eROSITA mirrors are done!

November 13, 2013
After more than seven years of development and construction, the last of the eight mirror modules for the eROSITA X-ray telescope was completed on Friday, 8 November 2013. The Max Planck Institute for Extraterrestrial Physics leads the development and construction of the entire telescope, including its various components, in an international cooperation.

eROSITA will be part of the Russian-German space mission Spektrum-Röntgen-Gamma. From 2016 onwards, it will map the entire sky with unprecedented sensitivity and for this task, eROSITA consists of seven identical X-ray mirrors with X-ray cameras developed at MPE at each focal point. The integration of these mirror modules is now complete and extensive tests demonstrated that the collecting area and resolution the project scientists aimed for have been achieved.

eROSITA mirror module (from front), being prepared for quality tests with X-rays in the PANTER test facility. The 54 individual mirrors are placed one inside the other like a Russian Matryoshka doll and have to be aligned with high precision.

"Over decades, we have accumulated experience with X-ray telescopes at MPE, but the relatively small size of the eROSITA mirrors posed completely new challenges," says Peter Predehl, Principal Investigator of the eROSITA project. "We could only overcome these technological problems with a huge, concerted effort by our team and our partners - and after many sleepless nights. But now we are all very proud and happy to conclude the mirror integration successfully!"

X-rays can be reflected on a mirror surface only at very small angles; therefore the mirrors of a so-called Wolter telescope (named after its inventor, the physicist Hans Wolter) are nested inside each other in a module (see Figure 1). For each of the 7 eROSITA mirror modules (plus a spare module), 54 mirror shells had to be inserted and aligned with high precision. In addition, the mirror surface of each mirror had to meet extremely high standards - the surface roughness is 0.3 nm (less than a millionth of a millimetre).

Mirror module with mounted aperture ("baffle"). The baffle itself had to be built with very high precision as well; all baffles were developed and built entirely at MPE. This image also shows the wiring of the heaters that keep the mirror at a constant temperature.

The mirror shells are made of nickel and each shell is produced by electroforming from a highly-polished negative form ("Mandrel"). Afterwards they get a gold coating to increase the reflectivity of their surface. This technique had already been successfully applied to the ESA XMM-Newton telescope, but the eROSITA mirrors are significantly smaller than those for XMM-Newton. Counter-intuitively, this relatively small size caused big trouble. Nevertheless, the developers were able to achieve the planned collecting area (about 350 cm^2 depending on energy) and resolution (roughly 16 arcsec).

eROSITA was developed in a collaboration with an industrial consortium, including at the beginning the companies Zeiss in Oberkochen and Kayser-Threde in Munich. The construction of the mirrors was then accomplished north of Milan by the Italian company Media Lario Technologies. eROSITA is funded by the German Aerospace Center DLR and the Max Planck Society.

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