ROSITA - ROentgen Survey with an Imaging Telescope Array

Hasinger G.$^1,2$, Predehl P.$^1$, and Tr\"umper J.$^1$

$^1$ Max-Planck-Institut f\"ur extraterrestrische Physik, Garching, Germany
$^2$ Astrophysikalisches Institut Potsdam, Germany

The current X-ray observatories Chandra and XMM-Newton are 
now successfully exploring the sky with pointed observations in 
the hard X-ray domain and have already yielded new classes of 
heavily obscured objects, which were not visible with previous 
X-ray missions. However, there is so far no sensitive all-sky survey 
planned in the same waveband. The experience with the ROSAT All-Sky
Survey and pointed observations has told us, that a complete and 
homogeneous all-sky survey with unlimited field of view is an 
essential tool to understand the large scale structure of the 
universe as well as the diffuse emission in our own galaxy. A 
wide-angle survey is also the only possibility to obtain 
large-volume local samples of active galactic nuclei and 
clusters of galaxies to be compared with the high-redshift samples
from pointed observations in order to determine their cosmological
evolution.

ROSITA (ROentgen Survey with an Imaging Telescope Array) is a 
project to accommodate an ABRIXAS-type telescope on the International
Space Station (ISS). Seven 27-fold nested Wolter-I mirror systems jointly
illuminate a single pn-CCD detector of the same type now very successfully  
operated aboard XMM-Newton. With a simple tilt mechanism perpendicular
to the natural scanning motion of the ISS the whole sky can be covered
with a sensitivity about 100 times fainter than the last
sky survey in trhe same energy band. About 100000 new obscured AGN and 
several ten thousand clusters of galaxies are expected during a 
3-year mission. The telescope is built in a modular fashion, such that the 
focal-plane detector can be replaced at a later time and therefore ROSITA 
can be used as a test bed for a new generation detector, e.g. for XEUS.