The science aims of ATHENA are highly demanding and require state-of-the-art instrumentation.

First of all, an X-ray telescope of at least 1 m² effective area at 1 keV with a spatial resolution < 10 arcsec, and a focal length of 12 m is required. The focal-plane plate-scale for such an optical system is 58 μm/arcsec.

Secondly, a single focal plane instrument cannot meet the requirements on spatial resolution, field-of-view (FoV), energy resolution, energy range, quantum efficiency and count rate capability. Therefore, the design uses two fixed, co-aligned telescopes, each feeding a fixed focal plane instrument.

The two instruments are:

• an X-ray imaging Microcalorimeter Spectrometer (XMS) that covers the 0.3–10 keV energy range with unprecedented energy resolution (∆E = 3 eV at E < 7 keV, E/∆E = 2300 at E > 7 keV), a 2.3 × 2.3 arcmin FoV and relatively modest count rate capability;
  
  
• a Wide Field Imager (WFI) covering the 0.1–10 keV energy range with a large (25 × 25 arcmin) FoV, excellent angular resolution (≤ 10 arcsec) and efficiency, good energy resolution (∆E ≤ 150 eV (FWHM) at 6 keV) and good time resolution (32 µs).
  
  

The XMS is a 2-D imaging camera, which will allow the identification and characterisation of the different ionisation stages in hot plasmas thanks to its excellent spectral resolution. This requires that the camera operate at cryogenic temperatures. Complexity and heat load considerations will limit the size of the camera to about 1000 pixels. Compared to the planned cryogenic spectrometer on ASTRO-H, the first mission that will include a calorimeter, the number of pixels will be increased by a factor 30, the spectral resolution by a factor of two, the effective area by a factor of ten and the angular resolution by a factor of eight.

The WFI is an imaging X-ray spectrometer with a large field of view with solid heritage from eROSITA and BepiColombo-MIXS. The purpose of the WFI is to provide X-ray images simultaneously with spectrally and time-resolved photon detection. The advanced random access pixel architecture allows a very rapid readout, to enable microsecond time resolution and very high count rate capability. This allows high quality X-ray spectroscopy on unprecedentedly bright targets.

(For more detailed information about  the ATHENA instruments, please consult the ATHENA assessment study report (Yellow Book) - see link in right-hand menu.)