Instrument
The WFI (Wide Field Imager) instrument is one of two complementary focal plane cameras planned for ESA’s ATHENA X-ray observatory. It combines unprecedented survey power through its large field of view of 40' x 40' with an excellent high count-rate capability.
The performance is accomplished by the combination of a 2x2 mosaic of large-area DEPFET (depleted p-channel field-effect transistor) active pixel sensor matrices with a single smaller gateable DEPFET sensor matrix optimized for the high count-rate applications.
Each DEPFET pixel is a combined detector-amplifier structure with a MOSFET integrated onto a fully depleted 450 µm thick silicon bulk with a sensitive area covering 130µm x 130µm. The energy resolution is state-of-the-art over a broad energy band (approx 0.2-15 keV) with an expected full width at half maximum at 7keV of ≤170 eV throughout the mission.
The WFI is composed of several subsystems. The filter wheel controls the photon flux arriving from the Athena mirror on the focal plane. The energy, incidence position, and arrival time of the X-ray photons are measured by the DEPFET active pixel sensors which are controlled and read out by the front-end ASICs. Together they form the camera head, surrounded by a graded Z-shield and a proton shield to minimize instrumental background and radiation damage, respectively. The camera head is cooled passively via heat pipes and radiators.
The analog signal from the camera head is transferred to the six detector electronic boxes (one for each large DEPFET and for each half of the fast DEPFET) where it is digitized and the events are pre-processed. The Interface Control and Power Distribution Unit (ICPU) is the communication interface between the Athena spacecraft and the WFI also controls the operation of the six detector units, merges their six data streams and is responsible for the filter wheel and thermal control.
The main characteristics of the WFI are summarized below.
| Parameter | Characteristics |
|---|---|
| Energy Range | 0.2-15 keV |
| Pixel Size | 130 µm x 130 µm (2.2"x2.2") |
| Operating Mode | rolling shutter (min. power consumption) full frame mode optional: window mode |
| High-count rate detector | 64 x 64 pixel, mounted defocussed split full frame readout time resolution: 80 µs 1 Crab: >90% throughput and <1% pile-up |
| Large-area DEPFET | 4 quadrants, each with 512x512 pixel (total FOV 40'x40') time resolution: <5 ms |
| Quantum efficiency incl external filter |
>20% @ 277 eV >80% @ 1 keV >90% @ 10 keV transmissivity for optical photons: 3 x 10-7 transmissivity for UV photons: <10 -9 |
| Energy Resolution | FWHM(7 keV)≤170 eV |
| Non-X-ray Background (L2 orbit) | <5x10-3 ct cm-2 s-1 keV-1 |
More details can be found in, e.g., Meidinger et al. 2015