XMM tests at CSL come to an end
11 November 1998After of two and a half years' work, the Centre Spatial de Lihge (CSL)in Belgium is coming to the end of the calibration tests it hasbeen carrying out on the Mirror Modules of XMM, the European SpaceAgency's X-ray space telescope. In mid-December, after somethermal hardware integration activities, the three flight modules willbe leaving for ESTEC for integration there early next year onto thesatellite platform.
On 12 October, the XMM project manager visited the ESA co-ordinated testing centre, created in 1959 as a laboratory of the Institute of Astrophysics of the University of Lihge. It was an occasion for Robert Laini to thank the CSL director Claude Jamar and his 80-strong staff for their contribution to the satellite. Present also was a film crew producing a documentary on the development of the mirror modules.
The need to test and calibrate the spacecraft9s mirror modules at a dedicated test facility was recognised very early in the XMM programme. The tests on the mirror assemblies had to be effected vertically to avoid the deformations resulting from gravity given their extreme flimsiness. Also, the test required the use of a highly accurate parallel beam to simulate the position of the stars at infinity and to eliminate the effects of the divergence of light (see below). In 1994 it was decided to build the Focal X9 vertical test facility at the Centre Spatial de Lihge.
The facility was financed jointly by ESA (under the XMM programme budget) and the University of Lihge. Only 26 months passed between the initial decision in December 1993, through construction, to the first XMM tests in March 1996.
The FOCAL-X installation (Facility for Optical Calibration at Lihge) consists of a vertical cylindrical vessel or chamber, 4.5m in diameter and 12 m high, and associated pumping systems to create a vacuum. The mirror module under test is entered through a door in the vessel and positioned within it on a supporting structure, the lower optical bench, whose axis can be modified. 10 m above is placed the upper optical bench with its detection instruments (cameras) at the focal point of the mirrors. The apparatus that provides the three optical stimuli at various wavelengths (visible, extreme ultraviolet and X-ray) in beams of different shapes (sizes) is situated at a depth of nearly 10 m below the level of the mirror module.
All handling of the mirror modules is carried out in an extremely clean environment (class 100 cleanroom). The tests take place under vacuum (10-6 mbar reached in about 4 hours). All the optical conditions of the satellite's operation in space are thus simulated. The XMM modules have also been submitted to vibration and thermal tests before and after the optical measurements.
The objective has been to obtain precise data on the optical characteristics of each mirror module. This will allow astronomers using XMM to make the best use of the telescope knowing all the performance aspects of its three mirror modules. The results of the tests also led to the choice of the three modules with the best specifications which will finally be embarked on the satellite.
The CSL has contributed to previous ESA science missions (Hubble, Giotto, Hipparcos, ISO, SOHO). It will be used to test the Integral gamma-ray telescope. With its FOCAL-X vertical test facility, it is today the only one of its kind in the world. It has well fullfilled its role in the XMM programme and, being adapted to other types of test and telescopes, could be the basis for next generation scientific programmes.
The XMM mirrors
Each XMM mirror is a 60 cm long cylinder approximately 1mm thin. 58 of them, of diameter from 700 mm to 306 mm, are nested inside each other like russian dolls. Their 'flimsiness' refers to the low thickness to diameter ratio of each mirror (thickness of 1 mm for diameter of 700 mm).
Manufacturing the mirrors by Media-Lario in Italy has necessitated the development of new cutting-edge production and integration techniques. Special procedures and tools have had to be devised to overcome all the problems encountered when handling these wafer-thin elements. The gold surface of each mirror is extremely smooth - to within a few atoms ! - a precision upon which the performance of XMM will largely depend.
Even when the 58 mirrors are integrated onto their support structure (or 'spider9), placing them in a horizontal position would inevitably entail some deformation. An essential confirmation during the tests was also that the effects of the divergence of a light beam compared to the parallel beam - preventing the illumination of the complete mirrors and the associated gratings - were even more important than those of gravity.