eROSITA consists of seven identical and co-aligned X-ray mirrors (see Gallery). Each of the mirrors comprises 54 paraboloid/hyperboloid mirror shells in a Wolter-I geometry, with an outer diameter of 358 mm and a common focal length of 1600 mm. The average on-axis resolution of the seven mirrors as measured on ground is 16.1'' Half-Energy Width (HEW) at 1.5 keV. However, to improve the angular resolution averaged over the field-of-view (FoV), each telescope was put out of focus, leading to a slight degradation of the on-axis angular resolution (18''). The resulting averaged-FoV resolution is about 26''.
Each eROSITA mirror has a charge-coupled device (CCD) camera in its focus. The eROSITA CCDs each have 384 x 384 pixels in an image area of 2.88 x 2.88 cm2. The circular FoV has a diameter of 1.03 degrees. Each pixel corresponds to a sky area of 9.6'' x 9.6''. The eROSITA CCDs contain a framestore area, which almost completely suppresses the amount of so-called ''out-of-time'' events. The image area is shifted within 0.115 ms into this framestore area and the read-out happens within 9.18 ms. The nominal integration time of the eROSITA CCDs is 50 ms.
In a nutshell, the basic eROSITA instrument parameters in launch configuration are:
| Instrument | 7 mirror assemblies | 7 camera assemblies |
|---|---|---|
| Size - 1.9 m ⌀ x 3.5 m | Diameter of outer shell - 358 mm | CCD image - 2.88 x 2.88 cm2 |
| Mass - 808 kg | Number of shells - 54 | Pixel size - 75μm x 75μm |
| Power - 522 W max. | Focal length - 1600 mm | Time Resolution - 50 ms |
| Data rate - 600 MB/day max. |
Each camera has its own filter wheel, which has four positions: