The PANTER X-ray test facility in Neuried of the Max-Planck-Institut für extraterrestrische Physik, Garching was designed and built at the end of the 70s for developing, testing, and calibrating the X-ray optics (focal length 2.4 m) and detectors for the ROSAT X-ray observatory. This included the setting up of a beamline with a long vacuum tube with (length 120 m and diameter 1 m) and a test chamber (length 6m and diameter 3.5 m). For testing the XMM-Newton optics (focal length 7.5 m) and detectors this chamber was enlarged (length 12 m but maintaining the same diameter). After XMM-Newton intra-focal tests on prototype modular optics for the proposed large new ESA X-ray observatories were performed at PANTER. This was done end of the 90s for XEUS  (focal length 50 m, effective area 6 m2 (30 m2 on ISS) at 1 keV) for Ni shells and starting in the mid 2000s for IXO  (focal length 20 m, effective area 3 m2 at 1 keV). A new optics technology, the Silicon Pore Optics (SPO) [7-8] was developed by ESA in conjunction with the company Cosine to make it possible develop light weight modular optics that can be used in large collecting area missions. From 2012 and extension to PANTER  made it possible to obtain in-focus measurements of 20 m focal SPO Mirror modules. After the IXO project was discontinued ATHENA (focal length 12 m, effective area 2 m2 at 1 keV, HEW 5”) was selected as the new ESA L2 mission. Since then developments and measurements have been going on.
Furthermore the same SPO mirror technology that will be used in ATHENA has been set as the baseline optics for the proposed high resolution spectroscopy NASA MIDEX mission Arcus which has currently finalized its Phase A study and is awaiting a decision.
PANTER has played an important role in the development process of the SPOs as it is currently the only place in Europe that performs tests requiring a full illumination of the mirror modules.
TYPES OF OPTICS MEASURED AT PANTER FOR ATHENA
Optics modules (elements) for ATHENA and its predecessor missions IXO and XEUS mirror assemblies have been developed and most of them tested and measured at PANTER.
For many years slumped glass optics (SGO) were considered a possible optics solution for ATHENA, these were tested at PANTER in many different configurations from single layer primary + secondary shell modules to modules with many co-aligned layers. Currently the main development efforts for ATHENA from which Arcus also benefits are concentrated on the Silicon Pore Optics (SPO).
The plan is that a completed ATHENA mirror assembly (MA) will contain between 700 and 1000 SPO mirror modules, in the case of Arcus it would be 4 mirror modules each containing 38 SPO mirror modules [9,10].
Before a complete mirror assembly or petal can be tested the SPO mirror modules have to be individually assembled at BESSY. The SPOs are made up from two co-aligned X-ray optical units XOUs and each XOU is made up of a primary and secondary high performance optical (HPO) stack (i.e. a parabola and a hyperbola in the case of a Wolter type I X-ray optic). These HPOs are made of 35 layers of silicon wafers that are diced, wedged and grooved, coated and stacked on a high precision mandrel. The HPOs once stacked have many pores (i.e. opical channels) the give the optics their name. In Figure 1 the process from Wafer to SPO Petal for ARCUS is shown.
To test X-ray optics and X-ray Telescopes in PANTER  a standard set of alignment and measurement sequences are performed to qualify the optics. These are adapted to conform with the needs and the status of the development of the different optics. Before the tests take place input from simulations  and if available measurements taken at other beamlines (i.e. BESSY) are used to prepare and optimize the measurement plan.
Laser Alignment of Optics Modules on Air
Once the optics are mounted in the large vacuum chamber either on the tip/tilt stage and rotary stage or the hexapods a laser beam that marks the geometric center of the beamline from the X-ray source to the center of the large vacuum chamber is used to do a preliminary alignment of the optic. This alignment entails checking that the optic is aligned in auto-collimation (sometimes using a reference mirror) and that the PSF is located on the detector. The separation of the X-ray detector to the intersection plane of the optic to the detector is set to value computed using the thin lense equation that takes into account that with a finite source distance, i.e. the image distance lies further from the optic than the true focal distance of the optic.
X-ray Alignment (pitch and yaw) under Vacuum
As a starting point in-vacuum alignment measurements are done using Al-K characteristic line at 1.49 keV as this is very efficient in producing X-rays that can also reflect nicely off uncoated glass and Si surfaces which are commonly used during the early development phases of X-ray optics. For optics modules such as the SPOs the best alignment reference we have is the self-shadowing by the SPO pores which produce nice quasi triangular (vignetting) functions of intensity vs. off axis angles.
X-ray Focus Search
Following the X-ray alignment of the optic a focus search is performed in which the location of the best HEW (important for ATHENA) and the best transverse PSF (important for Arcus) is determined: See the focus search shown in Figure 3
This test shows the quality of the mirror module as a function of azimuth. See Fig 4.
Point Spread function (PSF) Measurements
With these measurements the on and off-axis PSFs are characterized at different energies. The main parameter derived from these measurements is the Half Energy Width HEW (i.e. the 50% encircled energy). For ATHENA the requirement is that the average HEW for the SPO MMs is about 4.3 arcsec and 5 arcsec for the integrated Mirror Assembly.
For Mirror modules like the SPOs also the Full Width Half Maximum FWHM of the transverse PSF i.e. the waist of the bowtie shaped PSFs is measured. For the spectroscopic missions such as Arcus this important value should approach 1.5” so that the highest spectral resolution can be achieved.
Effective Area (EA) measurements
The best way of determining the effective area for the optics is done by measuring the countrate (cts/s) of the intrafocal image on the detector. Then without modifying the source settings that are a given energy you remove the optic from the beam and measure the flat field countrate per square cm (cts/s/cm2) impinging on the detector of a known size (cm2). The ratio of these two numbers corrected the distance between optic and detector gives the effective area (cm2) of the optic.
RECENT ATHENA AND ARCUS RELATED TESTS
Recently at PANTER tests were done to check two competing Assembly Integration and Testing (AIT) methods to mount and co-align the SPO modules into the mirror assembly in this case two optics into a mini petal. One AIT approach (Media Lario) [15,16] uses UV light to check the alignment of the optics into the petal. The other approach (Thales-CH) [17,18] used a metrology characterization of the individual optics at BESSY and then co-aligned them based on the metrology in PANTER. Both methods provided results that were within the required accuracy needed for ATHENA in September 2018.
Currently we now have a setup running at PANTER that allows the mounting of two HPOs into a double hexapod support structure. This allows the testing of primary plus secondary HPO pairs that have not been glued into brackets. This allows PANTER to be even more involved in the development of the ATHENA optics. A very good matching between the BESSY and PANTER measurements can be seen in Figure 4 central panel.
For ATHENA in the near future many more iterations of measurements at PANTER have to be performed on the XOU and SPO MM level to ensure that set HEW goal of 5 arcsec for mirror assembly can be achieved. Furthermore testing of ARCUS petals and later a 1/6 sector of an ATHENA mirror assembly can be tested at the existing PANTER X-ray test facility. To test and calibrate the complete mirror assembly a new facility will have to be built. The ATHENA  optics design requires an 800 m long beamline with an entrance aperture to the chamber between 2.8 m and 3.3 m depending on the final diameter of the Mirror assembly. A possible location of the new X-ray test facility at the Garching science campus is shown in Fig. 6.
The research in this paper has in part received funding from the European Union’s Horizon 2020 Program under the AHEAD  project (grant agreement n. 654215) and the ESA contract 4200023066/10/NL/AF