1. Interferometric measurements on mirrors
Idoia Freijo Martin
X-Ray Optics and Beam Transport, WP-73
TDR review meeting, Hamburg, 14th Sep. 2012
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2. Introduction
Interferometric measurements system to characterize the prototype and perform corrections in the mirrors.
Goal: accuracy 10nm / 10nrad (in positions / angles)
Pitch angle changes interferometer measures angles
Operational range for the mirrors:
- SASE1 and SASE2: 2.5 mrad
- SASE3: 14 mrad
Distribution mirrors: Angular range in pitch is enough Tilt of the mirrors and interferometer head vertically
Foreseen set-up for the mirror chamber
14 Sep. 2012, TDR review meeting.
Idoia Freijo Martin, WP-73

3. Test set-up Current set-up in the lab
Mirror dummy
Interferometer heads
Cooling plate
Rotation stage
Translation stage
Interferometer head
The laser beam is generated in a central He-Ne laser and split into two parts, which are guided by optical fibers to two interferometer heads Each head consists of a double pass interferometer and a detector.
14 Sep. 2012, TDR review meeting.
Idoia Freijo Martin, WP-73

4. Measurement method Δ(α) : distance measured by the interferometer
Aim: to find the angular acceptance of our set-up before losing the beam
Mirror
Mirror at 96mm distance from interferometer head
Δ(α) : distance measured by the interferometer
Each rotation stage is moved in steps of 0.01 and 0.1 deg (left and right direction)
Theoretically: interferometer head rotated out of perpendicular position, distance to mirror increases (positive parabola)
Experimentally: parabola is negative
cos α = D / (D + Δ(α) )
Δ(α) = D/cos α - D
14 Sep. 2012, TDR review meeting.
Idoia Freijo Martin, WP-73

5. Measurements’ results
Rotation of each interferometer head at the time
Observed negative distance change: due to measurement principle.
Slopes of parabolas: 1nm/(0.01°)2, so, a precision of 0.01° is enough for the goniometers to obtain 1 nm accuracy ( < 2nrad angular accuracy )
Deviations from parabolas:  50 nm, due to temperature fluctuation, and no backlash compensation of motors controller. Eccentricity of goniometers contribution: 50 nrad
14 Sep. 2012, TDR review meeting.
Idoia Freijo Martin, WP-73

6. Long-term measurements
Set-up enclosed by a Styrofoam cover, aluminum frame is temperature stabilized with a water chiller
Results of a 7 days measurement: Room temperature variation:  0.2K
-Temp. inside the enclosure:  0.1K
-Linear drift by one head:  0.5 μm
-Drift in differential signal: 50nm
(vendor specifications: for the laser unit, 50 nm/°C and for the interferometer heads, 100 nm/°C)
Observed drifts caused most likely by relaxation motions of aluminum frames or interferometer mounts.
14 Sep. 2012, TDR review meeting.
Idoia Freijo Martin, WP-73

7. Conclusions and outlook
The observed precision levels of the test setup are in the range of 50 nm  accuracy of an angle determination of about 60 nrad for an 800 mm long mirror.
With a granite support, better temperature and motion control this number is likely to improve. However, with a stability level of 60 nrad, the chamber deformations of 0.6 μrad should be easily detectable.
Optimization of the set-up using a granite support and an improvement of the temperature and motion control.
To perform similar tests with an autocollimator and compare both results.
Thank you for your attention
14 Sep. 2012, TDR review meeting.
Idoia Freijo Martin, WP-73

8. END
14 Sep. 2012, TDR review meeting.
Idoia Freijo Martin, WP-73

9. Drifts caused mainly by:
-Relaxation motions of the aluminum frame and of the interferometer mounts
-Temperature and pressure changes
14 Sep. 2012, TDR review meeting.
Idoia Freijo Martin, WP-73

10. Extra slide
L= 96 mm is the distance between the mirror and the center of rotation
d=0.1mm is what I supposed that I am apart from the COR.
X2= L – d*sin (α)
D(α) = L/ cos(α) – d *tan(α)
D(0)= L = 96 mm
If we are in the COR, then d=0 ­ D(α) = L/ cos(α) , D(α) = L + Δ(α)
Δ(α) = L/ cos(α) - L
14 Sep. 2012, TDR review meeting.
Idoia Freijo Martin, WP-73

11. Extra slide 14 Sep. 2012, TDR review meeting.
Idoia Freijo Martin, WP-73

12. 14 Sep. 2012, TDR review meeting.
Idoia Freijo Martin, WP-73