial software packages. This analysis is recorded, typically with a screen capture, to be later inserted into the report document. If analysis is needed that is beyond the capabilities of the provided software, then the data must be exported to another custom software package. A summary is then created manually, typically by tabulating all of the results. This process must be repeated for each optical component measured. It is prone to error and very difficult to automate.
The CLS Optical Metrology Facility has begun developing software to address the above mentioned deficiencies in our commercial software. The goal of this software is to provide advanced analysis functionality for all three metrology instruments, pre-/post-processing, as well as support for automation and report generation. The core functionality to be implemented includes error analysis, curve fitting, and power spectral density (PSD) for both profile (1D) and surface (2D) data. This software has been developed in C/C++ using the ROOT data analysis framework [Brun et al.]. ROOT is an open source package being developed at CERN and provides many advanced analytical and graphical features. Currently the software is in the development and testing stages with a focus on curve fitting for the analysis of LTP and FI data.
Figure 1 (on the right) shows the results of a spherical fit (actually a polynomial approximation) to FI height data for a CLS VESPERS beamline KB mirror. The surface height data was calculated by averaging three FI measurements acquired successively. The surface height uncertainty was calculated from the variation in the original measurements. A data window has been applied to select only the clear aperture of the mirror. The best fit piston (z = C), tilt (z = Ax + By +C),
power (z = D(x2 + y2) + Ax + By + C) and twist (z = Fxy + Ax + By + C + Dx2 + Ey2)) were then individually removed from both the height data and height uncertainty data. The resulting surface height plot is shown in the left graph of Figure 1, with a histogram of the surface height data below. The resulting surface height uncertainty is shown in the right plot, with a corresponding histogram below. The histograms are useful to verify whether the distributions are approximately normal or not. The root mean square (RMS) and mean values of the surface height plot are displayed in the upper left results box, along with the respective errors calculated from the height uncertainty plot. The upper right results box displays the radius of curvature and the fitted parameter results, all with their respective errors. This particular KB mirror has a height error of
0.0115 ± 0.0002 µm and a radius of curvature of 42591 ± 605 m. These results are formatted such that they can be easily printed, or combined with others to generate a comprehensive report.
Rene Brun and Fons Rademakers, ROOT - An Object Oriented Data Analysis Framework, Proceedings AIHENP'96 Workshop, Lausanne, Sep. 1996, Nucl. Inst. & Meth. in Phys. Res. A 389 (1997) 81-86. See also http://root.cern.ch/.
Figure 1: Spherical fit of Fizeau Interferometer data for a KB mirror (VESPERS beamline).
Canadian Light Source Metrology Facility – Hardware Development:
An optical tip/tilt stage that can handle large synchrotron mirrors has been designed and machined (See Figure 2 below). Commercial stages were considered but discarded later for a variety of issues - stability, high cost, insufficient mirror length capability, insufficient translation range, or deficiency in tip, tilt, pitch adjustments. This flexible design can be used with all three metrological instruments in the CLS Optical Metrology Facility.
The conceptual design was developed by Bruce Truax of Diffraction Limited Design LLC. This preliminary design was further refined and finalized by Siyue Chen within the CLS Engineering and Technical Services Division. The stage can handle optical elements up to 1400 mm in length and approximately 200 mm in width, and weights up to 200 kg. A lateral motion of ± 25 mm in (x,z) can be achieved using micrometers. The height (y) can be finely adjusted ± 10 mm using three finely threaded screws (750 mm per thread) that are mounted to 100 mm diameter air bearings (porous silicon carbide) from New Way. When these air bearings are pressurized the stage can be easily moved while loaded with a heavy optic, even on a tapped optical table. The stage has sufficient rotational degrees of freedom (tip, tilt, pitch) to allow for alignment of the optical element (See Figure 2). A retroreflector mount is provided to accommodate a distance measurement interferometer (DMI), which is useful for radius bench measurements on short radius spherical or cylindrical optics (R < 2 m).
Figure 3 shows the underside of the optical table and gives a good view of the air bearings and ribbed support structure that provides rigidity. Figure 4 shows the mounting base for the table – complete with the pivot post that holds the spherical bearing (Figure 5), micrometers to move the mounting base laterally (x,z), and cut-outs to allow for rotation of the table through 180°. A symmetrical design allows for two possible configurations, which provides for either clockwise or counter-clockwise rotation of the table. The lateral micrometers (x,z) can be removed to mount the radius bench measurement guide bar (2 m), which allows the table and mount to be moved longitudinally (z). Figure 5 shows the rail release mechanism and cam followers (see also Figure 4) used for the radius bench measurement.
Currently the stage is undergoing testing with the Fizeau Interferometer, and will be used for measuring the Phase II beamline optical components.
Retroreflector Mount (for DMI Laser + Radius Bench Measurement)
Cam follower (radius bench measurement)
New Way air bearing (100 mm) - SiC
Micrometers for lateral (x) and
along beam (z) adjustments
Cut-out to allow rotation of table
(New Way air bearing)
Rotational scale with pivot post below
(not visible, see Figure 4) –
pitch rotation of table used for
Pivot Post for spherical bearing
Tension spring required for micrometer movement
Mounting Base securing screw with teflon spacer interface
Micrometers for lateral
Figure 4: Mounting Base for Table, showing pivot post, translation micrometers, retroreflector mount.
Figure 2: CLS optical tip/tilt stage (top-down view) to handle large synchrotron optics.
Spherical Bearing (mounts to Pivot Post)
Rail Release – attaches to mounting
base for radius bench measurement
Fine Pitch adjustment for table
New Way air bearing height adjustment (750 mm per thread)
Figure 5: Miscellaneous components
Figure 3: Side View showing ribbed support structure and New Way air bearings.