2.1

Test Setup

For the sake of efficiency, a holder supporting the MM-0051 optic and the ATHENA “row-8” MM-0047, MM-0048, and MM-0049 optics was manufactured so that it could mounted on a hexapod on the extension optical bench in the PANTER 1-meter diameter beamline tube (see Figure 1). In Figure 2 a view of setup in the beamline tube is shown together with the coordinates system used for these optics in PANTER.

Figure 2.

(left) Photograph of the optic mounted inside the chamber with MM-0051 centred behind the moveable mask. The view is looking towards the X-ray source. The alignment laser can be seen going through MM-0051. (right) depicts the coordinate system of the optics when mounted in PANTER.

3.1

Alignment

Before the pump down of the vacuum chamber a pre-alignment in pitch and yaw of the optics was done using the alignment laser to ensure that the double reflection lands on the X-ray detector. The separation between the intersection plane of the optic and sensitive surface of the X-ray detector was set to the image distance of i = 13336 mm which corresponds to the nominal focal length of the optic of f = 12000 mm for a source to optic distance of s = 119814 mm at PANTER (for the divergent beam correction, the thin lens equation 1/f = 1/s + 1/i can be used).

Once the level of vacuum for operating the X-ray detector was reached first light images were taken to verify that the double reflection was in the expected position in X-rays using the 100% mask. Subsequently the position of the 100% mask was optimized and aligned with respect to the MM. In this 100% mask position a short focus search was performed followed by a pitch and yaw alignment of the complete mirror module (see Figure 3). The deviation from the laser pre-alignment is ~10 arcmin in pitch and ~1.3 arcmin in yaw.

Figure 3.

Shown are the pitch scan (left) and the yaw scan (right). Plotted are the data obtained using the 100% mask to illuminate the full MM with the TRoPIC camera at Al-K as well as the “best fit” hyperbola (red) and Voigt function (Cyan).

3.2

Focus Scan

After completing the pitch and yaw alignment of the mirror module MM-0051 a focus scan of the mirror module MM-0051, as well as for the individual X-ray optical units, the inner iXOU and outer oXOU was performed with the 100% mask in place. The results of the HEW and FWHM-X (the FWHM of the transverse PSF) fits of the focus search images are displayed in Figure 4. The focus searches were measured with both the TRoPIC and PIXI CCD cameras. The results of the best fits to the focus searches are summarized in Table 2 and Table 3.

Figure 4

Shown in the top row are the measurements obtained with the TRoPIC CCD camera and in the bottom row the measurements obtained with PIXI CCD camera. The left column shows focus scans for the MM-0051 SPO, the central column the focus scans of the inner XOU-0132, and the right column the focus scans of the outer XOU-0131B. The scans are taken from -270 mm to +190 mm around image position corresponding to the nominal 12 m focus. Shown are the best hyperbola fits to the HEW points (red) and to the FWHM-X points (green). The FWHM-Y points are shown for completeness but are not fitted. The results are given in steps (1 step corresponds to 1.25 μm) and arcsec.

Table 2

Summary of all best focus positions from focus searches with TRoPIC of MM, iXOU, oXOU with 100% mask at Al-K. The best fit values for the focus position are given in steps, the HEW and FWHM-X values in arcseconds.

*

Relative position is the position relative to the laser position (nominal 12 m focus at infinity).

Table 3

Summary of all best focus positions from focus searches with PIXI of MM, iXOU, oXOU with 100% mask at Al-K. The best fit values for the focus position are given in steps, the HEW and FWHM-X values in arcseconds.

*

Relative position is the position relative to the laser position (nominal 12 m focus at infinity).

3.3

Deep on-axis PSFs in best focus positions

Using the best focus positions determined in Section 3.2 for the MM-0051 mirror module, the iXOU and oXOU deep PSF exposures obtained using both TRoPIC and PIXI (see summaries in Table 4 and Table 5) where obtained at the so-called laser position, the image distance of i = 13336 mm that corresponds to at the nominal design focal length of f = 12000 mm. Then at the best HEW position for the MM and the individual XOUs. Finally, the PSFs where obtained at the best transverse PSF (FWHM-X) positions, these are important for the spectral resolution of the Arcus mission [8]. In Table 4 the HEW, FWHM-X values of the PSFs measured in the (laser) position are shown together with the corresponding images. For the Mirror Module (both XOUs), the inner iXOU and the outer oXOU with 100% mask (full illumination) we obtain HEWs of 11.4 arcsec, 9.6 arcsec and 11.9 arcsec respectively, these numbers are given for the nominal focal length (Table 4). In the measured best HEW focus position of the MM, the iHEW and oHEW focus positions of the iXOU and oXOU the measured HEWs are 11.5 arcsec, 9.3 arcsec, and 10.5 arcsec as measured with TRoPIC (Table 5).

Table 4

Overview of the deep on-axis PSFs imaged with TRoPIC at the image distance corresponding to the nominal 12-m design focus positions at Al-K for the MM, the iXOU, and the oXOU. The mask used and the HEW, and transverse PSF FWHM-X are displayed together with the image of the corresponding PSF. Images and data correspond to the laser position measurements in Table 5, where we also report values obtained during the characterization at the BESSY II synchrotron facility (1 keV).

Table 5

Summary of all deep TRoPIC images of the MM, iXOU, oXOU with 100% mask at Al-K. Also shown, where available, the results obtained at the BESSY II synchrotron (1 keV).

*

Relative position is the position relative to the laser position (nominal 12 m focus at infinity).

Looking at the 100% mask results, the two XOUs do not yet place their PSF at the same location in the focal plane, in this case the distance between the two PSFs is just under 180 μm at the nominal focal distance of 12 m: although this is the best result obtained so far, further work is being done to reduce this difference, so that the performance of each individual XOU contributes optimally to the performance of the MM.

3.4

Azimuthal (Hartmann) scan

During this scan a 3 mm wide mask is shifted across the inner and outer XOUs in order to monitor the quality of the PSF with respect to the azimuthal position. This method provides detailed knowledge on where errors in the optic mainly come from. A compilation of the images made at each azimuth position during the scan is displayed in Figure 5 and the HEW results obtained from the analysis of these PSFs are shown in Figure 6. Once again there is an excellent agreement between the measurements taken at PANTER and those taken at the XPBF 2 [9], showing that the assembly and characterization of a SPO using a pencil beam [10] is a reliable technique for the assembly and characterization of segmented X-ray optics. Figure 5 shows the images of the optics taken during the azimuthal scan. This “Hartmann test” indicates that the performance is better than 8” over more than 70% of the optic, and that the degradation of the sides is significantly reduced with respect to earlier samples. The numbers computed for the smaller regions around the center of the optic are summarized in Table 6. The iXOU was scanned in the focus position 152881 steps and the oXOU in the focus position 124580 steps, in the best TRoPIC iHEW and oHEW positions.

Figure 5.

Compilation of images obtained during the azimuthal scans of the inner XOU (top) and outer XOU (bottom) with 3 mm mask, the scan runs horizontally from the left side to the right side of the XOUs. In Figure 6 the analysis of the above PSFs is shown.

Figure 6.

Overview of the azimuthal scan results obtained using the PANTER TRoPIC (purple & yellow data points) and PIXI (red & green data points) detectors. The graphs show the results of the azimuthal scans with the 3 mm slit mask over (left) the inner XOU-0132 (iXOU) and (right) the outer XOU-0131B (oXOU). For comparison, the results obtained at BESSY are overlaid with a dotted blue line with ‘*’ symbols at the 22 azimuth positions.

Table 6

Here using the azimuthal scan PSF images taken with PIXI, summed up images have been created and analysed to obtain the HEW and the FWHM-X values for different sized azimuthal regions of the optic. All selected regions are centered on the middle of the optic. The 70% values obtained at BESSY are shown in parenthesis.

3.5

Effective Area

When computing the expected effective area for MM-0051 SPO one has to keep in mind that the optic is uncoated and therefore the surface reflectivity is that of SiO2. In Table 7 we show the measured effective areas for the MM and each XOU, together with the highest possible predicted effective area, assuming ideal XOUs. Pending more detailed modelling we take that the ideal effective area must be reduced by 10% to account for the alignment and manufacturing obscuration factors.

Table 7.

Results of the effective area measurements at Al-K for the MM-0051, as well as for the inner and outer XOUs with the 100% mask in place.

3.6

Vignetting curves

The data for the vignetting curves was obtained by stepping (4 arcmin steps) through a sequence of off-axis tilt (pitch) and rotation (yaw) angles which correspond to different off-axis positions. The curves displayed in Figure 7 and Figure 8 show the dependence of the effective area of the optic as a function of the pitch and yaw angles for both the inner and outer XOUs.

Figure 7

(left) Pitch and (right) Yaw scans of effective area vignetting curve of the iXOU

Figure 8

(left) Pitch and (right) Yaw scans of effective area vignetting curve of the oXOU

3.7

MM-0051 Measurements Summary

The MM-0051 optic is also one of the best measured mirror modules so far in full illumination. With 100% mask (full illumination) we obtain a HEWs of 11.4 arcsec, 9.6 arcsec and 11.9 arcsec respectively for the Mirror Module (both XOUs), the inner iXOU and the outer oXOU. These numbers are given for the nominal focal length. A 70% mask was not available but we are able to determine the PSF for the inner 70% of the optic by adding up the images from that region obtained during the azimuthal scan of the iXOU and oXOU. These summed images yield a HEW of 7.2 arcsec and 7.8 arcsec for the iXOU and oXOU. These values were obtained in the corresponding best HEW focus positions for each XOU.

The confocality of the system has been improved substantially, but it needs to be improved further to convert the performance of the individual XOUs into a comparable aggregate MM performance.

The effective areas measured, both at XOU and MM level, are compatible with expectations.

4.1

ATHENA petal opto-thermo-mechanical tests at PANTER

Currently tests are ongoing at PANTER in which MPE, ESA, cosine and Media Lario are performing two complex opto-thermo-mechanical tests of the two full scale 1/6th sectors of the final ATHENA mirror assembly structure produced by the potential ATHENA primes Airbus Defense and Space (ADS) and Thales Alenia Space (TAS). For these tests a set of 3 ATHENA row-8 SPO MMs have been produced following the flight configuration recipe. The characterisation of these optics prior to incorporating them into the optics in the ATHENA petals is described in Section 4. The SPO MMs will be mounted into each sector by Media Lario at PANTER shortly before each test begins. After the first petal test Media Lario will remove the optics from the first tested petal so that they can be reused in the second petal.

The setup of the opto-thermo-mechanical tests in the vacuum chamber is shown in Figure 9 together an image of the setup as mounted in the chamber. Frames have been manufactured to hold the shrouds on the space side and the spacecraft side as well as a kinematic mount for holding the petals during the test. As these frames are large and heavy (about 380 kg including the Petals) they have to be mounted on the movable optical bench (tilt and rotation movement of up to ±3°) in the large vacuum chamber. As the image distance is about 13.3 m from the optics the image is formed outside the large chamber. A smaller 3-m long extension chamber containing a 3-axis translation stage for the CCD camera can be attached to the main chamber via a DN250 flange. This requires a very precise positioning of the illuminated optics in the petal to ensure the focused images make it to the detectors. A gap between the shrouds both on the hot and cold side of the setup allows the X-ray to illuminate the optics and the reflected X-rays to form an image, the PSF, on the detector.

Figure 9.

(left) Overview of the thermal opto-thermo-mechanical test of the ATHENA petals that is due to take place in summer 2022 at PANTER. From the left the X-rays form the X-ray source enter the three ATHENA “row-8” optics that are mounted in the petal and are double reflected on to the detector which is mounted on an x-y-z-stage in an extension chamber (not shown) attached to the large PANTER vacuum chamber. (right) picture of the PANTER vacuum chamber with the shroud around the petal wrapped in multilayer insulation. The shroud is mounted on the movable optical bench that enables pitch and yaw movements of the setup. The three optics (top MM-0049, center MM-0048, bottom MM-0047) mounted in the petal are visible, with the setups as mounted in the vacuum chamber prior to testing the X-ray come from the rear side of the setup.

The main goal of these petal tests is to quantify the impact thermal gradients on the thermoelastic behavior of the sectors and compare the measurements with the model predictions. During these tests the location of the centroids of the 3 X-ray PSFs of the ATHENA “row-8” optics mounted in the petal will monitored as a function of the thermal load cases listed in Table 9.

Table 9.

Summary of the load cases for testing the ATHENA petals at PANTER. The direction -Z is towards the X-ray source.

The heaters and thermocouples were installed on the petal to ensure that together with the PANTER cooling and heating shrouds the required gradients can be generated and measured on the petals. For this in the most extreme case the cold shrouds will be cooled to -40°C and warm ones heated to +35°C. Furthermore, heaters installed on the petal close to the optics will allow an even larger enhancement of the load case gradients.