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The next component in the optical path is a flat folding mirror (R5), which reflects the light out of the slicer assembly towards the first collimator mirror (R6). To test the alignment of this mirror and the slit mirrors we used the alignment laser. The optical spectrometer axis is defined by the center of the Lyot-stop pupil and the center of the slicer mirror stack (see Fig. 4.5).

The laser was aligned with respect to this axis using the Lyot-stop pinhole mask. The position of the beam was measured directly behind the location of the fold mirror (R5). As can be seen in Figure 4.6, the beam was perfectly centered in the horizontal direction but its vertical position was about 1 mm too low compared to the expectations from the ray tracing simulation. This small deviation is acceptable, since it will only minimally influence the wavelength calibration and a correction would have required to enlarge the screw holes within the holding plate of the mirror (R5).

After the fit pins of the fold mirror (R5) were removed, the orientation of the mirror was aligned such that the beam hits the required position (indicated by the cross in Fig. 4.7) on the mirror holder of the first collimator (R6) with a precision of about 0.5 mm, corresponding to _∼ 1% of the beamsize at this position. From the first collimator (R6) the beam is reflected to the second collimator mirror (R7), which is located directly in front of the grating. At the second collimator (R7) the beam was vertically 1 mm higher than expected. This discrepancy is due to the fact that the beam is located 1 mm too low at the fold mirror (R5), an acceptable offset, since it will just cause a minimal shift of the recombined pupil at the grating position. To successfully recombine the pupil of the single configurations at the grating, the orientations of the slit mirrors have

Figure 4.6: Measurement of the laser beam position behind the fold mirror (R5). The horizontal position was found to be perfectly centered, while the vertical position was 1 mm too low.

to be precisely aligned. Small manufacturing errors of the slit mirrors lead to an offset of the single pupils from their pre-defined position, which is the center of the grating. The maximum deviation along the larger axis of the grating is 10 mm and 5 mm along the short axis, corresponding to about _∼ 8% of the beam size at this position. The magnification factor between the capture mirrors and the extended direction of the grating is 25.5. Since the pupil alignment at the capture mirrors has a precision of about 0.5 mm, the measured deviations of the pupil images at the grating are within expectations. This is also true for the short axis of the grating, since the anamorphic magnification relation is 2:1 between the long and short grating axis. Since the dimensions of the grating are oversized, the pupil misalignment at the grating position will not lead to a loss of light but may cause a small difference in the spectral calibration of the single configurations if the angle of the incident beam changes among the configurations.

In Figure 4.8, one can see the laser beam centered on the grating for the central configuration. The spectrometer is designed to closely resemble a Littrow- configuration, which means that the beam direction is reversed at the grating. The dispersed beam passes the two collimators and falls upon the detector via the exit optics. The exit optics produces an image of the pupil 240 mm in front of the detectors.

Figure 4.9 shows the alignment-verification of the detector-pupil for the central configuration. The yellow lines indicate the expected location of the laser spot in the horizontal and vertical directions. Whereas there is a small offset in the vertical direction of about 0.5 mm, the horizontal direction is perfectly aligned. The offset in the vertical direction of 0.5 mm is acceptable since it only amounts

Figure 4.7: The laser beam hits the desired position (for the central configuration) on the mirror holder of the first collimator (R6) (red spot + cross)

Figure 4.9: Verification of the detector-pupil alignment for the central configu- ration. The yellow lines indicate the horizontal and vertical expected location of the laser spot. While the pupil is perfectly aligned in the horizontal direction a small offset of 0.5 mm was found in the vertical direction. The precision of the measurement is also _∼0.5 mm.

to_∼11% of the pupil size at this position and the pupil mask is oversized in the vertical direction.

In Figure 4.10, a blaze reflex (for the central configuration) falling onto the detector is shown. A blaze reflex is a direct or multiple reflection at the grating grooves and appears for certain grating orientations. Figure 4.10, shows how the blaze reflex is perfectly centered on the central detector module. This was achieved by rotating the grating and the second collimator mirror (R7) both with their fit pins removed. Changing the grating angle (angle of incidence) by about 30◦

led to a second much weaker blaze reflex. It was verified that this second reflex was also centered on the central detector module, demonstrating that the rotational axis of the grating is well aligned with respect to the grating surface.

Next, the K-mirror was aligned with respect to the entrance optics. To achieve this, the alignment laser was centered with respect to the rotation axis of the K- mirror and then the reimager (E5) (with removed fit pins) was slightly rotated to align the beam with respect to the reference axis defined by the center of the Lyot-stop and the center of the central slicer mirror (Fig. 4.5). Rotation of the K-mirror did not change the beam alignment at any position within the optical path, confirming that the system was well aligned.

Figure 4.10: The laser beam hits the central detector module as expected for the image center.

Figure 4.11: Image of the pseudo slit in the detector plane compared to a ray tracing simulation. The black dots are simulated field points vertically centered on each slice. Measurement and simulation are in good agreement, and only a small rotational offset is measured.

Lyot-stop was homogeneously illuminated, and the grating was replaced by a flat mirror mounted into the holding structure of the grating. In Figure 4.11, the images of the slicer mirrors in the detector plane are compared with a ray trac- ing simulation (black dots). The black dots are simulated field points vertically centered on each slice. The images of the slices fit well the simulation, with only a small rotational offset that will minimally influence the spectral calibration.