3.1

Installation and Setup

The optic was installed on its side, as the ~12 m focal length of the optic restricted the setup to certain configurations. Figure 3-1a shows the optic installed in the PANTER chamber attached to a hexapod, which allows fine movements of the optic once under vacuum. The pivot point of the hexapod is set to the centre of the optic. To assist the reader in visualising the measurements in this paper, the ‘top’ of the optic is the edge of the optic closest to the roof of the facility, and the ‘bottom’ of the optic is closest to the ground. This coordinate system is indicated in Figure 3-1b.

A moveable mask was installed between the optic and x-ray source, with apertures capable of illuminating 100%, 48%, and 4% of the optic. These apertures were chosen to enable characterisation of the full optic and of specific sections of the optic. The mask is visible in Figure 3-1a.

Figure 3-1:

(a) The XOU-0055 mounted inside the PANTER chamber attached to a hexapod, to enable fine adjustment of the optic under vacuum. The moveable mask used to illuminate either the full optic or a chosen section of the optic is also shown. The MLI between the optic and the hexapod protects the optic from the heat produced by the hexapod motors. (b) The optic in its holding structure at the PANTER facility, showing the nomenclature for the measurements of the optic during this campaign. (Credit: MPE)

3.2

Alignment

Once under x-ray illumination, the 100% mask was aligned over the optic by observing the total x-ray count rate for the optic. The mask was moved incrementally and the corresponding change in count rate was observed using the online detector tool. Once the 100% mask was aligned, the 4% mask was aligned using geometric distances from the mask CAD drawing, then finely adjusted by eye to observe when the x-ray scattering around the point spread function (PSF) was and was not visible.

The optic was aligned in pitch and yaw using the 100% mask. This is part of the standard optical alignment method at PANTER, the aim of which is to minimise the HEW and maximise the flux over a range of pitch and yaw angles⁴.

Once the optic alignment in pitch and yaw was completed, a focus search⁴ was carried out to determine the best focus position for the detector. The detector was translated towards and away from the x-ray source ±50 mm around the initial in-chamber alignment position in steps of 5 mm. An exposure was made at each of these positions, and the corresponding HEW, x-FWHM, and y-FWHM calculated. Fitting a parabola to the data determined the minimum HEW and y-FWHM and the corresponding ‘best focus’ position. The y-FWHM was taken as the final fitting metric, due to the variance of the HEW across the optic surface.

Cosine had measured the optic at BESSY, at the PTB XPBF 2.0 beamline⁷, prior to this campaign. As such, it was known that the best focus position varied across the optic. It was decided that a second focus search should be carried out, for the bottom half of the optic, using the 48% mask, where the best HEW performance was expected. The bottom-of-optic best focus position deviated from the complete optic best focus position by 28.7 mm.

3.3

Measurements

All measurements were made at Al-K (1.49 keV).

4.1

HEW

Table 4-1 presents the HEW values measured during this campaign using both single exposures and pixel scans. The pixel scan method is more precise thus it was expected that it would yield a slightly different HEW value to that of the single exposure method. Figure 4-1 shows the resulting PSF for the two pixel scan measurements.

Table 4-1:

The HEW values of the optic for each measurement set, showing the mask used and the section of the optic imaged

Figure 4-1:

The best focus PSF image for (a) the full optic with the 100% mask, and (b) the bottom ~30% of the optic with the 48% mask. The corresponding HEW values are written on the appropriate image. (Credit: MPE)

Figure 4-2 shows the plot of all 24 azimuthal scan HEW values for the full optic. The azimuthal measurements were made prior to the pixel scans, thus it was possible to identify the region of lowest HEW – the bottom ~30% of the optic. It is believed that the HEW of the optics varies in the sagittal direction due to the meridional curvature of the optics varying across the sagittal direction. Why this occurs is ongoing, but the measurements taken at cosine are compatible with this probable cause.

Figure 4-2:

The HEW at each position of the azimuthal scan, aligning the 4% mask in front of the optic, starting at the ‘top’ of the optic, and measuring 24 separate positions (each one a separate exposure) to the ‘bottom’ of the optic. (Credit: MPE)

4.2

Intra-Focal/Extra-Focal Images

Figure 4-3 shows the intra-focal (220 mm) image for the full optic, using the 100% mask. Figure 4-4 shows the extra-focal at 250 mm and Figure 4-5 the extra-focal image at 340 mm for the full optic, using the 100% mask. It is clear in the distortion of the images that there is a difference across the reflecting surface of the optic. However, no unexpected features were observed in the images.

Figure 4-3:

Intra-focal 220 mm (a) full image and (b) x4 zoomed image of XOU-0055, at the best full optic focus position, taken at Al-K using the 100% mask.

Figure 4-4:

Extra-focal 250 mm (a) full image and (b) x4 zoomed image of XOU-0055, at the best full optic focus position, taken at Al-K using the 100% mask.

Figure 4-5:

Extra-focal 340 mm (a) full image and (b) x4 zoomed image of XOU-0055, at the best full optic focus position, taken at Al-K using the 100% mask.